foliar plasticity of hybrid spruce in relation to crown ... · a.d. richardson, g.p. berlyn, p.m.s....

Foliar plasticity of hybrid spruce in relation tocrown position and stand age

A.D. Richardson, G.P. Berlyn, P.M.S. Ashton, R. Thadani, and I.R. Cameron

Abstract: This study examined the foliar response of putative hybrid Engelmann × white × Sitka spruce (PiceaengelmanniiParry × Picea glauca(Moench) Voss ×Picea sitchensis(Bong.) Carr) needles in relation to crown posi-tion and across three stages of development (15, 55, and 145 years). We focused on the morphological and anatomicalresponse, and used physiological measures (photosynthesis and stomatal conductance) to emphasize the important rela-tionship between structure and function. We found that needles from the upper outer crown position were adaptated toreduce stress from evapotranspiration. Trees from the 15- and 55-year-old stands generally responded the most, andtrees from the 145-year-old stand responded the least. As they mature, these spruce trees may have reduced ability toadapt to their environment. Some of our results contradict what the literature considers “typical” for sun–shade dimor-phism in temperate forests. For example, in all stands, sun needles were broader than shade needles and, in the twoyounger stands, sun needles were larger in area, not smaller, than shade needles. Also, in the youngest stand, stomatalpores were longer on sun needles than on shade needles. There were no definite patterns in stomatal spacing with re-gard to crown position. Our results are indicative of the strategies adopted to increase competitiveness in a resource-limited environment. We suggest that, in the 15-year-old stand, neither water nor light is limiting; in the 55-year-oldstand, both water and light are highly limiting; and in the 145-year-old stand, water is most limiting.

Key words: drought, foliar plasticity, needle anatomy, photosynthesis,Picea, stand development, sun–shade.

Résumé: Les auteurs ont examineé la réaction foliaire des aiguilles de l’épinette hybride présumée Engelman ×blanche × Sitka (Picea engelmanniiParry × Picea glauca(Moench) Voss ×Picea sitchensis(Bong.) Carr), en relationavec la position sur la cime et selon trois stades de développement (15, 55, et 145 ans). L’accent est mis sur la réac-tion morphologique et anatomique ainsi que l’utilisation de mesures physiologiques (photosynthèse et conductance sto-matale) pour souligner la relation importante entre la structure et la fonction. Les auteurs ont constaté que les aiguillesde la partie supérieure de la cime externe sont adaptées pour réduire le stress par évapotranspiration. Les arbres desstations âgées de 15 et 35 ans réagissent généralement le plus fortement, et ceux de la station âgée de 145 ans, le plusfaiblement. Au cours de la maturation, ces épinettes peuvent développer une capacité réduite d’adaptation au milieu.Certains résultats obtenus contredisent ce que la littérature considère comme dimorphisme ombre-lumière “typique” enforêts tempérées. Par exemple, dans tous les peuplements, les aiguilles de pleine lumière sont plus larges que les ai-guilles d’ombre, et dans les deux peuplements les plus jeunes, les aiguilles de pleine lumière sont plus grandes en su-perficie, et non plus petites, que les aiguilles d’ombre. De plus, dans les stations les plus jeunes, les pores stomatauxsont plus longs chez les feuilles de lumière que chez celle d’ombre. Il n’y a pas de patron défini pour l’espacementdes stomates en relation avec la position dans la cime. Les résultats sont indicateurs des stratégies adoptées pour aug-menter la compétitivité dans un environnement où les ressources sont limitées. Les auteurs suggèrent que dans le peu-plement de 15 ans, ni l’eau ni la lumière ne sont limitantes. Dans le peuplement de 55 ans, l’eau et la lumière sontfortement limitantes. Dans le peuplement de 145 ans, l’eau est plus fortement limitante.

Mots clés: sécheresse, plasticité foliaire, anatomie des aiguilles, photosynthèse,Picea, développement du peuplement,ombre–lumière.

[Traduit par la Rédaction] Richardson et al. 317

Introduction

Leaf structure has both a genetic component and an envi-ronmental component. As it develops, the leaf responds to

its environment in a variety of ways. For example, pheno-typic plasticity can be expressed within an individual. Dif-ferences in light quality and quantity from the upper- to thelower-crown positions of a tree result in a plastic response

Can. J. Bot.78: 305–317 (2000) © 2000 NRC Canada

305

Received November 22, 1999.

A.D. Richardson,1 G.P. Berlyn, P.M.S. Ashton, and R. Thadani.2 Yale University, School of Forestry and EnvironmentalStudies, 370 Prospect Street, New Haven, CT 06511, U.S.A.I.R. Cameron.3 Research Branch, British Columbia Ministry of Forests, 515 Columbia Street, Kamloops, BC V2C 2T7, Canada.

1Author to whom all correspondence should be addressed (e-mail: [email protected]).2Present address: Central Himalayan Rural Action Group (CHIRAG), Sitla, P.O. Mukteswar, district Nainital, U.P. 263 138, India.3Present address: J.S. Thrower and Associates, Ltd., 103-1383 McGill Road, Kamloops, BC V2C 6K7, Canada.

J:\cjb\cjb78\cjb-03\B00-005.vpWednesday, April 19, 2000 9:08:00 AM

Color profile: DisabledComposite Default screen

of both leaf structure and function, i.e., morphological, ana-tomical, and physiological responses to the sun–shade con-tinuum. Hanson (1917) and Büsgen and Münch (1929) wereamong the early workers to enumerate differences betweensun and shade leaves.

High levels of solar radiation in the upper crown are asso-ciated with desiccation, and so the responses to intense lightand scarce water are often similar. Sun leaves tend to exhibittraits similar to those of drought-avoiding plants (Ashtonand Berlyn 1992), as ultimately there is a trade-off betweenmaximizing carbon gain (necessitating CO2 uptake and lightharvesting) and minimizing water loss (Givnish 1979, 1988)and photoinhibition. In practice, it is methodologically im-possible to conclusively separate the anatomical effects of aresponse to drought from those of a response to high lightlevels when field-grown specimens are being studied. Foliarmorphology may also be related to nutrient availability(Joffre et al. 1999; Meziane and Shipley 1999; Poorter andGarnier 1999), but light is the main environmental triggerfor changes in leaf morphology (Fitter and Hay 1987, p. 50).

The physiological and anatomical responses to light anddrought vary among tree species (according to the succes-sional status) and within a species (depending on tree ageand canopy position) (Wylie 1951, 1954; Jackson 1967; Car-penter and Smith 1975; Boardman 1977; Hinckley et al.1978; Lichtenthaler et al. 1981; Fetcher et al. 1983; Bahariet al. 1985; Abrams and Knapp 1986; Abrams 1988; Givnish1988; Lee et al. 1990; Strauss-Debenedetti and Bazzaz 1991;Ashton and Berlyn 1992, 1994; Strauss-Debenedetti andBerlyn 1994; Ashton et al. 1998). Relationships betweenanatomical and physiological parameters are important (re-viewed by Smith et al. 1997), because they can help to ex-plain why species are well suited to occupying particularsites (Ashton and Berlyn 1994).

This study compares the morphological, anatomical, andphotosynthetic characteristics ofPicea engelmanniiParry ×Picea glauca(Moench) Voss ×Picea sitchensis(Bong.) Carr(Engelmann × white × Sitka spruce hybrid) needles fromdifferent crown positions and across a forest chronose-quence. In northern and central British Columbia (between52° and 57°N), from the Rocky Mountains to the CoastalRange, most spruce stands show some evidence of hybrid-ization between white, Engelmann, and, occasionally, Sitkaspruce (MacKinnon et al. 1992).Picea engelmannii× Piceaglauca hybrids are commonly designated “interior spruce”(Parish et al. 1996). In our study area, however, which is partof the Nass Skeena Transition (Grossnickle et al. 1997), in-terior and Sitka spruce are known to hybridize (Sutton et al.1991, 1994). It is therefore realistic to refer to the spruce inour study area simply as hybrid spruce.

Our emphasis is on how a needle’s growth environmentresults in morphological and anatomical adaptations. Asphysiological parameters, measures of photosynthesis andstomatal conductance are used primarily to assist in our in-terpretation of the structural response to environmental con-ditions, e.g., relationships between form and function (Smithet al. 1997) in the context of crown position and tree age.

