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Properties of materials from Birch

Variations and relationships Part 1: Growth, wood density and biomass

Sven-Olof Lundqvist, Thomas Grahn, Lars Olsson

Innventia Report No.: 390

January 2013

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Properties of materials from Birch – Variations and relationships. Part 1: Growth, wood density and biomass Innventia Report No. 390

Acknowledgements This report has been produced within a project at Innventia with the objective to describe properties of wood, fibres, vessel elements, pulps and sheets originating from birch, as well as their variations within and among trees of different growth rates. The project has been funded by the “Södra Skogsägarnas stiftelse för forskning, utveckling och utbildning” and RISE.

The methods and data used for the work presented in the report have been successively built up by Innventia during the last seven years in a number of projects funded by different organisations and companies. The first sets of data, including the data on Scots pine used for comparison in this report, were compiled in the project “Innovood” within the Swedish-Finnish research programme “Wood Material Science and Engineering”, funded by VINNOVA, TEKES and a consortium of companies. The material on Eucalyptus urograndis (E. urophylla x E. grandis), also used for comparison, originates from a previous research cluster “Optimal Fibres for Pulp and Paper Production”, funded by Klabin Papéis, Stora Enso and Södra Cell.

The stands to sample were suggested by Anders Eklund, Södra. The sampling was performed by Asa Experimental Forest of the Swedish University of Agricultural Science (SLU).

In the start-up phase of this project, Innventia took up a discussion with the Linnaeus University in Växjö, Sweden, to see if there was an interest in working together to include also further properties related to solid wood for sawing, if it as a next step would be possible to expanded the study in that direction. The sampling was adapted to make this possible, and a year later further funding became available for such an expansion. The cooperation with our colleagues at the Linnaeus University has been very fruitful. The authors want to thank Harald Säll and Marie Johansson for inspiration and support.

All funding companies and organizations are acknowledged, and also all persons who have supported the development and application of the routines applied and the progress of the project.

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Table of contents Page

1 Summary ............................................................................................................... 4

2 Introduction .......................................................................................................... 5

3 Short on birch ....................................................................................................... 7 3.1 The species ................................................................................................. 7 3.2 Use of wood ................................................................................................ 7

4 Material and data .................................................................................................. 8 4.1 Birch stands sampled and other species used for comparison .................... 8 4.2 Sampling strategy ....................................................................................... 9 4.3 Sub-samples for different measurements and tests ................................... 10 4.4 Short on the measurements and tests performed ...................................... 11 4.5 The database ............................................................................................ 12

5 Growth ................................................................................................................. 14 5.1 Size and shape of the sampled trees ........................................................ 14 5.2 Annual growth ........................................................................................... 17 5.3 Diameter and height growth, averages up to age of sampling ................... 18 5.4 Cross-sectional development by age and radial growth rate...................... 20 5.5 Discussion on cross-sectional growth ........................................................ 23 5.6 Height development by age and height growth rate ................................... 24 5.7 Annual growth units ................................................................................... 25 5.8 Volume by age and volume growth rate .................................................... 25

6 Wood density variations within and between stems of trees .......................... 28 6.1 Short on fine-scale density variations and structures in growth rings ......... 28 6.2 Radial density variations at different heights of the stems ......................... 29 6.3 Comparison of wood densities in stands with different growth rates .......... 33 6.4 Property variations along stems and by log diameter ................................ 37 6.5 Statistical distributions for wood densities among stands .......................... 39 6.6 Summary on patterns of within stem variations in wood density ................ 40 6.7 Wood densities of growth units .................................................................. 41

7 Biomass growth.................................................................................................. 43

8 Bark ..................................................................................................................... 45

9 Comparisons of growth and density – Summary ............................................. 46

10 Conclusions ........................................................................................................ 48

11 References .......................................................................................................... 50

STFI-Packforsk Database information ..................................................................... 52

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1 Summary This is the first report in a series covering growth and properties and their variations in birch. In the study, samples from birch trees with different growth rates from 3 stands were investigated and compared: Two stands of birch trees growing within stands of pine and spruce (low and medium growth rates), and one experimental stand of improved birch trees (high growth rate). The ages of the trees were about 80, 60 and 20 years, respectively.

In the study, standardised routines were used for sampling, sample preparation and characterisation of a multitude of properties of wood, fibres, pulps and paper originating from the trees. The routines had previously been developed and applied by Innventia for compatible data on different spruce, pine and eucalypt species compiled in Innventia’s Benchmarking Database. In the report, the birch trees are compared to such data for Norway spruce and Eucalyptus urograndis. For the birch project, new routines for studies of growth were developed, and also for investigation of properties important for solid wood products.

In this report, growth, wood density and biomass are covered. Differences and variations of properties of wood, fibres, vessels and sheets are presented in other reports of the series. The average wood density of the trees from the slow-grown forest stand was about 20% higher than for the trees of the other stands. But there were large differences among individual trees. The density increased with increasing radius for the trees from the slow-grown stand, less so for the medium-grown stand, while it was rather constant for the most fast-grown birch stand. The density variations were normally smaller along the stems than in their cross-sections, with the result that differences among trees prevailed along the stem. The wood density was much higher for birch than for Norway spruce, and higher to similar for birch compared to E. urograndis. Birch and eucalypts have no latewood with high density. They are therefore more homogeneous in the growth ring scale than spruce and other softwoods. The annual growth was much higher for the improved birch trees than for those of the natural regenerated stands. This was valid for growth of breast height diameter, height and volume as well as for dry wood biomass. The trees of the slow-grown forest stand showed lower or similar values for all these growth rates compared to those of the medium-grown forest stand. It may however be a bit dubious to compare growth of trees of different ages, being in stages of different dynamics and forest regimes. The most fast-grown trees were already at the age of 20 years approached their maximum diameter growth rate, and this will soon start to retard. For these trees, the thickness of the bark was found proportional to the diameter of the stem up to a diameter on bark of 20 cm, with a larger factor for the slow-grown forest trees. These methods made it possible to apply new approaches in the studies of growth and properties, and for research on selection of suitable trees to plant on different sites and for various uses, on matching practices to growth conditions, on optimal use of forest resources, etc. It is, however, also important to consider that the current study was of limited size. For solid results, it would be useful with an expanded study, based on samples from more trees with more well-defined origins, growth conditions and forest treatments, applying the same standardised routines as in the current study, to allow comparison with results from this study and with data on other tree species available in Innventia’s Benchmarking Database.

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2 Introduction

A series of reports on properties of birch

This is the first part in a series of publications to be published on birch, covering many aspects relevant for different uses within pulp and paper, sawing and bio-energy. It is a result of cooperation between Innventia and the Linnaeus University (Lnu) in Växjö. The parts of the series will be:

Properties of materials from birch - Variations and relationships: • Growth, wood density and biomass This report by Innventia • Mechanical and physical properties (Johansson et al 2014) – Report from Lnu • Wood, fibres and vessels • Pulps and sheets • Microfibril angle, wood stiffness and spiral grain

The later parts will be published by Innventia. An early overview was given in (Lundqvist et al 2012). Results are also included in (Lundqvist et al 2013) and Grahn et al 2013).

Strategy applied

Innventia has during the last 8 years investigated property variations within and among industrially important tree species growing under various conditions. Differences in properties of wood and fibres among species, stands, trees and parts of trees have been analysed. For hardwood species, also the vessels and vessel elements have been characterized. Also properties of materials produced from the wood of different origins have been investigated. The work includes softwood and hardwood species from different continents. Examples are given in Figure 1. The material and data are successively expanded in new projects. All data are compiled in a “Benchmarking Database” to facilitate comparisons and evaluations.

Figure 1.Eucalypt stands sampled in southern Brazil (left) and stand of Black spruce (Picea mariana) sampled in Canada (right).

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One objective of these efforts is to learn about differences among major wood and fibre raw materials used in the industry world-wide, as well as the variations within the species and how their properties overlap. Another objective is to build up a database with wide spans in properties for use in investigations of how wood and fibre properties influences properties of products produced from the materials. Part of the concept is also to use the data in the development of models describing these relationships. The data and models developed may be used for information on properties of forest resources and how they affect various types of products, for more product-oriented use of wood raw materials, for product development and process control in the mill.

