Summary Leaf retention time increases with decreasing ir-radiance, providing an effective way of amortizing the costs of foliage construction over time. To elucidate the physiological mechanisms underlying this dependence, I studied needle life span, morphology, and concentrations of carbon, nitrogen and nonstructural carbohydrates along a gradient of relative irradi-ance in understory trees of Picea abies (L.) Karst. Maximum needle life span was greater in shaded trees than in sun-exposed trees. However, irrespective of irradiance, needles with maxi-mum longevity were situated in the middle rather than the bottom of the canopy, suggesting that needle life span is determined by the irradiance to which needles are exposed during their primary growth. Morphology and chemistry of current-year needles were adapted to prevailing light condi-tions. Current-year needles exposed to high irradiances had greater packing of foliar biomass per unit area than shaded needles, whereas shaded needles maximized foliar area to capture more light. Nitrogen concentrations were higher in shaded needles than in sun-exposed needles. This nitrogen distribution pattern was related to the high nitrogen cost of light interception and was assumed to improve light absorptance per needle mass of shaded needles. In contrast, in both 1- and 2-year-old needles, morphology was independent of prevailing light conditions; however, needle nitrogen concentrations were adjusted toward more effective light interception in 2-year-old foliage but not in 1-year-old foliage, indicating that acclimation of sun-adapted needles to shading takes more than one year. At the same time, needle aging was accompanied by accumulation of nonstructural carbohydrates (NSC), and increasing concen-trations of needle carbon, suggesting a shift in the balance between photosynthesis and photosynthate export. The accu-mulation of NSC and carbon resulted in a dilution of the concentrations of other needle chemicals and explained the decline in needle nitrogen concentrations with increasing age. Thus, although morphological inadequacy to low light avail-abilities may partly be compensated for by modifications in needle chemistry, age-related changes in needle stoichiometric composition progressively lessen the potential for acclimation to low irradiance. A conceptual model, advanced to explain how environmental factors and age-related changes in the activities of needle xylem and phloem transport affect needle longevity, predicted that adaptation of needle morphology to irradiance during the primary growth period largely determines
the fate of needles during subsequent tree growth and develop-ment.
Keywords: aging, carbon balance, needle longevity, needle morphology, nitrogen content, nonstructural carbohydrates, Norway spruce, shade tolerance.
Introduction
Plants acclimate to low irradiances in forest understories by enhancement of light capture as a result of plastic modifica-tions of foliar and canopy architecture and biomass allocation patterns (Givnish 1988, Pearcy and Sims 1994). Although increased assimilate investment in leaf area formation im-proves light interception (Pearcy and Sims 1994), the high carbon cost of this investment seriously limits enhanced foli-age production. Therefore, an increase in leaf longevity, result-ing in greater foliar biomass accumulation (Schulze et al. 1977, Reich et al. 1992), provides an effective way of amortiz-ing the costs of leaf construction over the long-term and en-ables construction of an efficient foliar display for light absorption in low-light environments. A wide range of species, including the herb Fragaria virginiana Duchesne (Jurik et al. 1979), the shrubs Rhododendron maximum L. (Nilsen et al. 1987), Daphniphyllum macropodum var. humile Rosenthal and Pachysandra terminalis Sieb. et Zucc. (Kikuzawa 1988), several tropical woody Piper species (Williams et al. 1989) and the conifers Abies mariesii Mast. (Kohyama 1980), Picea abies (L.) Karst. and Pinus cembra L. (Koike et al. 1994) are able to extend foliar retention time in response to increased habitat shading. Additionally, leaf life span may adjust to differences in irradiance across the canopy (e.g., Pinus con-torta Engelm.; Schoettle 1989). However, apart from pheno-menological observations, little is known about the physiological mechanisms underlying adjustments in foliage life span.