Crown position is related to drought stress and, most im-portantly, to the light environment in which the needle ex-pands and functions. Not only is the quantity of lightreduced in the lower crown positions (owing to shading by

the upper canopy), but the quality of light, as measured bythe red : far red ratio, is also altered. The integrated photon-flux density, rather than the peak density, has been shown tobe the main factor in determining the foliar response to light,and is positively correlated with several “sun-leaf” charac-teristics (Chabot et al. 1979). The red : far red ratio insidethe canopy is different (owing to the filtering effect ofleaves; see Lee and Graham 1986) from that of uninter-cepted sunlight (Richards and Lee 1986), and this ratio hasbeen related to foliar morphogenesis (van Hinsberg and vanTienderen 1997). Thus, needle structure can be interpretedas giving an index of the integrated light conditions at apoint in the canopy (Sprugel et al. 1996).

The chronosequence gives insight into the changes in nee-dle structure and photosynthetic parameters that may be as-sociated with tree ontogeny and stand development. Changesin the plastic response with tree age are not well understood.We find evidence of foliar differences between juvenile andmature trees, as different strategies are adopted to deal withcompetition. However, a complicating factor is that treesmodify their own environment. The availability of limitingabiotic factors, such as light, water, and nutrients, changes asstands pass through different stages of development (Oliverand Larson 1996).

Materials and methods

Study siteThe research site (which has been described in greater detail

elsewhere; see Ashton et al. 1998) was located in northwesternBritish Columbia, Canada (55°55′N, 126°10′W) in a moist coldsubzone of the Interior Cedar Hemlock biogeoclimatic zone(ICHmc2). The interior cedar–hemlock forest type (Meidinger andPojar 1991) occupying this area represents a transition betweentrue coastal and interior forests. Forest stands are dominated by thevery shade tolerant conifersTsuga heterophylla(Raf.) Sarg. (west-ern hemlock) andThuja plicata Donn. (western redcedar). Otherimportant species includeAbies lasiocarpa(Hook.) Nutt. (subal-pine fir), Picea engelmannii× glauca× sitchensis(hybrid spruce),Pinus contortaDougl. (lodgepole pine),Betula papyriferaMarsh.(paper birch),Populus trichocarpaT. & G. (black cottonwood),Populus tremuloidesMichx. (aspen), andSalix spp. (willows).

In an attempt to recreate the chronosequence of succession, westudied three stands of different ages, viz., 15 years (S15), 55 years(S55), and 145 years (S145). LePage (1995) documents stand his-tories in greater detail. These extensive stands are growing on simi-lar soils, experience similar climates, and are considered to be thesame forest type. All these stands were established following cata-strophic fires. Stem analysis and stand reconstruction has indicatedthat the stands we chose as a chronosequence are representativeof stand development patterns in this forest type. The two olderstands (S55 and S145) were located in the Date Creek ResearchArea, managed by the British Columbia Ministry of Forests, andthe youngest (S15) was on land bordering the research area.

The three stands fit into the “stand initiation” (S15), “stem ex-clusion” (S55), and “understory re-initiation” (S145) stages of Oli-ver and Larson’s (1996) stand development model. S15 had anopen canopy and abundant light reached the understory. Paperbirch and willow emergents stood above an irregular canopy oflodgepole pine and hybrid spruce and a subcanopy of western hem-lock and western redcedar. S55 had a closed canopy and a veryshaded understory. Hybrid spruce, lodgepole pine, and paper birchdominated the irregular canopy. There was a subcanopy of westernhemlock and some western redcedar. S145 had scattered emergentsof hybrid spruce, lodgepole pine, and subalpine fir. The canopy

© 2000 NRC Canada

306 Can. J. Bot. Vol. 78, 2000



was largely western hemlock and paper birch, with a subcanopy ofwestern hemlock and western redcedar.

Sutton et al. (1994) demonstrated that the trend in Sitka-interiorspruce introgression is consistent with topographical trends. Thereis a significant difference in the nature of hybridization betweenour study area, located in the Nass Skeena Transition, and thenearby Bulkley Valley (Sutton et al. 1994). A steep environmentalgradient from a coastal to an interior climate (in particular, temper-ature and precipitation) and strong selection pressures (Grossnickleet al. 1997) characterize the Nass Skeena Transition. Seeds fromthree areas immediately surrounding our site have a hybrid sprucefraction mix of 0.46±0.08 SE (Sutton et al. 1994), where 1.0 meansinterior spruce and 0.0 means Sitka spruce. The three stands westudied are located in close proximity to each other and occur ontopographically similar sites. This fact, combined with genetic andphysiological studies of seedlots in this region by Sutton et al.(1991, 1994) and Grossnickle et al. (1997), as well as the work ofFan et al. (1997, 1999) on Sitka × interior hybrids in general, sup-ports our contention that the nature of hybridization in S15, S55,and S145 is consistently the same. While some variation in hybrid-

ization no doubt exists in the area around our study site, it is actu-ally related to much greater scales of introgression when progress-ing from the sea across the mountains.

SamplingIn July 1994, photosynthesis was measured and needles were

sampled from four randomly selected trees in each stand (details ofthe experimental design are given in Ashton et al. 1998). Sampletrees were well dispersed in each stand to account for possible siteheterogeneity. The needles used for the study were located at sixdifferent crown positions that were characterized as upper outer(UO), upper inner (UI), middle outer (MO), middle inner (MI),lower outer (LO), and lower inner (LI) (Fig. 1). Because of thesmall size of trees in S15, only the UO, LO, and LI crown posi-tions were sampled.

PhotosynthesisOn the day of sampling, a selected tree was felled early in the

morning. From each crown position a branch was cut and immedi-

© 2000 NRC Canada

Richardson et al. 307

Fig. 1. Mean dimensions, by stand, of hybrid spruce, with mean crown position of foliage samples taken. “Base” measurements are thevertical distance from the sample to the base of the tree; “bole” measurements are the horizontal distance from the sample to the boleof the tree. Measurements are in metres unless otherwise stated. Standard deviations are given in parentheses. Note the differences invertical and horizontal scales. Vertical scale is compressed for the S145 and S55 stands.



© 2000 NRC Canada

308 Can. J. Bot. Vol. 78, 2000

(A) Morphology.

LI LO MI MO UI UO

Needle width (mm)S15 1.04±0.11 1.14±0.08 1.54±0.07S55 1.14±0.06 1.17±0.08 1.32±0.20 1.39±0.12 1.62±0.16 2.20±0.17S145 1.12±0.06 1.15±0.09 1.31±0.12 1.26±0.12 1.42±0.10 1.54±0.14

Needle thickness (mm)S15 0.87±0.06 0.91±0.06 1.13±0.06S55 0.94±0.04 0.94±0.05 0.99±0.07 1.12±0.12 1.18±0.09 1.24±0.06S145 0.96±0.05 0.89±0.03 0.98±0.02 0.92±0.04 0.94±0.02 1.01±0.05

Needle width to thickness ratio (mm/mm)S15 1.21±0.10 1.26±0.06 1.37±0.06S55 1.22±0.06 1.26±0.10 1.32±0.12 1.25±0.06 1.38±0.08 1.79±0.21S145 1.18±0.07 1.29±0.07 1.33±0.10 1.37±0.09 1.51±0.07 1.52±0.06

(B) Anatomy.