For such joint use of data from different species and projects, compatibility of data is obviously a crucial issue. To secure this, Innventia has developed standardised routines for the work all along the chain from sampling, via data collection on stands and trees, sample preparation and measurements to the structure of the Benchmarking Database and evaluation routines. The routines are also designed to provide related information about wood, fibres and pulps, along the value chain. The routines are described in chapter 4 of this report.

In 2010, information on 4 softwood species sampled in Sweden, Finland, Canada and Brazil and 4 eucalypts from South America had been compiled in the Benchmarking Database, but no European hardwoods were included. So far, the value chains from wood and fibres, via pulp to various types of fibre-based products had been emphasised. However, also properties like wood density, microfibril angle and wood stiffness relevant for wood products were available in the Benchmarking Database.

At that stage, an analogous project on birch was started at Innventia with funding from Södra’s Research Foundation and RISE. Innventia saw, however, large synergies within reach if also mechanical wood properties and growth could be studied from the same material and took up a discussion about this with the Linnaeus University. The sampling routines were modified to provide also samples for expanded investigations towards solid wood products. The year after, funding was made available from the same sources to take up such work on the sample material stored for this purpose. Thereby, also the research behind the fourth part in this series of reports on birch became possible.

This widened concept addressing both fibre based and solid wood products is now applied by Innventia and the Linnaeus University in a project in South Africa in cooperation with Stellenbosch University. Three draught resistant types of eucalypts are investigated and their potential use as raw material for different products is compared.

The Benchmarking Database now (December 2012) includes data on: • 2 spruces: Norway spruce (Picea abies) and Black spruce (Picea mariana) • 3 pines: Scots pine (Pinus sylvestris), Taeda pine1 (Pinus taeda) and Pinus maximinoii • 7 species and crossings of eucalypts • Birch (Betula) used in research and development with industry, universities and institutes. 1 In North America normally called loblolly pine

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3 Short on birch

3.1 The species Birch is a broadleaved deciduous tree or shrub of the genus Betula. Various types of Betula grow in a vast geographic area, and under a variety of growth conditions. In Sweden, birch stands for about 2/3 of the total volume of hardwood and 11% of the total wood volume of the forest resource, and it is the third most common tree species.

In Sweden the two species Silver birch (Betula pendula) and Downy birch (Betula pubescens) are usually mixed in the forest. Figure 1 shows a forest dominated by birch, which clearly visualizes the character of this tree species. However, the largest volume of birch in Sweden grows as individual trees mixed into forest stands dominated by spruce and other tree species.

Figure 1. A forest of birch trees in Värmland, Sweden, (photo: Sven-Olof Lundqvist)

The current work is dedicated to birch growing in boreal forest, typical for large parts of Scandinavia, Finland and Russia. All trees investigated in the projects are from the county of Scania in the south of Sweden.

3.2 Use of wood The pulp and paper industry is the largest consumer of birch wood in Sweden. It is also used for production of plywood and furniture. Birch is also popular as firewood for heating.

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4 Material and data Standardised procedures previously designed by Innventia have been applied, in order to provide data on birch fully compatibility with existing data, to allow comparison with properties of other species. The procedures cover routines for sampling, sample preparation and characterization of many properties of wood, fibres, vessels, pulps and sheets. The sampling scheme has, however, in this case been somewhat widened to provide materials also for investigations of mechanical wood properties. Most of the selected trees are from the species Silver birch (Betula pendula). The material investigated is limited. This is however to some extent compensated by the possibility to compare the results with patterns of property variations previously observed for other species in the Benchmarking Database. And one conclusion is that the layout of the study can be recommended for further investigations.

4.1 Birch stands sampled and other species used for comparison The sampling for the birch study was designed to reflect effects of different growth rates. As written above, the major part of the birch resource in Sweden is constituted by individual trees growing in stands dominated by spruce and pine, even though also single species stands of birch exist. There are also experiments with improved birch showing higher growth. To reflect typical growth rates within the existing and potential future resource, the following types of stands to sample and the sites were suggested by Anders Ekstrand, Södra Timber:

1. a softwood stand including birch trees with low growth rate 2. a softwood stand including birch trees with normal growth rate 3. a stand of improved birch with high growth rate managed by Skogforsk

Further data about the stands are given in table 1. The properties of this boreal birch material will in the set of reports be compared with data on properties samples from two other species investigated by Innventia in previous projects:

• Eucalyptus urograndis, representing fast-growing plantation hardwoods • Norway spruce, representing boreal softwoods

Some data on the origins of these materials used for this comparison are also given in table 1.

Table 1. Some data on the birches sampled and the stands of other species used for comparison.

Stand (plot)

Short name Growth rate (among stands)

Site index Country Age years

Birch (Betula pendula) SB1 Slow-grown Low Sweden 80 SB2 Normal-grown Normal Sweden 60 SB3 Improved High Sweden 20

Eucalyptus urograndis (E. urophylla x E. grandis) Urograndis low and high SI Brazil 7

Norway spruce (Picea abies) Norway spruce thinning, final cut Sweden 33 and 65

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Figure 2 illustrates birch trees from stand 1 (slow-grown) and stand 3 (fast-grown).

Figure 2. The photo to the left shows the “slow-grown” birch trees (SB1) to be samples, after the harvesting of the surrounding pine trees which dominated the stand. The photo to the right shows the managed stand of improved birch (SB3). (Photo: Elis Bengtsson, SLU)

4.2 Sampling strategy The birches of the stands were classified into three size classes based on breast height diameter. From each natural stand, 4 healthy trees were selected typical from the size classes: 1 large (fast-grown) tree, 2 medium-size trees and 1 small (slow-grown) tree, see Figure 3. For the improved birch, only one medium-size tree was sampled. Data on the stand and the trees were collected for reference. After felling, samples were cut at four sampling heights along the stems, indicated with the black rectangles: a) at breast height (1,3 m), b) at 1/4 of the tree height, d) at stem diameter 8 cm and c) at half distance between b) and d). (We will return below to the orange partly fillings of the rectangles at some of the positions.) From all of these positions, samples were cut in the form of short logs, minimum length 45 cm.

Figure 3. The thin black rectangles indicate positions where samples for measurements of wood and fibre properties are taken, and orange rectangles samples cut for pulping or testing.

Tree 1 2 3 4 number

Stem of Stems of Stem of large medium small tree trees tree

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Asa Experimental Forest, SLU, wase engaged to perform the sampling (Ola Langvall, Elis Bengtsson), with support from the Linnaeus University (Harald Säll). Figure 4 illustrates the sampling. The red lines along the samples indicate the northern side of the stem. The sampled material was then divided into subsamples for measurements at Innventia and at Lnu.

Figure 4. Cutting of samples in the stand of improved birch, SB3 (photo: Elis Bengtsson, SLU).

4.3 Sub-samples for different measurements and tests The sampling above resulted in 16+16+12 = 44 samples in the form of short log of minimum 45 cm length. These samples were then divided into subsamples for different types of measurements, again following Innventia’s standardized procedures, see Figure 5. The Figure illustrates a part of the stem and a short log cut from it between the red lines. This part is divided into the following parts counted from below and upwards:

1. A 50 mm thick disc available for measurement of fibre length and other properties

2. A 30 mm thick disc to be used for measurement of a multitude of properties with SilviScan

3. A 70 mm thick disc available for measurement of spiral grain (this disc has been added to the previously standardized procedures.

4. A minimum 300 mm thick piece available either for pulping or mechanical testing

The measurements performed are described in some further detail in chapter 4.4 below.

Pulp was produced and investigated only from a limited number of the samples, namely the samples marked with orange in Figure 3. These samples for pulping are distributed among the total number according to a design providing pulps from wood of different characteristics: from fast-grown and slow-grown wood, as well as materials dominated by juvenile wood and

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mature wood. The positions of the orange parts within the black-lined rectangles indicate that the upper part of the 45 cm sample is normally used for pulping and/or physical testing, with the exception of the positions close to the top.

Figure 5. Illustration of how the short log samples are divided into sub-samples for characterization of a multitude of properties of wood, fibres, vessels, pulps and sheets, according to the standardized procedures of Innventia.