Several studies on the acclimation of current-year needles of evergreen woody species to within-canopy light gradients have recently been described (Niinemets and Kull 1995b, Sprugel et al. 1996, Niinemets 1997a). In conifers, needle thickness generally increases with increasing irradiance (Aussenac 1973, Tucker and Emmingham 1977, Sprugel et al. 1996), resulting in a positive relationship between needle dry mass
Acclimation to low irradiance in
Picea abies:
influences of past and
present light climate on foliage structure and function
ÜLO NIINEMETS
Institute of Ecology, Estonian Academy of Sciences, Riia 181, Tartu EE 2400, Estonia
Received May 2, 1996
per area (LMA) and irradiance. Plastic modifications in needle morphology also appear to underlie the frequently observed strong linear relationship between needle nitrogen per area (Na) and irradiance (Brooks et al. 1996, Niinemets 1997a). Although changes in needle anatomy are associated with in-creases in Na with increasing irradiance in P. abies, needle nitrogen per dry mass (Nm) increases with decreasing irradi-ance in this species (Niinemets 1997a). This preferential allo-cation of nitrogen to needles at low irradiances improves the efficiency of light interception, which is expensive in terms of nitrogen (Evans and Seemann 1989).
Recent studies on the acclimation of older sun foliage to shading have demonstrated that morphological adjustments in leaf structure, which optimize carbon assimilation of current-year needles, are not reversible (Brooks et al. 1994, 1996). As a tree canopy gains size, needles developed in a particular light regime will be increasingly shaded by newly expanding branches and foliage. Therefore, the morphological structure of needles that are ≥ 1-year-old will almost always be more or less unmatched to the prevailing irradiance. Although these architectural insufficiencies may partly be compensated for by changes in needle biochemical machinery----e.g., chlorophyll concentration in older needles responds to increased shading on a time-scale of months (Brooks et al. 1994)----needle accli-mation to shading is likely to require reallocation of nitrogen to thylakoid proteins within the needle as well as enhanced nitrogen concentrations in foliage. However, with increasing age, needle nitrogen content becomes progressively decoupled from shading and the concentration of needle nitrogen de-creases (Gower et al. 1989, Schoettle 1989, Everett and Thran 1992, Helmisaari 1992). The age-dependent decline in needle nitrogen may seriously limit the potential of older sun-adapted needles to acclimate to shade conditions.
To gain insight into the interrelationships between needle life span and the adaptive modifications in needle structure to past and current light conditions, I studied the demography of needles, and their morphology and chemical composition in the evergreen conifer, P. abies. The following hypotheses were tested: (1) needle longevity is related to light conditions; (2) older foliage responds to decreased irradiance in the same manner as young foliage; and (3) needle morphology and life expectancy depend on the irradiance conditions prevailing when the needles were formed.
Material and methods
Characteristics of the study trees
The research was conducted in a planted 33-year-old P. abies stand located on a plateau-like crest of a hill with podzolic and brown pseudopodzolic soils formed on phyllite at Oberwar-mensteinach (49°59′ N, 11°47′ E; elevation about 760 m above sea level), Fichtelgebirge, Germany (Hantschel 1987, Türk 1992) in September--October 1991. The study was carried out in an unthinned part of the stand. The 0.25-ha plot had a density of about 8000 stems ha−1 with tree heights ranging from 1 to 8 m. Additional shading was provided by neighbor-ing P. abies trees, about 100 years old and 25 m high, and
several Fagus sylvatica L. trees, about 40 years old and 15 m high, so that relative irradiance (proportion of open sky) above the tree crowns was rarely greater than 0.3.
Thirty trees that differed in vigor and exposure to sun were selected. Although tree size generally alters foliar structure (e.g., Niinemets and Kull 1995a, 1995b, Niinemets 1997a), the effect was not significant in the current study (P > 0.05), probably because all of the sampled trees were within a height range of only 2.4 to 4.9 m (mean ± SE = 3.39 ± 0.14).
Estimation of irradiance conditions
Variation in relative irradiance (RI) among trees and among sampling locations was quantified by a hemispherical photo-graphic technique (Anderson 1964). The photographs were analyzed according to Nilson and Ross (1979). Relative amount of penetrating diffuse solar radiation (RI) was calcu-lated for uniformally overcast sky conditions from the relative area of canopy gaps measured from hemispheric photographs and corrected for cosine of incidence effects. The RI is an index that varies from 0 to 1. An RI of 1.0 corresponds to the diffuse irradiance of open sky, and an RI of zero corresponds to complete shade with no penetrating canopy gaps (cf. Salmi-nen et al. 1983).
