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Silva Balcanica, 16(1)/2015

HARVEST RESIDUES ASSESSMENT IN PINE PLANTATIONS HARVESTED BY WHOLE TREE AND CUT-TO-LENGTH HARVESTING

METHODS (A CASE STUDY IN QUEENSLAND, AUSTRALIA)

Mohammad Reza GhaffariyanForest Industries Research Centre, Faculty of Arts and Business, University of the

Sunshine Coast, Locked Bag 4, Maroochydore DC, Queensland

Robin ApolitUniversite de Lorraine ENSTIB, Épinal

Abstract

Forest harvesting residues can be an additional fibre source if recovered during harvest but it can also play an important role in maintaining soil quality over the next rotations if left on the cut-over area. The first step in properly managing harvesting residues is to know how much is left after harvesting operations and is potentially available for sustainable recovery. There has been little information available on quantity and quality of plantation harvest residues in Queensland, thus this project aimed to measure the average weigh of slash left on two harvesting sites; a whole tree extraction to road side and a cut-to-length harvest site. The results indicated a significant difference between average weights of residues from the two harvesting methods. The percentage of other elements of harvesting residues were measured and documented in this study. The results can help land owners and forestry planners to set a proper strategy for woody waste management and additional fibre recovery.

Key words: whole tree extraction, cut-to-length harvesting, t-test, residue composition

INTRODUCTION

Previous studies in Australian plantations (Ghaffariyan, 2013) have shown that there was a significant amount of harvesting residues left in different clear-felled pine plantations. These are a potential renewable, sustainable source of bio-mass currently in use in Europe and North America for bioenergy largely in biomass heating systems (Cuchet et al., 2004). The harvesting residues left on the site after merchantable wood extraction (left-slash) depend on various parameters, such as the harvesting method and equipment, stand species, site and stand quality. A previous study conducted in pine plantations indicated that processing trees at the stump (cut-

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to-length method (CTL)) can result in higher harvesting restudies than processing trees at road side (whole tree method (WT)) (Ghaffariyan, 2013).

Extracting whole trees from the site can reduce tree growth (Mann et al., 1988) due to high nutrient removal (Hendrickson et al., 1989; Ouro et al., 2001 cited in Hacker, 2005). Koelling et al. (2007) indicated that high level of biomass utilization may reduce soil fertility and uncontrolled biomass recovery may impact stand growth which results in reduction of long term site productivity. There is also concern regarding to potential damage to soil during the harvesting operations (Skin-ner et. al., 1989; Senyk, Smith, 1991 cited in Proe et al., 1994). Negative ecological impacts can be reduced by appropriate timing of operations, minimizing the nutrient removals from the forest sites and recycling of ash from combustion installation. The removal of forest residues from poor sites should be avoided in all cases, because this would further reduce the nutrients availability in these already nutrient poor sites (Burgers, 2002; Hakkila, 2002).

There have been several studies on quantifying the amount of harvesting resi-dues in softwood plantations. In Finland, Nurmi (2002-2004) studied the residue removal in mature Norway spruce stands (Pinus abies L.) harvested by a combina-tion of harvester and forwarder. The author used 2×2 m sample plots on a 30×30 m grid. Under different working methods the uncovered slash (remaining slash after biomass recovery) ranged from 19.4 to 49.9 GMt/ha. Steele et al. (2008) studied the post-harvest residue collection by slash bundler for southern pine species. For the first thinning, second thinning and clear cut the amount of recovered residues was 14.3, 11.7 and 26.9 GMt/ha respectively. Stem wood had larger share (74-93% of bundle weight) than needles and residuals in the bundle fraction test. The remain-ing slash after residue recovery was not reported in this study. Cuchet et al. (2004) used 1×1m sample square plot distributed at 10 m intervals along transects. They reported remaining slash varying from 12.3 GMt/ha (in Maritime pine stand) to 81.5 GMt/ha (for spruce stands) after residue recovery by a slash-bundler in France. The initial amount of residues varied from 22.1 to 178.1 GMt/ha. Watson et al. (1986) quantified the energy-wood biomass available on two 22-year-old slash pine planta-tions and a 45-year-old natural stand of mixed slash and loblolly pine in Alabama. All stands were clear-cut for pulpwood. Average harvesting residues weight for both pine plantation sites was 75.4 GMt/ha while for the natural mature stand the harvest-ing residue weight averaged at 61.5 GMt/ha. Smethurst, Nambiar (1990) reported a weight of 52 GMt/ha for the remaining slash in a clear-felled Pinus radiata D. Don plantation by cut-to-length harvest method (CTL) (harvester and forwarder) in Mount Gambier (South Australia). They compared their results with the range reported for other coniferous forests; 43 GMt/ha on a Pinus elliottii Englem.- Pinus palustris Mill. site in Florida (Morris, Pritchet 1982), 54.6 GMt/ha on a high-produc-tivity Douglas-fir site in Washington (Bigger, Cole 1983).