Vascular cylinder cross-sectional area (mm2)S15 0.035±0.009 0.042±0.007 0.083±0.009S55 0.066±0.010 0.061±0.005 0.076±0.013 0.095±0.022 0.121±0.017 0.169±0.009S145 0.066±0.008 0.061±0.008 0.087±0.012 0.078±0.010 0.096±0.011 0.107±0.019

Mesophyll cross-sectional area (mm2)S15 0.513±0.078 0.617±0.060 0.959±0.069S55 0.500±0.047 0.540±0.046 0.634±0.136 0.710±0.120 0.875±0.114 1.089±0.104S145 0.598±0.043 0.569±0.054 0.714±0.063 0.664±0.089 0.750±0.044 0.824±0.112

Vascular cylinder ratio (mm 2/mm2)S15 0.061±0.005 0.064±0.005 0.080±0.008S55 0.117±0.009 0.101±0.003 0.109±0.004 0.116±0.008 0.121±0.007 0.136±0.011S145 0.099±0.004 0.097±0.006 0.107±0.006 0.107±0.008 0.114±0.008 0.114±0.007

Average cuticle thickness (µm)S15 2.32±0.11 2.22±0.10 2.90±0.33S55 2.92±0.29 2.67±0.24 3.19±0.35 3.22±0.45 3.36±0.28 3.63±0.26S145 2.93±0.21 3.08±0.27 3.21±0.17 3.65±0.51 3.90±0.27 3.76±0.54

Average upper epidermal wall thickness (µm)S15 6.43±0.40 7.12±0.27 8.13±0.32S55 7.96±0.71 7.67±0.41 9.06±0.48 9.61±0.90 9.95±0.46 9.79±0.50S145 8.08±0.45 8.08±0.21 8.57±0.32 9.19±0.63 9.56±0.50 9.59±0.40

Edge–edge interstomatal distance (µm)S15 33.7±4.9 31.8±1.4 35.5±1.3S55 50.9±5.3 55.8±8.1 42.9±4.9 39.5±5.4 51.6±8.6 47.0±4.9S145 43.8±4.0 43.6±2.7 40.5±5.0 39.8±2.9 37.6±4.1 36.8±0.8

Mid–mid interstomatal distance (µm)S15 85.1±5.8 85.5±2.2 90.1±1.3S55 106.8±6.2 111.6±6.0 97.8±5.8 95.4±4.3 111.1±8.5 102.8±4.3S145 100.0±3.8 99.9±2.4 95.1±5.5 94.8±3.4 92.5±4.3 91.9±1.4

Stomatal pore length (µm)S15 51.4±1.0 53.7±0.9 54.6±0.9S55 55.9±0.9 55.8±2.3 55.0±1.3 55.9±1.6 59.5±1.5 55.8±1.0S145 56.2±0.6 56.3±0.4 54.6±0.8 54.9±0.6 55.0±0.7 55.0±0.9

Table 1. Arithmetic means (± SE), by stand and crown position, of some anatomical, morphological, and physiological measurementsmade on hybrid spruce needles.



ately immersed in water, where it was re-cut to maintain an unbro-ken water column. Our own field tests indicated that the cutting ofbranches from these hybrid spruce and submerging the cut end inwater had no measurable effect on rates of photosynthesis or sto-matal conductance over the time period relevant to our measure-ments. We have found the same to be true with bothB. papyrifera(Olander et al. 1995) andT. heterophylla(McGroddy et al. 1995).A portable closed system LI-COR 6200 infrared gas analyzer(Welles 1986) with a 1-L leaf chamber was used to determine therate of net CO2 assimilation (maximum net photosynthesis) andstomatal conductivity under full sun conditions. This instrumentwas always set up on an adjacent forest road, and photosynthesiswas measured only on clear, sunny mornings (8 a.m. to 12 noon).

For all measurements, photosynthetically active radiation (PAR)was 1200 µmol·m–2·s–1 or greater and CO2 levels averaged370 µL·L–1 (± 30 µL·L–1 SD). Mean chamber temperature was27°C (± 3°C SD) and mean relative humidity was 43% (± 12%SD). The needles on the branch placed in the chamber werecounted and needle area was measured with a portable leaf areameasuring device (CID Inc., Vancouver, Wash.). Needle freshweight was measured using a portable balance (Ohaus ModelCT200, Florham Park, N.J.). Five needles from each branch wereimmediately fixed in FAA (formalin – acetic acid – alcohol; Berlynand Miksche 1976).

Anatomy and morphologyThree of the five needles in each sample were used for anatomy

measurements. Only mature needles from previous years’ flusheswere used. Fixed axial sections were cut from the middle of theneedles and these sections were dehydrated in an ethyl alcohol –tert-butyl alcohol series and embedded in paraffin (Berlyn andMiksche 1976). Because of the toughness of spruce needles, thetissue blocks were soaked overnight at 35°C in a solution of water,glycerol, DMSO, and liquid detergent (88:10:1:1), to soften cellwalls. Tissue sections were cut on a cryotome at a thickness of 12µm. Ribbons were mounted on slides prepared with a gelatin –chrome alum adhesive. Slides were stained in Safranin O and fastgreen FCF and then dehydrated and mounted with synthetic resinusing calipered No. 1 1/2 coverslips (0.15–0.19 mm).

Two needles from each crown position were prepared separatelyfor stomatal measurements. Needles were rehydrated in an ethanolseries and then kept in a 5% solution (1.25 M) of sodium hydrox-ide at 50°C until the pigments were removed and the needles weretransparent (about 10 days). Samples were stained in a 1% solutionof Toluidine Blue O for about 20 s and temporarily mounted onslides with Karo light corn syrup.

Anatomical measurements were taken using electronic imageanalysis equipment (Leica Q500MC and QWin v1.00 software).Objectives of 2×, 3.5×, 6×, and 40× were combined with oculars of

5× and 6×, depending on the cellular or histological attributes to bemeasured. The camera–computer setup provided an additional 8×magnification.

On each slide, from three to five replicates of needle width, nee-dle thickness, vascular cylinder cross-sectional area, and cross-sectional area inside the epidermis (from which the mesophyllcross-sectional area was then calculated by subtracting the vascularcylinder cross-sectional area) were measured. Additionally, twosets of both cuticle and upper epidermal wall thicknesses weremeasured, one on each side of the needle. Stomatal length andinterstomatal distance were measured 30 times on each needle.

Statistical analysisThe measurements from each slide were averaged to give a sin-

gle set of measurements for each needle, and abaxial and adaxialmeasurements were averaged. Three needles from each crown po-sition were measured, and each needle was considered an observa-tional unit. The four trees sampled from each stand were taken tobe experimental units. Results are reported as mean±SE, unlessotherwise noted.

The GLM (generalized linear model) procedure from SAS ver-sion 6.12 (SAS Institute Inc., Cary, N.C.) was used to analyze theeffects of crown position (UO, UI, MO, MI, LO, and LI) and stand(S145, S55, S15), plus the crown position × stand interaction term,on morphological, anatomical, photosynthetic, and conductancedata. The interaction term was not significant, even at the 10%level, in any of the ANOVAs, so each ANOVA was re-run with justthe main effects. Least squares marginal means were calculatedfor the main effects. To test the null hypothesisH0: lsmean(i) =lsmean(j), t tests were conducted for all (i,j) pairs within each maineffect and used to detect differences. The significance of Pearson’scorrelation coefficients (r) was assessed by testing the null hypoth-esisH0: r = 0. A significance level of 5% was used for all tests.

An index of foliar plasticity,ϕ, is defined as:ϕ = (mean sun leaftrait – mean shade leaf trait)/mean sun leaf trait. Measurementsfrom the upper outer part of the crown were used for “sun nee-dles,” and those from the lower innner part of the crown were usedfor “shade needles.” In using this index, we are not assuming thatthe light gradient is the same across all three stands, we are merelyattempting to characterize the individual’s plasticity (which is anindex of its competitive ability in the environment it faces) in re-sponse to crown position.

Results

Along the gradient from UO to LI, needles from all threestands displayed similar trends in anatomy and morphology(Fig. 2; Table 1). Photosynthesis measurements, which were

© 2000 NRC Canada


(C) Photosynthesis and conductance.

LI LO MI MO UI UO

Photosynthesis/unit area (µmol·m–2·s–1)S15 4.19±0.63 4.23±0.36 4.48±0.50S55 1.97±0.38 1.31±0.44 2.50±1.00 1.62±0.55 1.65±0.34 1.65±0.40S145 1.89±0.51 2.43±0.46 0.98±0.33 2.07±0.38 1.15±0.16 1.03±0.43

Conductance/unit area (µmol·m–2·s–1)S15 0.11±0.03 0.10±0.02 0.21±0.09S55 0.05±0.01 0.03±0.01 0.08±0.04 0.04±0.02 0.07±0.01 0.13±0.04S145 0.08±0.02 0.07±0.03 0.06±0.02 0.09±0.03 0.04±0.02 0.03±0.01

Note: Abbreviations used: stands: S15, juvenile (15 years); S55, intermediate (55 years); S145, mature (145 years); crown positions: LI, lower inner;LO, lower outer; MI, middle inner; MO, middle outer; UI, upper inner; UO, upper outer; MI, MO, and UI crown positions were not used for S15 trees.

Table 1 (concluded).



often lower in the upper part of the canopy than in the lowerpart, did not follow expected patterns (Table 1).

With the exception of stomatal conductance (p = 0.057),all ANOVAs on measured leaf parameters were significantat p = 0.05 (Table 2). In 12 ANOVAs, the “stand” effectwas significant atp = 0.05 and in 9 ANOVAs, the “crown-position” effect was significant atp = 0.05. In seven of thesemodels, the two effects were both significant atp = 0.05.