Demographic study
Because maximum foliage age may be a good estimate of mean foliage age (Kohyama 1980, Koike et al. 1994), I studied variability in maximum needle age (NAmax) across tree crowns. Picea abies has a deterministic growth pattern and produces only one flush of needles annually, so that it was possible to follow the growth history of a branch in a given whorl by examining the bud scale scars on branch axes. However, to convert these observations to an absolute time-scale, it was necessary to identify the chronological age of whorls. Thus, the first step in foliage census was to determine the year of formation of every whorl of each tree. From every needled whorl, one to four representative branches were further chosen for determination of NAmax. The NAmax for a given whorl, NAmax(W), was calculated as the mean of the estimates of all sampled branches on the whorl. Only green and securely attached needles were included in estimations of NAmax. Alto-gether over 1000 branches were examined in 30 trees. Current-year needles were assigned an age of 0.
There was a common tendency for all needle-age classes to be present in the upper canopy. The distribution of NAmax(W) leveled off in the mid-canopy and decreased with canopy depth thereafter (see Figure 1). The distribution of maximum needle age was fitted with the Weibull function (Mori and Hagihara 1991, Niinemets 1996):
To obtain an estimate of maximum needle age for the tree, the distribution maximum (NAmax′(W)= 0, solved for W, see also Figure 1) was derived as:
Wmax=(Wc + 1)−α
1−
1
m
1
m
. (2)
Substitution of Wmax into Equation 1 gave NAmax for the whole tree. Maximum needle age calculated by Equations 1 and 2 was correlated with highest needle age for all sampled branches per tree (r2 = 0.68, P < 0.001).
An estimate of relative irradiance (RI) for each tree was obtained from RI measurements just above the canopy (RIa) and below the canopy at distances of 0.1 m and 1.2 m from the stem in both the north and south compass directions (RIb0.1 and RIb1.2, respectively). The RIb0.1 (0.136 ± 0.011) was signifi-cantly lower (paired t-test, P < 0.01) than either RIa (0.169 ± 0.015) or RIb1.2 (0.157 ± 0.011); however, RIa and RIb1.2 were not statistically different (P > 0.5). Relative irradiance for the tree was computed as [RIa +(RIb0.1 +RIb1.2)/2]/2.
Needle sampling for examination of foliage structure and chemical composition
Foliar samples of current-year and 1- and 2-year-old needles, consisting of 10--20 needles per age-class, were taken from the southern aspect of terminal branch positions from the fourth whorl from the top. To allow needle expansion to reach com-pletion and needle mineral content to stabilize (Weikert et al. 1989, author’s unpublished observations), sampling was con-ducted between 1500 and 1600 h on cloudy days during 3 weeks in October 1991. Hemispheric photographs were taken immediately after foliage sampling. To get a more exten-sive gradient in RI, needle samples were in several cases collected also across the crown from different whorls. There were no consistent effects of whorl age on foliage morphology and chemical composition in current-year needles when
differ-ences in irradiance between whorls were accounted for (one-way ANCOVA), but W was occasionally more strongly corre-lated with foliar parameters than RI in older needles.
Foliage structure and chemical composition
Total needle surface area (TLA) and needle volume (V) were calculated from measurements of needle thickness (D1), width (D2) and length (L) according to Niinemets and Kull (1995b). Needles were weighed after drying the samples at 70 °C for at least 48 h, and needle dry mass per TLA (LMA) was calcu-lated. The LMA was further expressed as the product of V/ TLA and needle density (ND, dry mass per volume) (Witkowski and Lamont 1991, Niinemets and Kull 1995b, Niinemets 1997a):
LMA=TLAV ND. (3)
Foliar carbon (Cm) and nitrogen (Nm) concentrations were determined with a C/N analyzer (Model 1500, Carlo Erba, Italy). Concentrations of ethanol-soluble carbohydrates (ESC) and starch (SC) were determined by anthrone reaction as described previously (Niinemets 1995, 1997a).
Parameters for nonstructural carbohydrate-free dry mass
There is evidence that the size of the foliar nonstructural carbohydrate (NSC, ethanol-soluble carbohydrates plus starch) pool varies with needle age in P. abies (Einig and Hampp 1990), resulting in a parallel variation in foliar mass per area (e.g., Chatterton et al. 1972) and the concentrations of other leaf components (Plhák 1984). Therefore, LMA, ND and the concentrations of leaf chemicals were also expressed on the basis of NSC-free dry mass (Wong 1990). Needle dry mass per area and ND were multiplied by 1− TNC (where TNC= ESC + SC, expressed here as the proportion of dry matter) to obtain LMA and ND on the basis of NSC-free dry mass (LMAc and NDc, respectively). Leaf nitrogen content expresssed on an NSC-free dry mass (Nmc) was obtained by dividing Nm by 1− TNC. Foliar carbon content expressed on the basis of NSC-free dry mass was calculated as:
C mc =Cm − NCC
1 − TNC , (4)
where NCC is the proportion of leaf carbon in NSC.