Retaining woody debris is an essential component to site fertility and planta-tion growth through nutrient recycling, conserving soil moisture and mitigating ero-

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sion (Schnepf et al., 2009; Graham et al., 1994; Evans, 2011; Thiffault et al., 2011) thus it is important to know the weight of the harvesting residues in order to define if sufficient level of slashes has been left to ensure future site sustainability. There has been little information available on quantity and quality of harvesting residues in south-east Queensland. This project aimed to meet the following objectives:

Measuring the average weight of harvesting residues (in green metric tonnes per hectare (GMt/ha)) in two harvesting sites operated by whole tree and cut-to-length methods);

Evaluating the composition of residues (such as branch, bark, cone …) for both sites;

Assessing the impact of harvesting method on quantity of left-slashes to iden-tify potential additional recovery of the residues.

STUDY AREA

The study sites were located in the Toolara Forest in Bungawatta plantation near Gympie (Queensland). Fig. 1 and 2 show the map for both harvesting sites in-cluding an indication of where one hectare study blocks were located within them. The species was mainly slash pine (Pinus elliottii Engelm.) and Honduras Caribbean Pine (Pinus caribaea var. hondurensis (Sénécl.) W.H.G.)). The stand was 30 years old. The average DBH in both sites was 31 cm. The small end diameter for log pro-

Fig. 2. One hectare study block for cut-to-lengthfor whole tree extraction

Fig. 1. One hectare study block

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cessing was 7 cm. WT harvest site was sampled on July 23th 2014 and CTL site on July 24th 2014.

The WT operation consisted of a feller-buncher to fell the trees and a skidder to extract whole trees to road side where the trees were processed into logs at the landing (Fig. 3). The CTL operation included a harvester to fell and process the trees to short logs at the stump while leaving residues on the site. Then a forwarder was used to extract the logs to the landing (Fig. 4). Both sites were harvested in April/May 2014. Sawlog and pulpwood were produced at both sites.

Fig. 3. Whole tree harvesting site Fig. 4. Cut-to-length harvesting site

STUDY METHOD

The method used in a previous pine residue study within a mobile chipper harvesting site (Ghaffariyan et al., 2014; Ghaffariyan et al., 2012b) was used to mea-sure the amount of left-slash after industrial wood recovery to improve our under-standing of its quality and quantity (Fig. 5). After a brief investigation of each site, a representative area was selected for sampling. An assessment of the left slash was conducted using line transects, within and adjacent to the study site, along which 18 plots (0.5×0.5m) were established where 5 of them were fractioned to evaluate the proportion of needles, bark, cones and branches (including small branches with di-ameter at midpoint 3 cm) (Fig. 6). The green weight of samples was measured with a portable scale that had an accuracy of 0.01 kg. The moisture content of the samples was not evaluated in this case study. To compare the average weight of harvesting residues in two harvesting site a Student’s t-test was applied to compare the means at α = 0.05. This test was applied to verify the impact of harvesting method on average weight of harvesting residues in pine plantations.

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RESULTSThe primary analysis for both sites was based on 18 samples collected from

each site. The average weight of harvesting residues in WT and CTL sites was 31.7 GMt/ha and 77.6 GMt/ha (Table 1). The weight of harvesting residues in CTL site ranged from 20.0 GMt/ha to 235.8 GMt/ha while for WT site the minimum weight of slash was 12.8 GMt/ha and maximum weight of slash was 64.0 GMt/ha.