Stands differed for most, but not all (i.e., not for width tothickness ratio or mesophyll cross-sectional area) parameters(Table 3). Plasticity indices (Table 3) were generally high( . )ϕ ≈ 030 for the morphological and some anatomical mea-surements, but were aroundϕ = 0 for the stomatal measure-ments, pore length and interstomatal distance. The twoyounger stands, S15 and S55, were more plastic for bothmorphological and anatomical parameters than S145. Photo-synthesis and conductance plasticity differed dramaticallybetween stands.

There were definite patterns along the gradient from upperto lower crown positions, but differences between crownpositions were larger for some parameters than others (Ta-ble 4). Notably, however, crown position was not a sig-nificant factor in either stomatal or gas exchange ANOVAs(Table 2), nor did the least squares means of these parame-ters display any obvious trends with respect to crown posi-tion (Table 4).

MorphologyNeedles from S55 were wider, thicker, had a larger needle

area, and were heavier than those from either of the othertwo stands (Table 3). In all stands, plasticity values werepositive for most of the morphological measurements, indi-cating that sun needles were bigger than shade needles (Ta-ble 3). In terms of needle area, sun needles were more than30% larger than shade needles in both S55 and S15. On theother hand, in S145, sun needles were 10% smaller in areathan shade needles. Sun needles were wider than shade nee-dles for all three stands. Needles varied in cross-sectionalshape with crown position: the needle width to thickness ra-tio was greater for sun needles than shade needles (Tables 3and 4). However, needles in all three stands were similarlyflat, as this ratio did not vary significantly between stands(Table 3).

AnatomyNeedle anatomy varied both with stand (Table 3) and

crown position (Table 4). The amount of conducting tissue,as measured by the cross-sectional area of the vascular cyl-inder, was lowest in S15 and highest in S55 (Table 3). Thevascular cylinder accounted for a greater proportion of nee-dle cross-sectional area (“vascular cylinder ratio”) in S55and S145 than in S15 (Table 3).

Although mesophyll cross-sectional area did not vary sig-nificantly between stands (average of 0.69 mm2 for all threestands,p = 0.77 in the ANOVA), mesophyll plasticity washigh in all three stands (Table 3) and there were distincttrends within the canopy (Table 4). There was consistentlymore mesophyll in sun needles than in shade needles. Meso-phyll-area plasticity was especially high in S55 (ϕ = 0.55).

Cuticle thickness was about 40% greater in S145 than inS15 (Table 3). Similarly, epidermal wall thickness was about

© 2000 NRC Canada

310 Can. J. Bot. Vol. 78, 2000

Pa

ram

ete

r

Mo

de

lE

rro

rO

vera

llm

od

el

Eff

ect

Sta

nd

Cro

wn

po

sitio

n

df

MS

df

MS

df

Fp

df

pd

fp

Ave

rag

en

ee

dle

are

a(m

m2 )7

1.2

5×

10–

24

24

.55

×1

0–3

49

2.7

40

.01

93

*2

0.0

61

35

0.1

81

9N

ee

dle

wid

th(m

m)

74

.82

×1

055

05

.96

×1

045

78

.08

0.0

00

1**

*2

0.0

34

8*

50

.00

01

***

Ne

ed

leth

ickn

ess

(mm

)7

7.0

4×

10

45

01

.42

×1

045

74

.97

0.0

00

3**

*2

0.0

06

4**

50

.00

11

**N

ee

dle

wid

th/t

hic

kne

ssra

tio(m

m/m

m)

71

.26

×1

0–

15

03

.41

×1

0–2

57

3.7

00

.00

27

**2

0.4

03

75

0.0

01

2**

Ave

rag

en

ee

dle

we

igh

t(m

g)

76

.19

×1

0–

54

28

.94

×1

0–6

49

6.9

30

.00

01

***

20

.00

12

**5

0.0

03

4**

Va

scu

lar

cylin

de

rcr

oss

-se

ctio

na

la

rea

(mm

2 )7

6.5

4×

109

50

6.2

2×

108

57

10

.51

0.0

00

1**

*2

0.0

00

5**

*5

0.0

00

1**

*M

eso

ph

yll

cro

ss-s

ect

ion

al

are

a(m

m2 )

71

.84

×1

011

50

2.6

4×

101

05

76

.98

0.0

00

1**

*2

0.7

71

65

0.0

00

1**

*V

asc

ula

rcy

lind

er

ratio

(mm2 /

mm

2 )7

3.1

0×

10–

35

01

.70

×1

0–4

57

18

.24

0.0

00

1**

*2

0.0

00

1**

*5

0.0

03

8**

Ave

rag

ecu

ticle

thic

kne

ss(

µm

)7

1.8

4×

100

50

3.4

2×

10–

15

75

.38

0.0

00

1**

*2

0.0

02

9**

50

.01

07

*A

vera

ge

up

pe

re

pid

erm

al

wa

llth

ickn

ess

(µ

m)

78

.23

×1

005

08

.05

×1

0–1

57

10

.22

0.0

00

1**

*2

0.0

00

7**

*5

0.0

00

1**

*M

id–

mid

inte

rsto

ma

tal

dis

tan

ce(

µm

)7

3.7

7×

102

48

8.1

2×

101

55

4.6

50

.00

05

***

20

.00

01

***

50

.36

70

Ed

ge

–e

dg

ein

ters

tom

ata

ld

ista

nce

(µ

m)

72

.66

×1

024

87

.89

×1

015

53

.37

0.0

05

3**

20

.00

01

***

50

.48

54

Sto

ma

tal

po

rele

ng

th( µm

)7

1.3

3×

101

48

4.6

8×

100

55

2.8

40

.01

46

*2

0.0

04

3**

50

.36

74

Ph

oto

syn

the

sis/

un

ita

rea

(µ

mo

l·m

–2 ·

s–1 )

79

.67

×1

005

11

.02

×1

005

89

.51

0.0

00

1**

*2

0.0

00

1**

*5

0.9

21

4C

on

du

cta

nce

/un

ita

rea

(µ

mo

l·m

–2 ·

s–1 )

76

.49

×1

0–3

40

2.9

8×

10–

34

72

.18

0.0

57

02

0.0

32

1*

50

.32

63

Not

e:In

tera

ctio

nte

rmw

asno

tsi

gnifi

cant

atp≤

0.05

for

any

ofth

eA

NO

VA

san

d,th

eref

ore,

was

drop

ped

from

each

mod

el:

*,

p≤

0.05

;**

,p

≤0.

01;

***,

p≤

0.00

1.T

here

are

thre

est

ands

inst

and

effe

ct(S

15,

S55

,S

145)

and

six

posi

tions

incr

own

posi

tion

effe

ct(L

I,LO

,M

I,M

O,

UI,

UO

).M

S,

mea

nsq

uare

.

Tabl

e2.

AN

OVA

resu

ltsfo

rth

eva

rio

us

pa

ram

ete

rso

fhy

bri

dsp

ruce

ne

ed

les

me

asu

red

.



© 2000 NRC Canada


15–20% greater in S145 and S55 than in S15. However,in all three stands, plasticity was positive (in the range of0.15 < ϕ < 0.20; Table 3) for both measurements. For bothmeasurements, there was a linear trend from lower to uppercanopy and there were significant differences betweencrown positions (Table 4).

Since stomata occurred in rows rather than in a uniformdistribution across the leaf surface, we concluded that in-terstomatal distance was a better measure of stomatal fre-quency than the usual stomatal-density measures (used, forexample, by Ashton et al. 1998). We measured two inter-stomatal distances: mid–mid interstomatal distance, which isthe linear distance between the midpoints of two adjacentstomata; and edge–edge interstomatal distance, which is thewidth of the “gap” between two adjacent stomata. Thusmid–mid interstomatal distance = edge–edge interstomataldistance + stomatal pore length. The same stand level pat-terns were apparent for both interstomatal distance measure-ments (Table 3).