For the number 4 sub-samples in orange, the full 300 mm short log was normally used for pulping. The number 4 sub-samples from all other positions were made available for production of battens for testing of physical wood material properties with mechanical and acoustic methods. If also the material from positions dedicated to pulping would be found to be of particular interest for testing of physical properties, the wood materials of these few sub-samples may be shared: Battens are cut and the wood in between is pulped.

The procedure applied was identical to the standard routines previously used by Innventia, except for the introduction of a disc for measurement of spiral grain and the shared use of disc 4. This widened scheme has now been applied also in a study of three different types of eucalypts in South Africa. In this case, a 5th disc has been added for CT-scanning and logs for sawing and grading of boards have been cut between some of the sampling positions.

4.4 Short on the measurements and tests performed The radial variations of the different properties have been analysed on the samples from different heights of the stems. This way, information has been provided about variations both in the radial and in the longitudinal direction of the stems:

+ of fibre length, etc.

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• From disc 2, radial samples were produced from pith to bark in the direction towards north. If features of interest were visually observed also in the opposite direction, samples were cut also towards the south. The radial samples were analysed with Innventia’s SilviScan instrument (Evans 1994; Evans 2006), providing information about the radial variations in wood density, fibre width and wall thickness, microfibril angle (MFA), sizes and numbers of vessels, wood stiffness and many other properties. Also variations in spiral grain were measured with a preliminary method under development. In this first report in the series from the birch investigations, SilviScan data on wood density are presented and used for calculation of biomass, etc. Other wood and properties determined with SilviScan is presented in the other reports.

• Disc 1 was used for measurements of radial variations in the lengths of fibres and vessel elements. Fibres and vessel elements were liberated chemically from 1 cm long radial intervals along the radius towards the north, same direction as analysed with SilviScan, but just below these samples. The fibres and vessel elements of these small samples were characterized with image analysis (Karlsson, Fransson, Mohlin 1999; Granlöf, Lundqvist, Hirvonen 2006), using a L&W FiberTester instrument. Results from these measurements are presented in the Parts 2 and 3 of the series of reports.

• Also from disc 3, radial samples were produced from pith to bark in the same directions, precisely adjacent (above) the SilviScan samples to allow comparison with the preliminary method. On these samples the radial variations in spiral grain are determined with the scribe test method (Säll 2002), a reference method. These results are presented in Part 4 of the series of reports.

• From some of the number 4 sub-samples, all the material or part of it was chipped and pulp was produced through laboratory kraft cooking. The pulps were milled to different refining levels and hand sheets were produced. The pulps were analysed for fibre dimensions and water retention, and the sheets were characterized for a large number of properties: Sheet density, strength properties, porosity, etc. Cooking yields and pulp and sheet properties are presented in Part 3 of the series of reports, including also differences between materials of different origins and influences of wood and fibre properties.

• From the majority of the number 4 sub-samples, battens were produced for mechanical and dynamic testing. The battens were tested for shrinkage, MOE and MOR. These battens were cut along the radii analysed with SilviScan to allow joint analysis of data from the different sources, as well as the comparison of data for the same properties but determined with different methods. The results are presented in Part 4 of the series of reports.

4.5 The database All these related data on properties of trees, wood, fibres, pulps and sheets were compiled in a database. Related data is thus compiled for important properties of materials along the chains stand – tree – wood – fibres – pulp – sheet as well as – solid wood. This facilitates evaluation,

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combination of various aspects and modelling of relationships and shared access to data within and between the project teams at Innventia and Lnu.

This database on birch is also fully compatible with Innventia’s “Benchmarking Database” with data the species listed in chapter 2, investigated in previous projects. The data on Eucalyptus urograndis and Norway spruce used for comparison in these reports on birch have been made available from this larger database.

Other benefits from the application of previously developed standardized routines and data structures are that already developed tools for evaluations, presentations, etc. may be reused, adding to the efficiency of the project.

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5 Growth

5.1 Size and shape of the sampled trees In Figure 6a, the size, taper and height of the trees sampled are shown, based on measurements performed in the forest of radii from pith to bark in the direction to the north and the south of each sampling height, as well as the tree height2. In this chapter only the wood under bark is presented. Some data on bark are given separately in chapter 9.

The Figure indicates that the stems were reasonably well tapered, but it also shows large deviations from the ideal taper. Reasons for this are illustrated in Figure 7 with images of 4 discs cut at different heights from the same tree. Disc 2 is distorted by a nearby branch. If the radius happens to be measured in the direction of the branch, this local effect will result in an unrepresentative deflection of the taper in Figure 6a. Asymmetries such as the kidney-shaped cross-section of disc 3 may give similar distortions of the taper.

Figure 7. Images scanned from discs cut at the four heights from one of the birch trees.

To reduce such effects when investigating taper and growth, the areas of each cross-section on bark and under bark were determined with image analysis. From these data, an “equivalent radius” was calculated for each cross-section = the radius of a circular cross-sections with the same area. Also the average bark thickness was calculated for each cross-section. In Figure 6b, the radii to the north and south shown in Figure 6a have been replaced with these equivalent radii. This Figure provides a more representative picture of the size, taper and growth of the trees investigated. It shows that all trees were well tapered, possibly with exception for one of the trees from the medium-growth stand SB23.

The trees 1-4 were selected to represent different sizes of trees, reflecting different growth rates within the stands. The selection was based on diameter at breast-height. Figure 6 shows that this classification did not always reflected the development of the trees further up the 2 It has not been within the scope of the study to investigate the straightness of the trees. In all the Figures, the pith is positioned along the centre line or at radius = 0. The stems of the trees investigated may thus have curvatures not shown in Figure 6. 3 The sample from close to the top from the large tree of SB1 was lost. Data from this position is missing.

Disc 1 Disc 2 Disc 3 Disc 4breast height

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Properties of materials from Birch – Variations and relationships. Part 1: Growth, wood density and biomass Innventia Report No. xx1

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Figure 6a. Size, taper and heights of the birch trees sampled in Sweden (Betula pendula), showing radii to the north and south from the pith.

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Figure 6b. Size, taper and heights of the birch trees sampled in Sweden (Betula pendula), showing equivalent diameters related to area of cross-section.

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stems. For stand SB1 a tree selected as a medium-size tree is actually the largest one, and for stand SB2 the “smallest” tree turned out to be the one with the largest volume. For the studies of properties of wood, fibre and vessels, this misnaming of individual trees is however of no importance, as the purpose of the classification was only to secure that part of the stand variation was represented in the sampled material.

5.2 Annual growth For softwood species like spruces and pines, the start of the growth seasons are clearly defined by the sharp transitions from latewood to earlywood. These transitions may easily be identified from different wood and fibre properties. Also from the images of the birch discs in Figure 6, one may in most cases easily imaging annual growth rings. Precise measurements of growth are however not as easy, because the starting point of the annual growth is not as obvious, not even under a microscope. Figure 8 shows an image of cross-section of fibres and vessels in a birch sample. The image is recorded with the video-microscope integrated in the SilviScan instrument (Lundqvist et al 2010). The thin vertical line is where the growth of the next year starts. It is not obvious how this line should be related to the ring visible on the disc. For precise information of annual growth, images like in Figure 8 have been used to determine annual growth.

Figure 8: SilviScan microscopy image of a section of a birch sample. The growth direction is left to right. The thin vertical line indicated with an arrow in the right half of the image is an annual ring boundary. (The blue and green classification of the vessels is part of the measurement of vessel number and size, see (Lundqvist et al 2010)).

There may also be other difficulties in the determination of annual rings and growth. In old birch trees, “false heartwood” (in Swedish rödkärna) and rot may occur, especially in the lower part of the stem. This is illustrated in Figure 9.

Figure 9. Images scanned from discs cut at the four heights from one of the birch trees sampled.

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As a consequence of this, it was for a small number of sample discs not possible to identify the growth rings in the inner part of the stem, mainly at breast height. In these cases, approximations were made to estimate the volume growth of the tree, etc.

The property measurements were performed along the radii towards the north from the pith, which is downwards in the images of Figure 7 and 9. On severe disturbances, such as in the inner part of the breast height disc in Figure 9, it was not only difficult to determine the rings, but also impossible to measure sound data for some properties. This is further commented below.