Statistical analysis
Linear correlation and regression techniques were used to analyze the relationships between needle morphological and structural parameters as well as the dependencies of these parameters on irradiance within an age-class. Differences be-tween age-classes were separated by the Bonferroni test after one-way analysis of variance. If RI significantly affected the dependent variable, it was included as a covariate in the statis-tical model. All effects were considered significant at P < 0.05 (Wilkinson 1990).
Results
Demographic study
The oldest needles were generally located in the middle of the living canopy (Figure 1), but maximum needle age was greater in shaded trees than in sun-exposed trees (Figure 2). Light availability also influenced maximum longevity of needles in different whorls. The age distribution maximum (Wmax, Equa-tion 2) tended to shift upward with increasing irradiance (r2 = 0.27), indicating that progressively younger annual shoot segments were defoliated at greater RIs (cf. Figure 1). The age
distribution maximum, Wmax, and NAmax were also correlated (r = −0.76, P < 0.001).
Relationships of needle morphology and chemical composition with irradiance in current-year needles
Because ND was independent of RI (P > 0.3), the increase in LMA with increasing RI was attributable to increases in needle width (P < 0.001) and volume to total area ratio (Figure 3).
The concentrations of ethanol-soluble carbohydrates and starch were weakly correlated with relative irradiance (r2 = 0.09, P < 0.01 for ESC and r2 = 0.06, P < 0.05 for SC, n = 105). These positive relations were more significant with LMA (r2 = 0.52, P < 0.001 for ESC and r2 = 0.08, P < 0.01 for SC). Needle nitrogen concentration was negatively correlated with RI (r2 = 0.12, P < 0.001) and LMA (Figure 4). Despite this, nitrogen content expressed on a TLA basis (Na) was positively correlated with both RI (r2 = 0.19, P < 0.01) and LMA (Figure 4), signifying the overwhelming importance of modifications in foliar morphology (Na = NmLMA) for the adjustment of foliar resources toward efficient use of different light microclimatic conditions. Expression of the variables on an NSC-free dry mass did not alter the conclusions qualita-tively (data not shown).
Dependencies of needle morphology and chemical composition on irradiance in 1- and 2-year-old foliage Needle age affected the relationships between morphological variables and irradiance qualitatively. In contrast to current-year foliage, neither needle width (P > 0.3), V/TLA (Figure 3) Figure 2. Dependence of maximum needle age per tree (NAmax) on
relative irradiance. Relative irradiance was calculated from estimates of relative irradiance above and below the tree canopy.
Figure 3. Relationships between needle dry mass per total area (LMA), volume to total area ratio (V/TLA) and relative irradiance. The V/TLA ratio describes the needle morphology-related variation in LMA (Equation 3). s = current-year needles, d = 1-year-old needles, and m =
nor LMA (Figure 3) was related to irradiance in 1- and 2-year-old needles. Despite the independence of RI and LMA in 1-and 2-year-old needles, whorl number 1-and LMA were posi-tively correlated in older needles (r2 = 0.25, P < 0.001 for 1-year-old and r2 = 0.15, P < 0.02 for 2-year-old needles), indicating that previous rather than actual light climate deter-mined LMA in older needles. Needle density was not depend-ent on irradiance in old foliage (P > 0.7).
The concentrations of ethanol-soluble carbohydrates and starch were not correlated with irradiance in 1-year-old need-les (P > 0.3 for both), but they were positively influenced by RI in 2-year-old needles (r2 = 0.24, P < 0.005 for ESC and r2 = 0.21, P < 0.005 for SC). Both ESC and SC were positively correlated with LMA in 1-year-old needles (r2 = 0.21, P < 0.01 for ESC and r2 = 0.10, P < 0.05 for SC), but not in 2-year-old needles (P > 0.05). In 1-year-old needles, Nm was negatively related to LMA (Figure 4) and independent of RI (P > 0.4), but it was dependent on RI (Figure 5) and independent of LMA in 2-year-old needles (Figure 4). The conclusions were not quali-tatively altered by using the parameters for NSC-free dry mass.