S15 had stomata that were both smaller and more closelyspaced than either of the two older stands (Table 3). For ex-ample, edge–edge interstomatal distance was 50% greaterand stomatal pore length was 6% greater in S55 than in S15.Although the crown position effect was not significant in thestomatal pore length (p = 0.37) and interstomatal distance(mid–mid, p = 0.37; edge–edge,p = 0.49) ANOVAs, therewas still evidence of within-crown stomatal plasticity (Ta-ble 3). This plasticity was generally much lower than thatfor other anatomical features or for the morphological mea-surements. In S15, sun needles had larger stomata thanshade needles (ϕ = 0.07) and there was some evidence thatstomata on sun needles were spaced more closely together(Tables 1 and 3). In S145 and S55, however, there were nodefinite trends within the crown for either interstomatal dis-

tance or stomatal pore length (Table 1).The phenotypic response to crown position occurs over an

entire suite of characteristics: cuticle thickness, epidermalwall thickness, vascular cylinder cross sectional area, meso-phyll cross sectional area, vascular cylinder ratio, needlewidth, and needle thickness were all positively correlatedwith each other atp < 0.01 (Fig. 3). Stomatal pore lengthand the interstomatal distance measurements were generallynot highly correlated with anatomical measurements, al-though they were correlated with needle weight, area, andthickness (allp ≤ 0.05). Stomatal pore length was positivelycorrelated with the vascular cylinder ratio (r = 0.39, p <0.01), vascular cylinder cross sectional area (r = 0.37, p <0.01), and epidermal wall thickness (r = 0.29,p = 0.034).

PhotosynthesisPhotosynthesis rates in S15 were roughly twice as high as

those in either S145 or S55, as were rates of stomatal con-ductance (Table 3). Both physiological measurementsshowed strongly negative plasticity in S145 (Table 3). Thismeans that, on a per unit area basis, the lower inner canopypositions in S145 were photosynthesizing and exchanginggas at a rate several times greater than the upper outer can-opy positions. On the other hand, stomatal conductance plas-ticity was positive in both S55 and S15.

Significant correlations were found between the physio-logical and several of the anatomical parameters (Fig. 3).Photosynthesis was negatively correlated with vascular cyl-inder ratio (r = –0.51, p < 0.01), edge–edge interstomataldistance (r = –0.48, p < 0.01), mid–mid interstomatal dis-tance (r = –0.48,p < 0.01), epidermal wall thickness (r =−042. , p < 0.01), vascular cylinder cross sectional area (r =−038. , p < 0.01), and cuticle thickness (r = –0.37,p < 0.01).

Stomatal conductance was positively correlated with nee-

Stand average across all crown positions Plasticity (ϕ)

S15 S55 S145 S15 S55 S145

MorphologyAverage needle area (cm2) 0.16±0.02ab 0.22±0.02a 0.17±0.01b 0.45 0.31 –0.10Needle width (mm) 1.25±0.08b 1.46±0.05a 1.30±0.05b 0.32 0.46 0.26Needle thickness (mm) 0.98±0.04ab 1.06±0.03a 0.95±0.02b 0.23 0.25 0.05Needle width to thickness ratio (mm/mm) 1.28±0.06 1.36±0.04 1.37±0.04 0.12 0.28 0.22Average needle weight (mg) 8.9±1.1b 13.2±0.8a 9.7±0.6a 0.54 0.46 0.29

AnatomyVascular cylinder cross-sectional area (mm2) 0.056±0.008b 0.097±0.005a 0.082±0.005a 0.59 0.62 0.34Mesophyll cross-sectional area (mm2) 0.718±0.053 0.719±0.035 0.687±0.033 0.46 0.55 0.24Vascular cylinder ratio (mm2/mm2) 0.069±0.004c 0.116±0.003a 0.106±0.003b 0.23 0.13 0.12Average cuticle thickness (µm) 2.61±0.19b 3.17±0.13a 3.42±0.12a 0.16 0.24 0.18Average upper epidermal wall thickness (µm) 7.62±0.29b 8.99±0.19a 8.85±0.18a 0.21 0.16 0.15Edge–edge interstomatal distance (µm) 31.7±3.0c 47.9±1.9a 40.3±1.8b 0.04 0.00 –0.19Mid–mid interstomatal distance (µm) 85.0±3.0c 104.2±2.0a 95.7±1.8b 0.06 –0.01 –0.09Stomatal pore length (µm) 53.3±0.7b 56.4±0.5a 55.3±0.4a 0.07 –0.02 –0.02

Photosynthesis and conductancePhotosynthesis (µmol·m–2·s–1) 4.27±0.33a 1.78±0.21b 1.57±0.21b 0.03 –0.55 –2.34Stomatal conductance (µmol·m–2·s–1) 0.13±0.02a 0.07±0.01b 0.07±0.01b 0.43 0.53 –1.14

Note: Values for leaf attributes among stand ages followed by different letters (a > b > c) are significantly different (p ≤ 0.05) based on leastsquares estimates of marginal means andt test of the hypothesisH0: LSM(i) = LSM(j) for all crown position pairs (i,j). Significant differences arenoted only for those cases where the crown position effect was significant (p ≤ 0.05) in the original ANOVA. S145, mature (145-year-old) stand;S55, intermediate (55-year-old) stand; S15, immature (15-year-old) stand. Plasticity index (ϕ) is calculated as described in Statistical analysis.

Table 3. Least-squares means (± SE) and associated crown plasticities, by stand, of the various measurements made on hybridspruce needles.



dle width to thickness ratio (r = 0.39, p < 0.01), needlewidth (r = 0.35,p = 0.02), and mesophyll area (r = 0.33,p =0.02) and negatively correlated with edge–edge interstomataldistance (r = –0.29, p = 0.05). Stomatal conductance andphotosynthesis were positively correlated (r = 0.63,p < 0.01).

Discussion

Needle structure and function varied with crown positionand stand age for these hybrid spruce. Needles from trees inS145 displayed less plasticity than needles from the twoyounger stands. Previous work onPicea abiesby Kull andKoppel (1987) agrees with our result that older trees are lessable to adapt to environmental conditions. Niinemets (1996)has suggested that, as trees age, the relative importance ofdifferent competitive strategies changes, with branching pat-terns becoming more important and foliar plasticity less im-portant. Another explanation is that, regardless of light orwater, physiological and morphological differences persistbetween mature and juvenile trees. For example, Gould(1993) documented the heteroblasty ofPseudopanax crassi-

folius, which results in dramatic morphological differencesbetween the foliage of juvenile and mature trees.

It is important to be aware that our study was undertakenin an introgression zone and we cannot rule out the possibil-ity that some of this variation might be attributed to dif-ferences in hybridization. However, patterns in dimensionalchanges in needle structure with regard to crown positionwere consistently similar across the three stands, and the dif-ferences we have noted are consistent with our interpretationof the stand development patterns in this area. Furthermore,the three stands were located close to each other in the mid-dle of the Nass Skeena Transition area, and topography forall three stands was similar. Therefore, we strongly suggestthat environment overrides any detectable differences in hy-brid variation among trees and among age classes for thisstudy.

MorphologyThick leaves occur in sunny dry habitats and where fo-

liage is long-lived (Givnish 1979). Sun needles should bethicker than shade needles to minimize (via attenuation

© 2000 NRC Canada

312 Can. J. Bot. Vol. 78, 2000

LI LO MI MO UI UO

MorphologyAverage needle area

(cm2)0.14±0.03 0.15±0.02 0.18±0.03 0.19±0.03 0.22±0.03 0.20±0.02

Needle width (mm) 1.10±0.07c 1.15±0.07c 1.27±0.09bc 1.28±0.10bc 1.48±0.09b 1.73±0.07aNeedle thickness (mm) 0.92±0.03c 0.91±0.03c 0.98±0.04bc 1.00±0.05abc 1.05±0.04ab 1.12±0.04aNeedle width to thick-

ness ratio (mm/mm)1.20±0.05c 1.27±0.05bc 1.30±0.07bc 1.29±0.07bc 1.42±0.07ab 1.54±0.06a

Average weight/needle(mg)

7.9±1.2d 8.9±1.0cd 9.8±1.4bcd 11.4±1.1abc 12.4±1.1ab 13.3±0.9a

AnatomyVascular cylinder cross

sectional area (mm2)0.056±0.007c 0.055±0.007c 0.070±0.009c 0.075±0.010bc 0.097±0.009ab 0.117±0.008a

Mesophyll cross-sectional area (mm2)

0.537±0.047c 0.575±0.047c 0.679±0.061bc 0.691±0.064bc 0.817±0.061ab 0.946±0.049a

Vascular cylinder ratio(mm2/mm2)

0.092±0.004bc 0.087±0.004c 0.094±0.005bc 0.097±0.005abc 0.103±0.005ab 0.109±0.004a

Average cuticle thickness(µm)