5.3 Diameter and height growth, averages up to age of sampling Figure 10 shows the diameters of the trees sampled from the three stands plotted versus the number of growth rings at breast height (in cases like in Figure 9, the number is estimated). The age to reach breast height is not accounted for. For comparison, corresponding data are enclosed from previous investigations on trees of Eucalyptus urograndis harvested at 7 years of age and Norway spruce harvested at 33 years (thinning) and 65 years (final cut). Average diameter growths per year at breast height are indicated with the dashed lines fanning out from the origo of the plot.

Figure 10. Diameter at breast height versus age for the trees of different origins (for Norway spruce, age is replaced for number of annual rings at breast height). The dashed lines indicate different levels of radial growth per year for comparison. (Two symbols for SB2 trees are hidden behind others).

Figure 11 shows similar data for the total height of the trees of the three species plotted versus the tree age, as well as the average annual height growth.

Numbers for the average annual diameter and height growths are given in table 2.

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Figure 11. Tree height versus age for the trees of different origins (for Norway spruce, age is replaced for the number of annual rings at breast height). The dashed lines indicate different levels of height growth per year for comparison. (One symbol for a SB2 tree is hidden behind others.)

Table 3 Annual growth of diameter at breast height and tree height for the 3 species and types.

Tree species and type of stand

Diameter growth Breast height, mm/year

Height growth m/year

Average Average

Birch SB 1 1,23 0,27 SB 2 1,86 0,32 SB 3 4,28 0,83

Eucalyptus urograndis 9,85 3,39 Norway spruce 2,25 0,46

Up to the age of sampling, the fast-grown birch trees had on average grown 2-3 times faster than the slow-grown ones, both by diameter and by height. It may however be a bit dubious to compare this way, as the fast-growing trees are younger, smaller and in a more dynamic phase of growth. We will investigate this more thoroughly below, based on measurements of annual growth during different phases of age and radius.

The spruce trees sampled in the previous study had grown with rates between these extremes. The life-time growths of the eucalypts showed diameter and height growth rates up to 10 times faster than the slow-grown birches, compared at somewhat smaller breast-height diameters but larger tree heights for the eucalypts.

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5.4 Cross-sectional development by age and radial growth rate Figure 12 shows the development with age of the radius at breast height. The data originate from the identification of annual rings from SilviScan data and images of the locations of the individual growth rings, as shown in Figure 8, performed along radii towards the north at breast height. Due to this origin of data, we will throughout the rest of the report discuss cross-sectional growth in terms of radius. The diameter, which otherwise is more commonly used, is thus approximately twice the radius shown in the Figures.

The thin lined curves represent the radial development of each tree sampled from different stands and the thick dashed curves the averages for the sampled trees of each stand. The shapes of the average curves are similar but the growth rates differ greatly. The derivatives of the average graphs, representing the annual growth rate, show the expected development: A short initial phase of limited growth, followed by the most dynamic phase of growth from ring number 5. One may also see that the growth rates (the derivative) start to decline from this maximum rate already at an early age.

Figure 12. Radius versus number of annual rings at breast height of all the birch trees sampled (thin solid curves) and averages for each stand (thick dashed curves).

Another way of comparing growth, and sometimes more relevant, is to compare how many year it takes the trees reach a certain diameter. An indication of this may be obtained from the Figure 12 by identifying at what numbers of growth rings at breast height the trees on average have reached a diameter of for example 14 cm: For the improved birch trees this happens at about 20 rings, for the others at about 30 and 50 rings.

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The full detail of annual growth and its variability is illustrated in Figure 13, using SilviScan data from the trees of stand SB2 with the medium growth rate as an example. The thin lines represent the ring width developments at breast height of the four individual trees. The ring widths vary a lot from year to year because of annual variations in weather, but also due to local irregularities in the wood4. The thick black line shows the averages for rings of the same number counted from the pith.

Figure 13. Annual ring width developments at breast height for the four birch trees sampled from the stand SB2 with medium growth rate (thin lines), and the average growth rate development for the stand at breast height (thick black line).

Figure 14 a-c shows these average developments of the ring widths at different heights of the stems for all the stands. The differences in growth rate among the stands are evident, with SB3 showing extreme growth. A closer look reveals similar patterns among the stands: At all heights, the ring widths decrease from ring 5 and outwards. This is natural, as the material produced by the photo synthesis is allocated for each year along a larger perimeter. If compared at the same ring number, the growth is highest or among the highest at breast height during the phase when the tree is established. When the tree has reached a fair size, the radial growth further up the stem approaches and in some cases it becomes larger than the radial growth at breast height, when an increasing amount of the material by the photo synthesis is allocated further up the stem. Behind these averages are large ring-to ring-variations variations. In table 4, these variations are described with the total spans of variation in annual ring widths and internode lengths, as well as with the average ring widths for the groups of rings 1-20, 21-40 and 41-60. 4 The number of rings at breast height differs among the trees of the stand. One reason for this may be that trees of same age have reach breast height at different ages, another that the birches of the stand are not totally even-aged. Birch seeds may have rooted different years while the surrounding softwood trees were still young and small

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a. Slow-grown birch stand, SB1

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Figure 14. Annual growth at different heights of the stems of the trees sampled from birch stands of different growth rates.

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Table 4. Statistics on ring widths and height internodes of the trees sampled from the 3 stands.

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SB 1 1,23 1,65 1,01 0,95 0,05-5,9 0,27 0,24-0,30 SB 2 1,86 2,27 1,30 0,82 0,03-4,85 0,32 0,30-0,34 SB 3 4,28 4,28 - - 0,58-13,58 0,83 0,74-0,90

5.5 Discussion on cross-sectional growth The comparison of cross-sectional growth will now be discussed from further perspectives. Plot a in Figure 15 shows the radial developments at breast height for the stands, presented as radius versus growth ring number. The data are based on the thick dashed curves in Figure 12. From these data, the annual radial growths were calculated and plotted versus the annual ring number, see plot b of the Figure. The fast-growing improved birches of stands SB3 may show annual rings 3 times wider than the forest trees of SB1, but they have already at 20 years of age passed their most dynamic phase.

Figure 15. Cross-sectional growth at breast height presented from different perspections, providing a basis for the comparison among the stands.

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Already at 20 years, the ring width is reduced to the same width as for SB2 and is rapidly falling. Therefore, it is not proper to compare growth based on average cross-sectional growth over the time up to sampling. If the aim is to reach a certain larger diameter, you will have to wait much longer than this average indicates.

Plot c in the Figure shows the area added to the cross-section at breast height with each annual ring, calculated from the same data, and in plot d versus the current radius. Plot d indicates that the maximum cross-sectional growth by area is reached at similar radii of 70-80 mm, for all the stands of different growth, at a diameter of about 15 cm that is, but the areas added at the maxima differ. The forest birches passed their maxima at 40-60 years of age. The improved birches seem to approach their maximum already at the age of 20.

One rationale behind the similar radii of the maxima could be that, when a stem diameter sufficient for carrying its crown has been reached, it may be more beneficial for the tree to allocate biomass for height growth, in order to compete for sun light, rather than to fatten the stem.

5.6 Height development by age and height growth rate Figure 16 shows the development of the tree height with age, in analogy with Figure 12 for radius at breast height. The height-to-age curves are based on estimates of the ages at which the trees have reached the sampling heights in the trees.

Figure 16. Development of tree height with age of tree for the birch trees sampled (thin solid lines) and average growth for the trees sampled from each stand (thick dashed lines).

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The height growth is only slightly larger for SB2 than for SB3, but much faster for the improved birch tree of SB1.

5.7 Annual growth units From the data on the widths of growth rings formed every year at the multiple heights in the stems where the samples were cut, annual units of growth in the radial and longitudinal direction of the stem may be estimated. The location and sizes of these growth units within a vertical cross-section of a stem is illustrated in Figure 17 for one of the medium size trees of stand SB1.

Figure 17. Location and size of annual growth units in the radial and longitudinal direction of a stem for one of the medium size trees sampled from the slow-growing stand SB1.

This type of growth patterns are calculated with use of interpolation for the parts of the stem located between the heights of sampling. For the parts below breast height and in the top of the tree extrapolation has been applied (extrapolation below breast height not shown in the Figure). The growth ring widths were analysed only at 4 heights and local irregularities in the wood have caused some distortion of the growth pattern. But even so, this is a useful representation of growth, as it allows the calculation of different types of averages and statistical distributions, as well as combined use of different types of data.