Age effects on needle structure
Needle dry mass per TLA increased with increasing needle age (Table 1). This was attributable to greater needle density, needle thickness and needle width in older needles (Table 1). The increase in NSC content with needle age paralleled the increase in ND. Therefore, when ND was calculated per NSC-free dry mass (NDc), it did not differ between the age-classes (Table 1). However, as a result of contrasting needle
morphol-ogy, both LMA and LMAc were greater in older needles than in current-year needles (Table 1).
Analysis of the increase in NSC with needle age indicated that ESC varied little between age-classes, whereas starch concentration increased with needle age in parallel with the increase in NSC (Table 1). Although NSC did not only alter needle density, it diluted other leaf compounds, and most of the decline in needle nitrogen concentration during aging was atttributable to the accumulation of NSC (Table 1). Inasmuch as needle carbon concentration increased with increasing age, a larger fraction of carbon-rich compounds in older needles may also have diluted other leaf chemicals, in particular leaf nitrogen (Table 1). For the whole material, Nm was negatively Figure 4. Correlations between LMA and amounts of needle nitrogen per needle dry mass (Nm), and per total surface area (Na). Coefficients for the hyperbolic relationships between Nm and LMA were obtained from the linear relationships between Na and LMA (Na=aLMA +b, Nm=b/LMA +a). The intercept denoted by the asterisk did not differ significantly from zero.
correlated with both carbon (r2 = 0.15, P < 0.001) and non-structural carbohydrate concentrations (r2 = 0.13, P < 0.001).
Discussion
Changes in needle structure, composition and life span in relation to irradiance
The finding of increased needle longevity in P. abies growing in environments with low irradiance (Figure 2) is in agreement with other studies showing that stressful environments, e.g., high altitude (Schoettle 1990, Reich et al. 1996) or low-light habitats, result in an increase in leaf life span. Chabot and Hicks (1982) hypothesized that leaf longevity should adjust to amortize the costs of leaf construction. Although Williams et al. (1989) rejected this hypothesis, they demonstrated a positive relationship between leaf life span and its payback time. Inasmuch as lower carbon gain under limited irradiances results in a longer payback time, leaf life span should be greater at low irradiance. Although this relationship appears to hold in P. abies (Figure 2), the correlation between longevity and payback time alone provides no insight into why sun-adapted needles abscise earlier than shade-sun-adapted needles. The explanation may lie in the light acclimation process itself. There are two components of the needle’s response to its light environment: acclimation to irradiance during its primary growth, and its ability to adjust as the light environment changes. Many adaptations of leaf structure and function to irradiance are irreversible (Brooks et al. 1994, 1996). Further-more, the light environment for needles that developed in a
high-light regime changes more quickly than that for needles that developed in shade. Therefore, the lower life expectancy of sun needles compared with shade needles may largely depend on their low potential to adjust to shade and on the time constraints imposed on the acclimation process.
Generally, needle thickness increases with increasing irradi-ance in conifers (Aussenac 1973, Tucker and Emmingham 1977, Sprugel et al. 1996, Niinemets 1997a); however, in P. abies, needle width is considerably more responsive to irra-diance than needle thickness (Aussenac 1973, Greis and Kel-lomäki 1981, Niinemets and Kull 1995b, Niinemets 1997a). Because needle density is not correlated with irradiance in P. abies (Niinemets and Kull 1995b, Niinemets 1997a), the light-induced changes in needle width underlie the correlation between V/TLA and LMA in current-year needles (Figure 3, Equation 3). Adaptation to low irradiance seems also to involve modifications in leaf chemical composition as indicated by the increases in foliar nitrogen concentrations with decreasing irradiance (Figures 4 and 5, Niinemets 1997a).
Because the nitrogen requirement increases disproportion-ately with increasing leaf absorptance (Evans and Seemann 1989), it has been hypothesized that this pattern of nitrogen distribution represents an investment strategy inherent to shade-tolerant species (Kull and Niinemets 1993, Niinemets 1995, 1997a, 1997b). Because chlorophyll concentrations (Heinze and Fiedler 1976) and photosynthetic capacity per needle dry mass (Schulze et al. 1977) increase with decreasing relative irradiances in P. abies, I postulate that the extra nitro-gen is invested in compounds that improve light interception of incident irradiance as well as in needle compounds that Table 1. Needle chemical composition and morphology in relation to needle age1.