2.72±0.17b 2.66±0.17b 2.97±0.22ab 3.22±0.23ab 3.40±0.22a 3.42±0.18a

Average upper epidermalwall thickness (µm)

7.49±0.26d 7.62±0.26cd 8.38±0.33bc 8.95±0.36ab 9.32±0.33a 9.16±0.27ab

Mid–mid interstomataldistance (µm)

97.5±2.8 98.7±2.7 91.9±3.6 90.1±3.4 96.8±3.4 94.9±2.6

Edge–edge interstomataldistance (µm)

42.9±2.8 43.3±2.7 37.9±3.5 35.5±3.5 40.5±3.3 39.8±2.6

Stomatal pore length(µm)

54.6±0.7 55.3±0.7 54.0±0.9 54.6±0.8 56.4±0.8 55.1±0.6

Photosynthesis and conductancePhotosynthesis per unit

area (µmol·m–2·s–1)2.68±0.29 2.66±0.29 2.60±0.38 2.66±0.40 2.26±0.38 2.38±0.29

Conductance per unitarea (µmol·m–2·s–1)

0.08±0.02 0.07±0.02 0.09±0.02 0.08±0.02 0.07±0.02 0.13±0.02

Note: Values for leaf attributes among crown positions followed by different letters (a > b > c > d) are significantly different (p ≤ 0.05) based on least-squares estimates of marginal means andt test of the hypothesisH0: LSM(i) = LSM(j) for all crown position pairs (i,j). Significant differences are notedonly for those cases where the crown position effect was significant (p ≤ 0.05) in the original ANOVA. Crown positions are LI, lower inner; LO, lowerouter; MI, middle inner; MO, middle outer; UI, upper inner; UO, upper outer.

Table 4. Least squares means (± SE), by crown position, of the various anatomical, morphological, and physiological measurementsmade on hybrid spruce needles, for all stands.



within the leaf; see Larcher 1995) photoinhibition occurringas a result of intense radiation high in the crown (other fac-tors such as reflectivity may also be important), and indeed,this was true for all three stands. In this way, high levels oflight can be used most efficiently, and little radiation is lostto saturation. Although we did not measure needle orienta-tion, we observed that shade needles tended to be nearlyhorizontal to the branch (to maximize the amount of lightintercepted), whereas sun needles were nearly vertical (tominimize excessive heating), especially in the upper outercanopy position. This is consistent with McMillen andMcClendon (1979).

Thick sun needles have a high carbon cost of construction,but the benefit of reduced photoinhibition and increasedphotosynthate should make this a good investment (Sprugelet al. 1996). On the other hand, the low light levels founddeep in the canopy result in thin needles that are compara-tively inexpensive to produce but can still efficiently harvestthe available light. However, in S55, photosynthesis per unitarea was negatively correlated with needle thickness (r =−050. , p = 0.02). In both S15 (r = 0.23,p = 0.47) and S145(r = –0.13,p = 0.54), the correlation was not significantlydifferent from zero. These results contradict predictionsbased on cost–benefit economics (Givnish 1979, 1988).

Needles from S55 were larger in every dimension thanthose from the other stands. In particular, most sun needles

from S55 featured well-developed lobes emanating from thecentral rib of the vascular cylinder. These are clearly visiblein Fig. 2. The cause of the lobes is unknown (increased lightcollection is one possible explanation), although they resultin a flatter needle more characteristic ofP. sitchensisthan ofthe other spruces (Hitchcock and Cronquist 1976). Similarlobes were found on some sun needles from both S15 andS145, although not all sun needles from these two stands(indeed, not even all sun needles from a given tree) featuredsuch lobes. Since we have not noticed these lobes onP. glaucasun needles from Ontario, this supports our asser-tion that all three stands are composed of Sitka × interiorspruce hybrids.

Xeromorphic leaves have a small ratio of external surfacearea to volume (Fahn 1982). However, because volume in-creases more rapidly (as the third power) than external sur-face area (as the second power) with increased needle size,large needles from S55 also had a comparatively small sur-face area to volume ratio. Thus, on a relative basis (with re-spect to construction cost or photosynthetic capacity), theselarger needles should lose less water to cuticular transpi-ration than smaller needles (Ashton and Berlyn 1994).Drought could thus be extremely important in S55. Largerneedles also have thicker boundary layers, which result ingreater resistances to both water loss and CO2 uptake(Givnish 1979).

© 2000 NRC Canada


Fig. 2. Needle cross-sections from four crown positions (UO, upper outer; MI, middle inner; LI, lower inner (see Fig. 1 for more in-formation)) within the crown of 55-year-old hybrid spruce. Scale bar = 1000µm.



Although sun leaves are typically thicker than shadeleaves, sun leaves are generally smaller (in surface area)than shade leaves of temperate zone species (Hanson 1917;Stover 1944; Givnish 1979; Lichtenthaler 1985; Osborn andTaylor 1990; Sprugel et al. 1996). This held true only inS145. Where moisture is not limiting, tropical leaves mayhave larger sun leaves (Ashton 1995); this is also the casefor paper birch leaves from S15 (Ashton et al. 1998).

AnatomyThe amount of vascular tissue is an indicator of both the

evaporative demands on the needle (Stover 1944) and theamount of photosynthate being produced. Thus, sun foliagehas more conducting tissue than shade foliage (Larcher 1995).Needles from S145 and S55 had more vascular tissue thanthose from S15 and are, hence, more xeromorphic (Fahn1982; Larcher 1995). Drought stress in these two olderstands could be frequent, owing to foliar overheating and thedifficulty of transporting water up a tall trunk (Kozlowski etal. 1991). The four trees studied in S145 averaged over 34 min height. In S15, shorter stems (average of just over 2 m inheight) may make drought stress less a factor.

The rationale for the high degree of mesophyll plasticityis the same as that for sun needle thickness, since the twomeasurements are related. Needles with more mesophyll (the

center of photosynthetic activity) can spread the incomingradiation out over a greater volume of tissue, thereby morecompletely harvesting all the available light and reducinglosses to saturation. Such needles are no doubt costly both toproduce and to maintain but may be essential for increasingthe competitiveness of canopy trees.

The cuticle and upper epidermal wall minimize water lostto evapotranspiration. High levels of light and moisturestress are both positively correlated with cuticle thickness(Martin and Juniper 1970), and sun leaves generally havethicker cuticles than shade leaves (e.g., Hanson 1917; Stover1944; Ashton and Berlyn 1994). The thick cuticles found inS55 and S145 needles are, therefore, further indicators of xe-ric conditions in these stands.

We cannot easily explain the between-stand dichotomy instomatal measurements. Sun leaves generally have smallerbut more abundant stomata (Hanson 1917; Dengler 1979;Lichtenthaler 1985; Osborn and Taylor 1990; Ashton andBerlyn 1992, 1994), but this pattern was not evident in ourresults. In S15, stomata were larger on sun leaves. Li et al.(1996), in a study of paper birch, found that polyploid popu-lations were more drought tolerant and yet had much largerstomata than the less-tolerant diploids. Crown position wasnot clearly related to interstomatal distance in any of thethree stands we studied. In a study of semi-arid grassland

© 2000 NRC Canada

314 Can. J. Bot. Vol. 78, 2000

Fig. 3. Correlations between various anatomical, morphological, and physiological parameters of hybrid spruce needles. Note that theunits for photosynthesis and conductance are micromoles per metre2 per second. Best fit linear regression lines are calculated using or-dinary least squares. Abbreviations used: X-C, cross section; dist., distance.



species, stomatal density was not a useful parameter forassessing drought tolerance (Hardy et al. 1995). However,Ashton and Berlyn (1992) have suggested that low stomatalfrequency is correlated with drought tolerance. Thus, mid-dle-aged trees (such as those in S55, with stomata spaced farapart) could be more drought tolerant than either the youn-gest (S15) or oldest (S145) trees. Even in the moist bio-geoclimatic zone we studied, the competition for water isprobably high: there are many competitors, all closelyspaced, in S55. Drought tolerance could be essential in S55,because of the intense competition in this “stem exclusion”phase (Oliver and Larson 1996).

PhotosynthesisAlthough Givnish (1988) has suggested that photosynthe-

sis should be considered on a per unit needle weight basis(because weight is more readily interpreted as a cost of con-struction), we found this approach no more informative thanthe standard per unit area basis. The high rates of photosyn-thesis and stomatal conductance in the youngest stand are inagreement with previous work on a related species,P. abies(Kull and Koppel 1987).