5.8 Volume by age and volume growth rate The volume of the wood under bark at different ages may be calculated by integrating the volumes of the growth units being part of the stem at different ages. By doing this for all ages, the development over age of the wood volume is obtained. This is shown in Figure 18 for all trees sampled, thin lines. The stand averages are shown with thick

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dashed lines, in analogy with radial growth in Figure 12 and height growth in Figure 15. Also in this case, the tree age is approximated with the number of rings at breast height.

As expected, the Figure reflects large differences in the volume developments among the stands, and also among the trees within stands. (The discontinuities in the thick dashed curves for the average volume development of each stand are artefacts, which occur when the age of one tree is reached and the number of trees for averaging changes.)

Figure 18. Development of stem wood volume under bark, m3, with tree age for the birch trees sampled (thin solid lines) and average growth for the trees sampled from each stand (thick dashed lines). Figure 19 presents the annual volume growth of all trees sampled and the averages for the stands. Here the differences in growth become even more evident: The average growths of wood volume under bark are about 0,003 m3/year for the forest birches (0,0031 for SB1 and 0,0030 for SB2), and about 0,006 m3/year for the improved trees.

As expected, the Figure shows that the annual volume growth increases during the early life of the trees, after which it for most trees start to level out and later even may decline. There are however exceptions from this pattern: Two of the trees sampled from the most slow-grown stand stay at a moderate volume growth through its mid-age and end even increase it at high age, possibly due to improved growth conditions after thinning of the stand. This is the reason why the average annual volume growth of the trees sample from the stands SB1 and SB2 are so similar.

The improved birches of stand SB3 are still young and in their most dynamic phase, even though they have already reached a reasonable volume. They leave the trees of the

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forest stands far behind during the first 20 years. But they cannot continue like this to a high age due to competition for light, water, etc., at least not all of them, and due to various constrains (mechanical, hydraulic, etc.). Figure 15 illustrated that the annual ring widths at breast height had already started to decrease drastically in the stand of improved tree (plot b), even if they still are the trees with the largest annual increase of their cross-sectional area at breast height.

Figure 19. Annual growth of the stem wood volume under bark, m3/year, with tree age for the birch trees sampled (thin solid lines) and average volume growth for the trees sampled from each stand (thick dashed lines).

It may at first sight seem strange that the improved birch trees of SB3 have 2-3 times higher average for both radial and height growth as compared to the forest trees of SB1 and SB2, but “only” about twice the average volume growth, all averages calculated up to the age of sampling. This is again an example of the difficulty to compare populations of different ages and sizes of trees. The improved trees may each year add a 5-8 times larger volume of wood at the age of 20 years when they were felled. But the older trees from SB1 and SB2 are larger. Even though their growth rings are thinner and their annual height growths are shorter, the volumes of shell of wood added to their stems each year are close to 50 % of that added to the smaller but very fast-growing trees when they are 20 and felled. And this is the case over a long period of years.

As an alternative, one may compare the volume growth by looking at the ages at which a certain wood volume has been reached. From Figure 18, we can read out that an average stem wood volume of 0,2 m3 was reached after about 20 years at breast height for the improved birch trees samples, whereas the same volumes were reached after about 45 and 65 years for the trees sampled in the natural stands.

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6 Wood density variations within and between stems of trees

6.1 Short on fine-scale density variations and structures in growth rings Figure 20 illustrates a typical radial variation in density of wood from birch measured with SilviScan. The blue graph shows the density data presented as averages for radial intervals of 25 µm. The resolution is thus 40 measurements per mm.

Figure 20. Radial variations in wood density (conditioned), presented as averages for radial intervals of 25 μm (blue curve) and 2 mm (red curve) for one of the trees.

All measurements with SilviScan are performed in the laboratory on samples in equilibrium with a conditioned atmosphere of 23 ºC and 43 % RH. At these conditions, the moisture content of the samples is about 8 %, corresponding to a moisture ratio (in Swedish: fuktkvot) of about 9 %. The wood density presented in Figure 20, as well as in all other Figures and tables in the report, is thus related to conditioned weight / conditioned volume. These density values will differ from those of the commonly used basic density: Oven-dry weight/raw volume. As a rule of thumb, the conditioned density is about 25 % higher than the basic density.

Annual rings

In softwood species like Norway spruce and Scots pine, there are large differences in fibre dimensions and wood density among earlywood and latewood. A plot of wood density corresponding to figure 20 would show high peaks in wood density for each band of latewood and steep decreases in density would indicate the interfaces between the growth seasons. Figure 20 illustrates that the radial variation in wood density of

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birch does not show such clear indications of the start of the growth seasons. This is true also for other hardwood species. There is instead a wavy pattern reflecting differences in the sizes and numbers of fibres and vessels formed over the growth season.

An example of this detailed structure with small fibres and large vessels was illustrated in Figure 8. The thin vertical line in the right half of the image is the interface between wood formed during two consecutive years. Pattern of annual rings can normally be easily perceived in cross-sections of birch. However, the precise position of these transition between years cannot easily be identified, neither by the eye nor from wood density variations, as shown in Figure 20. These precise positions may in most cases not be necessary to know, but in this study we have gone to the end of identifying them by using images like in Figure 8 as a complement to density graphs like in Figure 20.

Radial resolution of data

The blue curve in Figure 20 with radial resolution of 25 µm shows a “noisy” density variation. The major reason for this is that most vessels in the wood are much broader than 25 µm. When measuring the density with a spatial resolution smaller than the widths of these vessels, the result will obviously be a large variation. This very small scale variation is not of interest for the current study, it would only make graphs on property variations “noisy” and hard to interpret. Therefore, no data with resolution higher than 2 mm, corresponding to the red curve in Figure 20, will be shown beyond this point of the current report.

6.2 Radial density variations at different heights of the stems In Figure 21 a-c, the radial variations at different heights in wood density (air-dry) are compiled for all the trees investigated, each Figure presenting the trees from one site on a full separate page:

a. the slow-grown stand SB1 in Figure 21a b. the medium-grown stand SB2 in Figure 21b c. the fast-grown stand SB3 in Figure 21c

The upper quartet of plots shows the radial variations at different heights, one plot for each tree. In the lower quartet of plots, the same curves are rearranged to show the density variations at each sampling height in the same plot for all trees of each stand. The radial variations are presented using averages for the individual annual rings.

These plots are shown to illustrate the basic variations and their irregularities. From these plots only, it is not possible to visually draw any clear conclusions about general patterns in variation. Large variations locally in the wood are observed. Reasons may be presence of knots, local rot, asymmetries of the stem, etc. Observations of larger practical implications will be made below after aggregation and evaluation of the data.

For some heights, the data show values = 0 for part of the radius close to the pith. These are samples for which it was not possible to measure with SilviScan close to the pith, due to rot, false heartwood or other severe disturbances of the wood structure.

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Figure 21c. Radial variations in wood density within and between the birch trees of stand 3 (improved).

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6.3 Comparison of wood densities in stands with different growth rates In Figure 22, the radial variations of density of the samples cut at ¼ of the total height of the birches are compiled for comparison. The data shown are averages for radial intervals of 10 mm, used to reduce the noise and lift forward the differences among trees and stands.

Figure 22. Wood density versus radius at 1/4 height for the birch trees aggregated as 10 mm averages. Round symbol = large tree, square = medium tree, triangle = small tree. The trees marked with un-filled symbols are breaking the pattern common for the rest.

At first sight, the figure appears as quite hard to interpret, very likely due to the facts that the material investigated is limited and that the variations in wood density are influenced by several factors in an integrated manor. Two trees differ from the other trees by showing very high wood densities close to the pith and decreasing density outwards: One slow-grown (SB1) tree and one fast-grown (SB3) tree. They are marked with un-filled symbols. We leave these trees temporarily outside the discussion.

The data for the rest of the trees provide an overall impression indicating:

• For the slow-grown trees of SB1, the density increases outwards with radius

• For the medium-grown trees of SB2, the density is rather constant or even slightly decreasing close to the pith, but it increases further out

• For the fast-growing trees of SB3 there are only two remaining trees. They show different patterns close to the pith, but rather constant densities further out.