Variable Definition (unit) Mean ± SE
Current-year needles 1-Year-old needles 2-Year-old needles
Cm Carbon per total dry mass (%) 47.79 ± 0.14 a 48.17 ± 0.15 ab 48.56 ± 0.16 b C mc Carbon per NSC2-free dry mass (%) 48.57 ± 0.19 a 49.04 ± 0.21 a 49.75 ± 0.19 b D1 Thickness (mm) 0.948 ± 0.022 a 0.995 ± 0.019 a 1.080 ± 0.016 b D2 Width (mm) 0.446 ± 0.014 a 0.480 ± 0.011 a 0.570 ± 0.013 b ESC Ethanol-soluble carbohydrates per 4.86 ± 0.24 a 5.53 ± 0.27 b 6.17 ± 0.29 b
total dry mass (%)
L Length (mm) 13.16 ± 0.24 a 13.34 ± 0.31 a 13.22 ± 0.30 a
LMA Total dry mass per TLA (g m−2) 36.3 ± 0.9 a 41.2 ± 0.8 b 48.3 ± 1.1 c LMAc NSC-free dry mass per TLA (g m−2) 33.3 ± 0.8 a 36.1 ± 0.7 a 40.5 ± 0.8 b Na Nitrogen per NSC-free dry mass (g m−2) 0.705 ± 0.018 a 0.704 ± 0.013 a 0.817 ± 0.019 b Nm Nitrogen per total dry mass (%) 1.866 ± 0.030 a 1.773 ± 0.023 b 1.726 ± 0.026 b Nmc Nitrogen per NSC-free dry mass (%) 2.105 ± 0.034 a 2.037 ± 0.033 a 2.029 ± 0.034 a ND Density (total dry mass per V, g cm−3) 0.365 ± 0.005 a 0.377 ± 0.005 a 0.385 ± 0.006 b NDc NSC-free dry mass per V (g cm−3) 0.324 ± 0.005 a 0.328 ± 0.004 a 0.322 ± 0.005 a SC Starch per total dry mass (%) 6.72 ± 0.36 a 8.10 ± 0.23 b 10.20 ± 0.33 c TNC ESC + SC (%) 11.61 ± 0.48 a 13.82 ± 0.42 b 16.30 ± 0.46 c TLA Total area (mm2) 27.9 ± 1.0 a 29.7 ± 1.0 ab 32.5 ± 1.0 b V Volume (mm3) 2.88 ± 0.19 a 3.26 ± 0.15 a 4.13 ± 0.18 b V/TLA V to TLA ratio (mm3 mm−2) 0.1002 ± 0.0027 a 0.1077 ± 0.0022 a 0.1255 ± 0.0026 b 1 Means designated by the same letter are not significantly different (P > 0.05). Data were subjected to ANOVA and the means separated by the
improve the utilization of intercepted light. High concentra-tions of NSC concentration at high irradiances is compatible with earlier results for this species (Einig and Hampp 1990, Niinemets 1997a) and other conifers (Gholz and Cropper 1991). A positive relationship between NSC and irradiance probably results from increased daily photosynthetic produc-tion at high irradiances (Hendrix and Huber 1986, Niinemets 1995).