In S55 and S145, photosynthesis under saturating condi-tions was lower in the higher crown positions (negative plas-ticity), although normally sun leaves have much highermaximal rates of photosynthesis than shade leaves (Larcher1995). Our data may be a manifestation of the fact thatplants have not been able to adapt fully to high levels oflight. Nishio et al. (1993) demonstrated that most carbon isfixed in the middle of a leaf, rather than near the upper leafsurface where light levels are highest. Additionally, a highlevel of light is known to cause chlorophyll loss and photo-inhibition (Larcher 1995). In shade plants, chloroplast move-ment is believed to be a photoprotective strategy (see Park etal. 1996).

Stomatal conductance patterns varied across stands. Con-ductance plasticity was negative in S145 but positive in S55and S15. The latter two stands are therefore adapted to max-imize gas exchange. The negative plasticity of both photo-synthesis and conductance in S145 could be an indicationthat, in this stand, there is less water stress lower in thecrown. Consequently stomata are able to open wider andstay open longer without desiccation occurring.

The high rates of gas exchange in the young stand are inaccordance with our hypothesis that neither light nor wateris limiting in this stand and, hence, that efficiency is not im-portant for competition. Rather, the most competitive treesare the ones that can photosynthesize at the highest rate, re-gardless of nutrient or water-use efficiency.

SummaryA competitive plant must be able to adapt to its abiotic en-

vironment, which changes (water, nutrients, and light, inparticular) with stand development. A plant’s individual or-gans (i.e., needles) must also be able to adapt. Althoughplasticity varies among populations within a species, it is,in the words of Schlichting (1986), “a characteristic of theindividual.” The data presented here give evidence of aphenotypic response to crown position in spruce needle mor-phology, anatomy, and photosynthesis.

However, differences between the three stands suggestthat a tree’s capacity for phenotypic plasticity changes as thetree ages: the oldest stand appeared to be the least plastic. Ingeneral, changes in leaf structure and physiology with treeontogenesis are not well understood (Schlichting 1986; Ash-ton and Berlyn 1994). Light-demanding species usually dis-play the greatest foliar plasticity (Jackson 1967; Ashton andBerlyn 1992). Although seedlings are considered more shadetolerant than mature trees (Kramer and Kozlowski 1979,p. 634), we believe that older hybrid spruce actually loseplasticity with age. The parent types of these hybrids are onthe tolerant end of the shade-tolerance spectrum: Sitka andEngelmann spruce are tolerant and white spruce are interme-diately tolerant (Burns and Honkala 1990). Kobe and Coates(1997) describe the tolerance of the hybrid spruce in ourarea as being intermediate betweenA. lasiocarpa andP. contorta. Selection for this trait may vary within the sev-eral biogeoclimatic zones in which these hybrids are found.The relationships we observed probably do not apply acrossdifferent stand types, for example, the relationships in open-canopy stands like those of ponderosa pine are likely to bequite different from the relationships in closed canopy standslike those of cedar–hemlock.

Understanding how changes in plasticity are related to agemay be important for understanding competition, as wellas stand dynamics. More stands, of both different ages anddifferent forest types, from different biogeoclimatic zones,should be studied so that a more complete picture can be ob-tained. We need to better understand the relationship be-tween plasticity and ontogeny.

Acknowledgements

We are greatly indebted to the British Columbia Ministryof Forests, who provided funding, and especially to KenMitchell for his support and assistance. We also thank ourcolleagues who aided us in the field work: Patrick Baker,Allan Shanfield, Greg Wright, and Phil LePage. JeffBendelius and Chris Elwell made some of the anatomicalmeasurements.

References

Abrams, M.D. 1988. Comparative water relations of three suc-cessional hardwood species in central Wisconsin. Tree Physiol.4: 263–273.

Abrams, M.D., and Knapp, W.K. 1986. Seasonal water relations ofthree gallery forest hardwood species in northeast Kansas. For.Sci. 32: 687–696.

Ashton, P.M.S. 1995. Seedling growth of co-occurringShoreaspe-cies in the simulated light environments of a rain forest. For.Ecol. Manage.72: 1–12.

Ashton, P.M.S., and Berlyn, G.P. 1992. Leaf adaptations of someShoreaspecies to sun and shade. New Phytol.121: 587–596.

Ashton, P.M.S., and Berlyn, G.P. 1994. A comparison of leaf phys-iology and anatomy ofQuercus (section Erythrobalanus—Fagaceae) species in different light environments. Am. J. Bot.81: 589–597.

Ashton, P.M.S., Olander, L.P., Berlyn, G.P., Thadani, R., andCameron, I.R. 1998. Changes in leaf structure in relation to crownposition and tree size ofBetula papyriferawithin fire-origin standsof interior cedar–hemlock. Can. J. Bot.76: 1180–1187.

© 2000 NRC Canada




© 2000 NRC Canada

316 Can. J. Bot. Vol. 78, 2000

Bahari, Z.A., Pallardy, S.G., and Parker, W.C. 1985. Photosynthe-sis, water relations, and drought adaptations in six woody spe-cies of oak–hickory forests in central Missouri. For. Sci.31:557–569.

Berlyn, G.P., and Miksche, J.P. 1976. Botanical microtechniqueand cytochemistry. Iowa State University Press, Ames.

Boardman, N.K. 1977. Comparative photosynthesis of sun andshade plants. Annu. Rev. Plant Physiol.28: 355–377.

Burns, R.M., and Honkala, B.H. 1990. Silvics of North America.Vol. 1. Conifers. Agriculture handbook No. 654. U.S. Depart-ment of Agriculture, Forest Service, Washington, D.C.

Büsgen, M., and Münch, E. 1929. The structure and life of foresttrees. Chapman and Hall, London.

Carpenter, S.B., and Smith, N.D. 1975. A comparative study ofleaf thickness among southern Appalachian hardwoods. Can. J.Bot. 59: 1393–1396.

Chabot, B.F., Jurik, T.W., and Chabot, J.F. 1979. Influence of in-stantaneous and integrated light-flux density on leaf anatomyand photosynthesis. Am. J. Bot.66: 940–945.

Dengler, N.G. 1979. Comparative histological basis of sun andshade leaf dimorphism inHelianthus annuus. Can. J. Bot.58:717–730.

Fahn, A. 1982. Plant anatomy. Pergamon, Oxford.Fan, S., Grossnickle, S.C., and Sutton, B.C.S. 1997. Relationships

between gas exchange adaptation of Sitka × interior spruce geno-types and ribosomal DNA markers. Tree Physiol.17: 115–123.

Fan, S., Grossnickle, S.C., and Sutton, B.C.S. 1999. Relationshipsbetween gas exchange and carbon isotope discrimination ofSitka × interior spruce introgressive genotypes and ribosomalDNA markers. Tree Physiol.19: 689–694.

Fetcher, N., Strain, B.R., and Oberbauer, S.F. 1983. Effects of light re-gime on the growth, leaf morphology and water relations of seed-lings of two species of tropical trees. Oecologia,58: 314–319.

Fitter, A.H., and Hay, R.K.M. 1987. Environmental physiology ofplants. 2nd ed. Academic Press, San Diego.

Givnish, T.J. 1979. On the adaptive significance of leaf form.InTopics in plant population biology.Edited byO.T. Solbrig, S.Jain, G.B. Johnson, and P.H. Raven. Columbia University Press,New York. pp. 375–407.

Givnish, T.J. 1988. Adaptation to sun and shade: a whole plant per-spective. Aust. J. Plant Physiol.15: 63–92.

Gould, K.S. 1993. Leaf heteroblasty inPseudopanax crassifolius:functional significance of leaf morphology and anatomy. Ann.Bot. (London),71: 61–70.

Grossnickle, S.C., Sutton, B.C.S., Fan, S.H., and King, J. 1997. Char-acterization of Sitka × interior spruce hybrids: a biotechnologicalapproach to seedlot determination. For. Chron.73: 357–362.

Hanson, H.C. 1917. Leaf structure as related to environment. Am.J. Bot. 4: 533–560.

Hardy, J.P., Anderson, V.J., and Gardner, J.S. 1995. Stomatal char-acteristics, conductance ratios, and drought-induced leaf modifi-cations of semiarid grassland species. Am. J. Bot.82: 1–7.