• Within each stand, there is a tendency that smaller trees have higher wood density than larger trees, meaning higher density for more slow-grown trees.

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To improve the understanding a bit beyond these rather vague results from visual inspection, we go to the level of annual rings. In Figure 23, the average wood densities of the rings are plotted versus the widths of the rings. The figure shows very clearly that a large part of the variation among and within the trees is related to differences in growth. And the growth is in turn strongly related to cambial age, as summarised in Table 4.

Figure 23. Wood density (averages for the annual rings) versus ring width at 1/4 height for the birch trees investigated. Round symbol = large tree, square = medium tree, triangle = small tree. The trees marked with un-filled symbols are breaking the pattern common for the rest.

Apart from most rings of the very fast-grown tree among the improved SB3 birches, unfilled red circles, and a limited number of growth rings from the deviating SB1tree, unfilled blue squares, the large majority of all the rings are within a band of decreasing density with ring width, levelling out at about 500 kg/m3. The band is about ±100 kg/m3 wide among the thinnest annual rings and narrower for broad rings.

Figure 24 shows how the wood densities of the different trees develop with cambial age. The average wood densities for the annual rings are plotted versus their ring numbers. If the two pattern-breaking trees marked with un-filled symbols are ignored, the data indicates the following pattern:

• Unstable densities in the most juvenile wood, often with elevated densities in some rings

• Outside this very juvenile wood, a sequence of rings with densities varying around a rather constant level.

• Then follow rings with densities varying around a slope of increasing wood densities.

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Figure 24. Wood density (averages for the annual rings) versus annual ring number at 1/4 height of the birch trees investigated. Round symbol = large tree, square = medium tree, triangle = small tree. The trees marked with un-filled symbols are breaking the pattern common for the rest.

The Figures 23 and 24 thus show that the major part of the differences between trees in density at the same radius observed in Figure 22 can thus be explained by differences in cambial age when the wood was formed. The differences are thus to a large extent an age effect, induced by the large differences in growth rate:

• The SB1 trees grew slowly and had already at small diameters reached the phase of increasing densities.

• The very fast-grown SB3 trees were despite their considerable diameters still in the phase of rather constant density when they were sampled.

• The SB2 trees were in between, but closer to SB1.

There are of course also other sources of differences in wood density than those caused by growth-induced variations in cambial age. One example is the differences seen in Figure 24 among trees from the same stands: Differences of up to 150 kg/m3 at the same ring number prevailing for decades. Another example is the two trees breaking the pattern. The wood density is also influenced by factors like genetics, site conditions, forest management and weather. When comparing individual trees, more or less random differences may occur for numerous reasons, including damages from weather and animals. These differences will increase even more when it comes to cross-sections of stems or increment cores, with influences from knots, asymmetry of the stem, etc. The deviation of the SB1 tree marked above with unfilled blue symbols is very probably caused by such a deformation. Figure 6a shows that the stem of this tree is very

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asymmetric, which must have resulted in locally much narrower rings precisely at the position and the radial direction where this sample was cut. The sample is thus atypical.

Due to factors mentioned above, the cause of variation can normally be only partly understood, and this is especially true when working with a limited sample material, as we do in this study. Therefore, it does not make sense based on this material to look deeper into reasons to why the two trees marked with un-filled symbols are breaking the pattern. We are, however, satisfied that the methodology applied has the strength to reveal relationships as shown above, despite the limited material used. And we believe that the approach applied can be very useful as a template for an enlarged study. Such widened studies with sampling and analysis of a large set of stands and trees are recommended to arrive at fully reliable conclusions.

Comparison with other tree species

A comparison with similar data from E. urograndis and Norway spruce is made in Figure 25. It shows, as is well known, that the densities of the two hardwood species are clearly higher than that of the softwood species Norway spruce. The reason why no latewood peaks are seen for spruce is the averaging performed for intervals of 2 mm.

The total variation among all trees investigated is the largest for birch. This is, however, no surprise as stands of different growth rates and ages have been selected for the birch material, while the eucalypt material originates from two even-aged plantations of the same ages. Further, for the eucalypt species, the increase in density with radius is very evident. (It should be easier to see, as the eucalypt material is more homogeneous.). It starts at a clearly lower density in its pronounced juvenile wood close to the pith, and it reaches similar densities as birch at a radial location of about 5 cm. Comparison by numbers will be shown in chapter 9.

Figure 25. Wood density versus radius for the samples cut at 1/4 height for the birch trees, compared with similar data from E. urograndis and Norway spruce.

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6.4 Property variations along stems and by log diameter In Figure 26, the cross-sectional averages of the wood density of the trees from the different stands are plotted versus height above ground, in order to visualize density variations along the stems. Also in this case it is hard to see clear patterns, but the data from these few trees indicate that:

• For the old and slow-grown trees of SB1, the variation of the cross-sectional density averages along the stem was smaller than the variation with radius within cross-sections; for the trees of SB2 and SB3, it was smaller or similar.

• In most cases, the differences in density among the trees of different origins more or less prevailed along the stem, with the highest densities for the slow-grown trees of SB1 and clearly lower densities for the SB2 and SB3 trees, with the SB3 trees on the low side.

Figure 26. Wood density variation along the stems (averages for cross-section) for the different stands and trees. Round symbol = large tree, square = medium tree, triangle = small tree. (* The sample from the uppermost sample position of the large tree from stand SB1 is missing, which makes this tree look shorter than it was in reality).

In Figure 27, variations along the stems of the birch trees sampled are compared with similar data for the other tree species. As a result of averaging of cross-sections, the patterns of variation have become more visible than in Figure 25. The slow-grown birches (SB1) show the highest densities of all. Those from SB2 and SB3 show densities similar to the eucalypts.

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Figure 27. Wood density variations along the stems (averages for cross-section) for trees of different species.

The eucalypt trees show a longitudinal density variation with the highest averages for stem cross-sections in the mid part of the stem. This may be related to the combined effect of two aspects of wood juvenility: 1) The radial development from juvenile to mature wood, evident for E. urograndis in Figure 25, which causes decreasing density when approaching the top of the trees. 2) A less know longitudinal juvenility, causing lower density close to ground. It cannot be ruled out that similar patterns would be found also for birch if a larger study would be performed.

In the plots of wood density versus height in tree above, a forestry-oriented perspective is applied. A more industrial perspective is to look at wood densities of logs with different diameters. When the logs are delivered to a mill, their precise position in the stem is most often not know, even if one may conclude that large diameter logs most often originate from the lower part of the stem. Figure 28 is shown to throw some light on the correlation between wood density and log diameter. It indicates that for birch the log diameter does not provide useful information about the wood density of logs from trees of un-known origins.

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Figure 28. Wood density (cross-sectional averages) of logs with different diameters. Round symbol = large tree, square = medium tree, triangle = small tree.

6.5 Statistical distributions for wood densities among stands As a final comparison of the wood density and their variations among each stand investigated, the cross-sectional averages shown in Figure 26 were used to calculate volume-weighted density averages and statistical distributions, first for all individual trees, then for each one of the stands, sampled to represent different growth rates5. The averages and standard deviations presented in Table 5 and the statistical distributions are shown and compared in Figure 29.

Table 5. Averages and standard deviations of wood density for comparison of the materials from all trees of the stands sampled

Wood density

kg/m3 Birch SB1

slow-grown

Birch SB2

medium-grown

Birch SB3

fast-grown

Average (volume-weighted) 702 607 594 Standard deviation (volume-weighted) 76 61 86

Approximate age 80 60 20 5 Volume-weighted averages and distributions for individual trees are calculated by weighting the cross-sectional averages from different heights along the stem with the relative parts they represent of the total volume of the stem. The averages and distributions for the stands are calculated by weighting these tree data in relation to the volumes of the individual stems.

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Figure 29. Statistical distributions for wood density (volume-weighted), for overall comparison among the stands samples.

The Figure and table show that the average wood density was rather similar for the most fast-grown improved birch trees sampled from stand SB3 and the medium-grown birches of stand SB2 grown imbedded among softwood trees, despite the differences in growth rate as well as age among the stands. It further shows that the wood density was about 100 kg/m3 higher for the slow-grown birches of stand SB1, also grown imbedded among softwoods. The internal variations with the stands, expressed as standard deviation, were however rather similar for all the three stands investigated.