Using the Student’s t-test (independent samples) the average weight of left-slash on both sites was compared. From Levene’s test (Table 2), as the significant level of 0.01 is less than α = 0.05 threshold, the variance of both groups was signifi-cantly different. This indicated to use the significance level of 0.01 (t=-2.85) in the t-test result (Rezai, A. 2008. http://academic.udayton.edu/gregelvers/psy216/spss/ttests.htm). Since the computed value is less than α = 0.05 there is a significant dif-ference between the average weight of residues in the two harvesting sites (Table 2). This demonstrates the harvesting method can significantly affect the weight of

Fig. 5. Weighing residues in each plot

Fig. 6. Fractioning method

Table 1 Results of left-slash assessment in both sites

Whole tree method Cut-to-length methodNumber of plots 18 18Min (GMt/ha) 12.8 20.0Max (GMt/ha) 64.0 235.8

Average (GMt/ha) 31.7 77.6

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harvesting residues. Processing trees at stump resulted in higher harvesting residues scattered on the site compared to whole tree harvesting where most of the residues were piled at road sides while little were scattered on the site. The geometric volume of one pile close to the one hectare study area in the WT operation at road side was measured about 110 m3. To convert volume of residue piles to green metric tonnes further studies on slash density of road side piles will be required.

Fig. 7 and 8 show the composition of residues for the WT and CTL harvest-ing methods, respectively. The CTL harvesting method resulted in a higher weight of residues especially in the amount of small and large branches compared to WT method. In CTL site 41% of residues were large branches (about 31.5 GMt/ha) while in WT site (Fig. 7, 8 and 9, 10) there was no large branch, only small branches

Table 2Independent samples t-test result

Levene’s Test for Equality of Variances

t-test for Equality of Means

F Sig. t df Sig. (2-tailed)

Mean Differ-ence

Std. Error Differ-ence

95% Confidence Interval of the

DifferenceLower Upper

Equal variances assumed

21.3 0.00 -2.85 34 0.007 -45.87 16.07 -78.53 -13.22

Equal variances not assumed

-2.85 18.89 0.010 -45.87 16.07 -79.52 -12.23

Fig. 7. Residues of whole tree extraction Fig. 8. Residues of cut-to-length extraction

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(diameter

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was less than for a Spruce case study in France (259.6 GMt/ha) reported by Cuchet et al. (2004) and another case study of radiata pine in Tasmania harvested by CTL operations (238.7 GMt/ha) (Ghaffariyan et al., 2012a). The weight of residues from the WT site (31.7 GMt/ha) is relatively high compared to a whole tree operation in Western Australian Eucalypt plantation, where left slash was found to be 6.1 GMt/ha. The difference can be explained by the high amount residue needles in this case study compared to leaves in Eucalypt trees (Fig. 7) and no other study on pine whole tree operation was available to compare.

Considering the level of slash removal on both sites meets or exceeds avail-able recommendations for maintenance of site quality (Table 3) and there is a rela-tively large quantity of residue logs and branches on the CTL site (31.5 GMt/ha) there may scope for additional fibre recovery (Walsh, Strandgard, 2014; Ghaffariyan et al., 2015) using conventional and modified bin-load forwarders (Ghaffariyan et al., 2012c) and mobile chippers (Desrochers et al., 1993). Recovered fibre have po-tential use in producing chips for medium-density fibreboard (MDF) or biorefinery/bioenergy depending on the quality of fibre and local markets. If bioenergy is a target application for the recovered residues, the material can be stored to dry naturally on site to reduce moisture content and increase calorific value (Acuna et al., 2012; Ghaffariyan et al., 2013) while all fine materials such as needles, small branches or barks can be retained on the site to maintain soil fertility for next rotation (Rothe, 2013).

CONCLUSIONS

Harvesting method significantly impacts the weight of harvesting residues in pine plantations. In this study cut-to-length harvesting yielded significantly higher residue levels compared to whole-tree harvesting due to leaving large branches on the site when processing trees at the stump. The weight of large branches left follow-ing CTL harvesting was 31.5 GMt/ha in this case study. Depending on the economics of harvesting and haulage operations, this may represent a viable resource of fibre to be extracted for industrial use.

Table 3Wood recovery and left-slash percentage

Industrial wood recovery (GMt/ha)

Left slash (GMt/ha)

Total biomass (GMt/ha)

Left sash share of total biomass (%)

Whole Tree 262.2 31.7 293.9 10.8

Cut-to-length 334.5 77.6 412.1 18.8

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Acknowledgments: The authors would like to acknowledge Mr Ian Last, Mr Tim Don and Mr Tim Lee (from HQP) for the assistance with this project by providing information and access to the plantations. They also thank Prof. Mark Brown for editing the English language of this article.

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