Hinckley, T.M., Aslin, R.G., Aubuchon, R.R., Metcalf, C.L., andRoberts, J.E. 1978. Leaf conductance and photosynthesis in fourspecies of the oak–hickory forest type. For. Sci.24: 73–84.

Hitchcock, C.L., and Cronquist, A. 1976. Flora of the PacificNorthwest. University of Washington Press, Seattle.

Jackson, L.W.R. 1967. Effect of shade on leaf structure of decidu-ous species. Ecology,48: 498–499.

Joffre, R., Rambal, S., and Damesin, C. 1999. Functional attributesin Mediterranean-type ecosystems.In Handbook of functionalplant ecology.Edited byF.I. Pugnaire and F. Valladares. MarcelDekker, New York. pp. 347–380.

Kobe, R.K., and Coates, K.D. 1997. Models of sapling mortality as

a function of growth to characterize interspecific variation inshade tolerance of eight tree species of northwestern British Co-lumbia. Can. J. For. Res.27: 227–236.

Kozlowski, T.T., Kramer, P.J., and Pallardy, S.G. 1991. The physio-logical ecology of woody plants. Academic Press, San Diego.

Kramer, P.J., and Kozlowski, T.T. 1979. Physiology of woodyplants. Academic Press, New York.

Kull, O., and Koppel, A. 1987. Net photosynthetic response tolight intensity of shoots from different crown positions and agein Picea abies(L.) Karst. Scand. J. For. Res.2: 157–166.

Larcher, W. 1995. Physiological plant ecology. 3rd ed. Springer–Verlag, Berlin.

Lee, D.W., and Graham, R. 1986. Leaf optical properties of rain-forest sun and extreme shade plants. Am. J. Bot.73: 1100–1108.

Lee, D.W., Bone, R.A., Tarsis, S.L., and Storch, D. 1990. Corre-lates of leaf optical properties in tropical forest sun and ex-treme-shade plants. Am. J. Bot.77: 370–380.

LePage, P. 1995. The structure and development pattern of mixed-species forest stands in the Interior Cedar – Hemlock zone:moist cold subzone of northwestern British Columbia. M.Sc.thesis, Oregon State University, Corvallis.

Li, W.L., Berlyn, G.P., and Ashton, P.M.S. 1996. Polyploids andtheir structural and physiological characteristics relative to waterdeficit in Betula papyrifera(Betulaceae). Am. J. Bot.83: 15–20.

Lichtenthaler, H.K. 1985. Differences in morphology and chemicalcomposition of leaves grown at different light intensities andqualities.In Control of leaf growth.Edited byN.R. Baker, W.J.Davies, and C.K. Ong. Cambridge University Press, Cambridge.pp. 201–221.

Lichtenthaler, H.K., Buschmann, C., Döll, M., Fietz, H.J., Bach,T., Kozel, U., Meier, D., and Rahmsdorf, U. 1981. Photo-synthetic activity, chloroplast ultrastructure and leaf characteris-tics of high-light and low-light plants and of sun and shadeleaves. Photosynth. Res.2: 115–141.

MacKinnon, A., Pojar, J., and Coupé, R. 1992. Plants of northernBritish Columbia. Lone Pine, Vancouver.

Martin, J.T., and Juniper, B.E. 1970. The cuticles of plants. St.Martin’s Press, New York.

McGroddy, M.E., Berlyn, G.P., Ashton, P.M.S., Baker, P., andCameron, I.R. 1995. Changes in needle morphology of westernhemlock (Tsuga heterophyllaSarg.) in relation to crown posi-tion and tree age. Bull. Ecol. Soc. Am.76(Suppl. 2, part 2):200 (Abstr.).

McMillen, G.G., and McClendon, J.H. 1979. Leaf angle: an adap-tive feature of sun and shade leaves. Bot. Gaz.140: 437–442.

Meidinger, D., and Pojar, J. 1991. Ecosystems of British Columbia.Research Branch, British Columbia Ministry of Forests, Victoria.

Meziane, D., and Shipley, B. 1999. Interacting determinants of spe-cific leaf area in 22 herbaceous species: effects of irradiance andnutrient availability. Plant Cell Environ.22: 447–459.

Niinemets, Ü. 1996. Importance of structural features of leaves andcanopy in determining species shade-tolerance in temperate de-ciduous woody taxa. Ph.D. thesis, Institute of Botany and Ecol-ogy, University of Tartu, Tartu, Estonia.

Nishio, J.N., Sun, J., and Vogelmann, T.C. 1993. Carbon fixationgradients across spinach leaves do not follow internal light gra-dients. Plant Cell,5: 953–961.

Olander, L.P., Ashton, P.M.S., Berlyn, G.P., Thadani, R., andCameron, I.R. 1995. Patterns in leaf morphology and physiologyof paper birch (Betula papyriferaMarsh.) with relation to crownposition across a forest chronosequence. Bull. Ecol. Soc. Am.76(Suppl. 2, part 2): 177 (Abstr.).

Oliver, C.D., and Larson, B.C. 1996. Forest stand dynamics. Up-date ed. Wiley, New York.



© 2000 NRC Canada


Osborn, J.M., and Taylor, T.N. 1990. Morphological and ultra-structural studies of plant cuticular membranes. I. Sun and shadeleaves ofQuercus velutina(Fagaceae). Bot. Gaz.151: 465–476.

Parish, R., Coupé, R., and Lloyd, D. 1996. Plants of southern inte-rior British Columbia. Lone Pine, Vancouver.

Park, Y.-I., Chow, W.S., and Anderson, J.M. 1996. Chloroplastmovement in the shade plantTradescantia albiflorahelps protectphotosystem II against light stress. Plant Physiol.111: 867–875.

Poorter, H., and Garnier, E. 1999. Ecological significance of inher-ent variation in relative growth rate and its components.InHandbook of functional plant ecology.Edited byF.I. Pugnaireand F. Valladares. Marcel Dekker, New York. pp. 81–120.

Richards, J.H., and Lee, D.W. 1986. Light effects on leaf morphol-ogy in water hyacinth (Eichornia crassipes). Am. J. Bot. 73:1741–1747.

Schlichting, C.D. 1986. The evolution of phenotypic plasticity inplants. Annu. Rev. Ecol. Syst.17: 667–693.

Smith, W.K., Vogelmann, T.C., DeLucia, E.H., Bell, D.T., andShepherd, K.A. 1997. Leaf form and photosynthesis. BioSci-ence,47: 785–793.

Sprugel, D.G., Brooks, J.R., and Hinckley, T.M. 1996. Effect oflight on shoot geometry and needle morphology inAbiesamabilis. Tree Physiol.16: 91–98.

Stover, E.L. 1944. Varying structure of conifer leaves in differenthabitats. Bot. Gaz.106: 12–25.

Strauss-Debenedetti, S.I., and Bazzaz, F.A. 1991. Plasticity and ac-

climation to light in tropical Moraceae of different successionalpositions. Oecologia,87: 377–387.

Strauss-Debenedetti, S.I., and Berlyn, G.P. 1994. Leaf anatomicalresponses to light in five tropical Moraceae of differentsuccessional status. Am. J. Bot.81: 1582–1591.

Sutton, B.C.S., Flanagan, D.J., and Elkassaby, Y.A. 1991. A simpleand rapid method for estimating representation of species inspruce seedlots using chloroplast DNA restriction fragmentlength polymorphism. Silvae Genet.40: 119–123.

Sutton, B.C.S., Pritchard, S.C., Gawley, J.R., Newton, C.H., andKiss, G.K. 1994. Analysis of Sitka spuce – interior spruceintrogressions in British Columbia using cytoplasmic and nu-clear DNA probes. Can. J. For. Res.24: 278–285.

van Hinsberg, A., and van Tienderen, P. 1997. Variation in growthform in relation to spectral light quality (red/far-red ratio) inPlantago lanceolataL. in sun and shade populations. Oecologia,111: 452–459.

Welles, J. 1986. A portable photosynthesis system.In Advancedagricultural instruments.Edited by W.G. Gensler. MartinusNijhoff, Amsterdam. pp. 231–236.

Wylie, R.B. 1951. Principles of foliar organization shown by sun–shade leaves from ten different species of deciduous dicotyle-donous trees. Am. J. Bot.36: 355–361.

Wylie, R.B. 1954. Leaf organization of some woody dicotyledonsfrom New Zealand. Am. J. Bot.4: 186–192.



foliar plasticity of hybrid spruce in relation to crown ... · a.d. richardson, g.p. berlyn, p.m.s....

Documents