6.6 Summary on patterns of within stem variations in wood density The patterns of variations of wood density within the stems sampled may be summarised as follows:

• In the microscopic scale, parts of mm, the vessels introduce large fine-scale variation in wood density. In larger scales, the wood density of birch is however more homogeneous than that of the softwoods. For example, no major within ring wood density variations among seasons were observed, in contrast to the clear differences between earlywood and latewood of softwood species. There may however be some effects of radial variations in number and size of vessels. This will be further commented on in part 2 of this series of reports on birch.

• The radial density variations were studied at ¼ of the tree height. Large variations with radius were observed, and large differences among trees and stands. A study of density variations with cambial age revealed that a major part of these differences were caused by age effects. Close to the pith were a few

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growth rings with unstable densities. Outside these rings with very juvenile wood followed a sequence of rings with density variations around a rather constant level. The followed rings where the density increased on formation of more mature wood. For the most fast-grown trees, wood with constant density was still formed when the trees were sampled. For the most slow-grown trees, increasing density was observed already from the first centimeter outside the pith. For trees of the same stand, there was a tendency that more slow-grown trees had higher density than more fast-grown.

• The variation of the cross-sectional density averages along the stem was smaller than the variation with radius within cross-sections for the trees of SB1, and smaller to similar for those of SB2 and SB3. This meant that in most cases the differences in density among the trees of different origins more or less prevail along the stem, with the highest densities for the slow-grown trees of SB1, with clearly lower densities for the SB2 and SB3 trees, and with the SB3 trees on the low side.

• The differences between trees and stands become most visible in the plots of density variations along the stems. There it is clearly seen that the density was clearly highest for the trees sampled from the most slow-grown stand SB1. The trees sampled from the more fast-grown forest stand SB2 and the stand SB3 of improved birches showed in a common span, at a lower level than SB1.

• The comparison of wood densities at same and different stem diameters, corresponding to logs of different diameters, showed the same pattern: No obvious systematic variations with diameter, but large differences among the stems, with the stems/logs from the most slow-grown stand at the highest densities and the other two stands in a common span below.

As said also above, it should be kept in mind that the set of tree sample is limited and that there are factors not fully described, such as growth conditions, which may have influenced the growth and properties of the sampled trees. It would be in place to complement the current study with more data, such as growth. For more solid results, it would be favourable with en enlarge study, applying the same methods.

As expected, the wood density of birch trees sampled was much higher than for the Norway spruce trees, and also higher than the E. urograndis trees used for comparison.

6.7 Wood densities of growth units The wood densities of the growth units at different heights in the trees are shown in Figure 30, in analogy with the presentation of the volumes of the growth units and for the same tree as in Figure 15. The Figure illustrates that the within stem variations in wood density of the tree are rather limited for birch.

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Figure 30. Wood density of the growth units in the birch tree used for illustration.

A basis for development of general model structures

The interpretations of the Figures 21-28 are based on visual judgements rather than scientific evaluations, and with averages given in table 5. The data are also used in other projects for the development of models describing wood and fibre properties and their variations within and between trees and species.

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7 Biomass growth Figure 31 illustrates how the total biomass under bark of a stem, its biomass at different ages and the annual growth of stem biomass may be calculated based on information from the different growth units about annual growth, as presented in Figure 17, and the within stem wood density distribution, as shown in Figure 30.

Figure 31. Illustration of how the total biomass of a stem, its biomass at different ages and its annual growth may be calculated from growth units, as shown in Figure 17, and the wood density distribution, as shown in Figure 30.

The dry biomass under bark was calculated for all growth units (not shown), including an approximate correction for the moisture content of the conditioned wood samples by multiplying the density with a factor of 0,92. As a next step, these data were used to calculate the dry biomass under bark of the stems at different ages for all the sampled trees investigated, see Figure 32, which also includes the average developments of biomass by age for the sampled trees of each stand. The annual growths in dry biomass under bark of the stems of the different trees and stands are shown in Figure 33.

The Figures show similar patterns as the analogous Figures 20 and 21 for volume growth. The dry biomass under bark of improved birches from stand SB3 develop much more during their first 20 years than that of the forest trees.

There is, however, one clear difference: The development of dry biomass for the slow-growing forest trees of SB1 is closer to those of the trees from the other stands, thanks to the higher wood density of the SB1 trees. The average dry biomass growths up to the ages of sampling are about 2,5 kg/year for the trees sampled from the two forest stands, and about 4 kg/year for the improved birch trees, less than a factor 2.

The same comments about using such averages for comparison of growth of trees with different ages and sizes may be made also for wood biomass. As an alternative, one may compare the biomass growth by looking at the ages at which a certain dry mass has been reached. Figure 30 shows that an average stem wood mass of 120 kg was reached after about 20 years at breast height for the improved birch trees, whereas the same masses were reached after about 50 and 60 years for the trees from the natural stands.

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Figure 32. The total dry biomass under bark, kg, of the stems at different ages of the trees investigated, and the averages for each stand. (The downward “steps in the graph for the averages occur when the number of trees is stepwise reduced)

Figure 33. Annual growth in dry biomass under bark, kg/year, of the stems for all the trees investigates, and the averages for each stand.

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8 Bark The bark is also valuable biomass from the trees. The thickness of bark was measured with image analysis on the wood discs as in Figure 7. In Figure 34 the thickness of the bark has been plotted versus the diameter on bark of the discs.

Figure 34. Bark thickness versus stem diameter on bark for the birch trees sampled (thin solid lines). Averages for the different stands are also shown (thicker dashed lines)

A very simplistic analysis of the averages for the trees sampled indicates that, for these trees, the thickness is approximately proportional to the diameter up to a stem/log diameter on bark of about 20 cm. The bark thickness (single-sided) was about 3% of the diameter for the trees of the stands SB2 and SB3. For the somewhat larger, older and more slow-grown trees from stand SB1, the bark thickness was about 5% up to of the diameter on bark of about 20 cm, and with a successively higher proportion at larger diameters closer to the ground.

If translated into proportions of the cross-sectional area of the stem, also representing the local volume proportion in a log, the percentages above would correspond to:

• 3% Area proportion of bark: +about 12% in relation to stem area (on bark) +about 13% in relation to wood area (under bark)

• 5% Area proportion of bark: +about 19% in relation to stem area (on bark) +about 24% in relation to wood area (under bark)

The percentages above shall not be seen as solid facts, as they are based on a limited sample material. They are rather indications and an illustration of what can be produced with the methods applied in the study.

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9 Comparisons of growth and density – Summary The averages for wood density and different aspects of growth, based on data to the age of sampling, are compiled in table 6. The averages for each stand are calculated by weighting the averages from each position with its cross-sectional area. This corresponds to volume-weighting of data for logs cut at these positions. In this way, averages representing the collective of investigated stems are obtained. Tree age is approximated with the number of growth rings at breast height.

Table 6. Comparison of average wood density and growth, based on data of all trees sampled from the stands investigated.

Property Unit Birch SB1

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Wood density kg/m3 709 604 605

Diameter growth mm/y 2,78 4,52 8,24 Height growth m/y 0,27 0,33 0,83 Volume growth m3/y 0,0031 0,0030 0,0059 Biomass growth kg/y 2,52 2,61 4,06 The differences between the averages are visualised graphically in Figure 35, with the averages for Birch SB2 as the reference.

Figure 35. Compilation of averages for wood density and different aspects of growth for the birches with different growth rates, with the averages for Birch SB2 as the reference ( = 100 %).

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In table 7, the different types of growth are instead compared among the trees of three origins based on the approximate times they used to reach a certain diameter, volume or biomass.

Table 7. Comparison of different aspects of growth, based on data of all trees sampled from the stands investigated.

Aspect of growth Limit to be reached

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Diameter growth 14 cm 50 years 30 years 20 years Volume growth 0,2 m3 65 year 45 years 20 years Biomass growth 120 kg 60 year 50 years 20 years

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10 Conclusions Growth and properties of birch trees with different growth rates from 3 stands were investigated and compared: Two stands of birch trees growing within stands of softwoods (low and medium rate), and one stand of an experiment of improved birch trees (high rate). The stand ages were about 80, 60 and 20 years, respectively.

The study has been based on use of standardised routines for benchmarking wood, fibres, pulps and paper of different origins previously developed by Innventia for sampling, sample preparation and characterisation of a multitude of properties and their variations. This approach has made it possible to incorporate the birch data into Innventia’s Benchmarking Database with compatible data on different spruce, pine and eucalypt species, and to make comparisons also with other species. The birch trees were compared to corresponding data for Norway spruce and Eucalyptus urograndis.

For the birch project, new routines for studies of growth have been developed, including not only diameter at breast height, tree height and volume, but also biomass. New routines have also been added for investigations of further properties of interest for solid wood products, in cooperation with the Linnaeus University. The previously developed routines have served well together with the new ones also in the birch project.

Wood density

The overall average wood density of the trees from the slow-grown forest stand was about 20% higher than for the trees of the other stands. There were, however substantial differences among individual trees.

Large radial density variations were observed, and large differences among trees and stands. These were to a large extent related to differences in cambial age. The annual rings first formed showed unstable density, followed by a sequence of rings with rather stable density. Then followed annual rings with increasing density. This combined with large differences in radial growth rate gave the result that the wood density started to increase close to the pith for the most slow-grown trees, while the density was rather stable and on a much lower level in the most fast-grown trees. For trees from the same stands, slower-grown trees tended to have higher density than more fast-grown trees.

The variation of the cross-sectional density averages along the stem was smaller than the variation with radius within cross-sections for the trees of SB1. For SB2 and SB3, this variation along the stem was also smaller than or in some cases similar to the variations within the stem cross-sections. This meant that in most cases the differences in density among the trees of different origins more or less prevailed along the stem.

The wood density was much higher for birch than for Norway spruce. E. urograndis had lower density close to the pith, but reached similar levels as birch at the bark. Birch and eucalypts do not have latewood bands with high density. Therefore the wood properties are more homogeneous in the annual ring scale than spruce and other softwoods.

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Diameter, height and volume growth Using the average for the trees of the medium-grown forest stand as the reference, the diameter growth at breast-height was 38% lower for those of the slow-grown stand and 82% higher for the improved birch trees, compared over the life time of the sampled trees. The diameter growth may also be compared by the age when a certain diameter is reached: 14 cm was reached after 50, 30 and 20 years, respectively.

It may however be a bit dubious to compare growth of trees of different ages, being in stages of different dynamics. A detailed study showed however that the most fast-grown trees already approached their maximum diameter growth and cannot be assumed to continue to grow this fast for long.

If compared in the same way, the height growth was 16% lower for the slow-grown stand and 160% higher for the improved birches.

A similar comparison of volume growth showed similar growth for the two forest stands, but 97% higher growth for the improved birches. The average volume of 0,2 m3 was reached after 60, 50 and 20 years, respectively. The same comment about growth dynamics is valid.

Biomass growth For biomass growth, the difference between the improved birches and the forest trees will be smaller than for volume growth due to the lower density of the most fast-grown birches. By combining local within-stem data on growth and wood density, the dry wood biomass of the stem and its annual growth may be calculated. A similar comparison as above of biomass growth showed similar growth rates for the two forest stands and 56% higher growth for the improved birch trees. The average mass of 120 kg dry wood under bark was reached after 60, 50 and 20 years, respectively.

Bark The bark is also valuable biomass from the tree. For these trees, the thickness of the bark was found related to the diameter of the stem up to a diameter on bark of 20 cm: 3% for the trees of the medium-grown and fast-grown stands, 5% for the slow-grown forest trees, which also showed increasing ratios for larger diameters close to the ground

New opportunities The methods used have made it possible to apply some new approaches in studies of growth and properties, offering new opportunities for research on selection of trees to plant on different sites for various uses, on matching of growth conditions and forestry practices, on optimal use of resources, etc.

Enlarged study useful It is, however, important to consider that the current study is of limited size. For solid results, an expanded study based on sampling of more trees with more well defined origins and growth conditions would be useful, applying the same standardized routines for compatibility with the current study as well as with data on other species in Innvenia’s Benchmarking Database.

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11 References Evans R (1994) Rapid measurement of the transverse dimensions of tracheids in radial wood sections from Pinus radiata. Holzforschung 48:2 pp. 168-172.

Evans R (2006) Wood stiffness by x-ray diffractometry. In: Chapter 11 “Characterisation of the Cellulosic Cell Wall”. Proceedings of the workshop 25-27 August 2003, Grand Lake, Colorado, USA. Southern Research Station, University of Iowa and the Society of Wood Science and Technology. Ed.: Stokke D and Groom L. Blackwell Publishing.

Grahn T, Lundqvist S-O, Olsson L (2013) Measurement of radial variations in spiral grain with X-ray diffraction on SilviScan Presentation at the IUFRO MeMoWood, Nancy, October 1-4, 2013

Granlöf L, Hedenberg Ö, Lundqvist S-O, Olsson L, Thomsson L (2003) Vessel elements in hardwood pulps - appearance and measurements STFI Report PUB 14

Karlsson H, Fransson P-I, Mohlin U-B (1999) STFI FiberMaster. 6th Int. Conference on New Available Technologies, SPCI, Proceedings, pp 367-374. Karlsson H (2006) New technique for measurement of fibre properties including vessel cells and mix of fibre species 60th Appita Annual conference and Exhibition, Melbourne, Australia 3-5 April 2006, Proceedings, pp 497-502. Lundqvist S-O, Olsson L, Evans R, Chen F F, Vapaavuori E (2010c) Variations in properties of hardwood analysed with SilviScan – Examples of wood, fibre and vessel properties of birch (Betula) 4th Conference Hardwood Science and Technology, Sopron, Hungary, May 2010

Lundqvist S-O, Grahn T, Olsson L, Wallbäcks L (2012) Properties of wood and fibres, pulps and sheets from birch Presentation at the 2nd Avancell Conference, CTH Gothenburg, 2-3 October 2012

Lundqvist S-O, Grahn T (2013) Framework for multi-species comparison and relationships between wood, fibre and product properties Presentation at the IUFRO MeMoWood, Nancy, October 1-4, 2013

Johansson M, Säll H, Lundqvist S-O (2014) Properties of material from Birch – variations and relationships Part 2: Mechanical and physical properties Report no 23, Faculty of technology, Linnaeus University. ISBN: 978-91-874274-73-2

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Säll H (2002) Spiral grain in Norway spruce Ph.D thesis Acta Wexionesia 22, Vaxjo University, 171 p.

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STFI-Packforsk Database information

Title

Properties of materials from Birch - Variations and relationships Part 1: Growth, wood density and biomass

Author

Lundqvist S-O, Grahn T, Olsson L

Abstract

This is the first report in a series covering growth and properties and their variations in birch. Samples from birch trees with different growth rates from 3 stands were investigated and compared, using standardised routines to allow comparison with data on other species in Innventia’s Benchmarking Database. In this report, growth, wood density and biomass are covered. The average wood density of the trees from the slow-grown forest stand was about 20% higher than for the trees of the other stands. There were, however, large differences among individual trees. The annual growth was much higher for the improved birch trees than for the trees of the two forest stands. This was valid for growth of breast height diameter, height and volume as well as for growth of dry wood biomass. The trees of the slow-grown forest stand showed lower or similar values for all these growth rates compared to those of the medium-grown forest stand. The current study was of limited size. For solid results, it would be useful with an expanded study, based on samples from more trees with more well-defined origins and growth conditions.

Keywords

Biomass, birch, database, eucalyptus, growth, hardwood, Norway spruce, variability, wood density, wood species.

Classification

1130, 1131, 1230

Type of publication Innventia Report

Report number

390

Publication year

2013

Language

English

INNVENTIA AB Box 5604, SE-114 86 Stockholm, Sweden Tel +46 8 676 70 00, Fax +46 8 411 55 18 [email protected] www.innventia.com

INNVENTIA AB is a world leader in research and development relating to pulp, paper, graphic media, packaging and biorefining. Our unique ability to translate research into innovative products and processes generates enhanced value for our industry partners. We call our approach boosting business with science. Innventia is based in Stockholm, Bäckhammar and in Norway and the U.K. through our subsidiaries PFI and Edge respectively.

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