Changes in needle morphology with aging
Increasing LMA with increasing needle age (Tucker and Em-mingham 1977, Benecke 1979, Borghetti et al. 1986, Ohmart and Thomas 1986, Oren et al. 1986, Gower et al. 1989, Gil-more et al. 1995) is a central modification in needle structure. Expression of LMA as the product of needle density and V/ TLA ratio (Equation 3) appears useful for a mechanistic understanding of the causes and effects of the age-related changes in LMA. In most cases, changes in needle density (Beets 1977, Tucker and Emmingham 1977, Shelton and Switzer 1984, Whitehead et al. 1994) underlie the increase in LMA with aging in conifers. However, in the current study, changes in needle width and thickness (Table 1) also contrib-uted toward the increase in LMA with increasing needle age. Because LMA of older needles was significantly larger in whorls that were previously exposed to higher irradiance, the lack of correlation between LMA and irradiance in older needles (Figure 3) may be associated with the adaptation of needle morphology to preceding rather than to prevailing light climate. Similarly, LMA in Pinus radiata D. Don was inde-pendent of depth in the canopy for needles ≥ 2 years old, whereas it was related to canopy position in younger needles (Ohmart and Thomas 1986). However, in many conifers, e.g., Pinus radiata (Benecke 1979) and Pinus sylvestris L. (van Hees and Bartelink 1993), LMA of older needles is related to depth in the canopy. By the same token, LMA of older needles is dependent on irradiance in Pseudotsuga menziesii (Mirb.) Franco (Del Rio and Berg 1979) and P. abies (Koppel and Frey 1984). These data suggest either that the location of needles in the crown is a poor estimate of actual light climate, or that the lack of correlation between needle structure and irradiance in the present study (Figure 3) was a consequence of the limited range of irradiances examined.
Because the morphology of needles acclimated to high-irra-diances has developed toward high photosynthesizing mass per unit surface area in current-year needles (Figure 3; Niine-mets and Kull 1995b, Niinemets 1997a) rather than to high efficiency of light interception, the structure of sun-adapted older needles becomes increasingly less advantageous as they become shaded. Moreover, needle thickness and width in-crease with needle age (Table 1), probably as a result of secondary phloem growth (Ewers 1982, Gilmore et al. 1995). Because secondary growth does not affect needle length, it leads to further reductions in the effectiveness of light intercep-tion, which is inversely related to needle thickness to length ratio (Takenaka 1994).
Age effects on nitrogen partitioning
The priority of increasing efficiency of light interception gains in relevance with aging and shading. Despite little flexibility in adjusting sun-adapted needle architecture to shade condi-tions, the stoichiometry of needle chemicals can be modified fairly rapidly in response to low irradiances (Brooks et al. 1994, Brooks et al. 1996). Thus, needle chlorophyll concentra-tions increase with increasing needle age (Linder 1972, Brooks et al. 1994, Mandre and Tuulmets 1995), resulting in enhanced light utilization efficiency (Teskey et al. 1984). Similarly, the observed alterations in needle nitrogen partitioning with age demonstrate (Figures 4 and 5, and Table 1) the potential for acclimation of needle function toward lower quantum avail-abilities. Nitrogen allocation within leaves can be related to lowered irradiance within a season (Brooks et al. 1994), how-ever, as is suggested by the close association between nitrogen in 1-year-old needles and previous light climate (cf. Figure 4), the redistribution of foliar resources among needles occurs over longer periods.
Aging and the stoichiometry of needle carbon constituents
Increases in nonstructural carbohydrates with increasing need-le age (Tabneed-le 1) have been observed in P. abies (Einig and Hampp 1990, Egger et al. 1996) and Pinus elliottii Engelm. (Gholz and Cropper 1991). Generally, a balance exists be-tween carbon acquisition in photosynthesis and carbohydrate export from leaves (Hendrix and Huber 1986). Thus, increas-ing concentrations of NSC with needle agincreas-ing (Table 1) may be related to decreasing activities of enzymes responsible for carbon translocation (e.g., sucrose phosphate synthase; Hen-drix and Huber 1986, Egger et al. 1996), or to a decreasing ability of the phloem to export photosynthates from the need-les. There is evidence that the increment of secondary growth of phloem declines as needles age (Ewers 1982).
Interactive effects of current and past light climate on needle function
Age-related changes in the concentration of needle chemicals, which are responsible for light capture and photosynthetic function, indicate that aging may be a crucial factor in setting constraints on subsequent shade acclimation. Besides lesions that accumulate over time (cf. Sutinen 1987), availability of needle chemicals for acclimation to shade also decreases with aging. In this respect, it may be the initial adaptation state, rather than needle chronological age, that is the primary factor influencing needle acclimation and longevity.
Preferential abscission of the most shaded needles may improve the capacity for light interception of the remaining needles, especially in sun shoots that not only have more needles per unit stem length than shade shoots (Niinemets and Kull 1995a) but also have higher self-shading within the shoot. The observed age-related changes in shoot morphology are in good agreement with the hypothesis that needles in unfavor-able positions in terms of light interception become selectively abscised (Norman and Jarvis 1974, Oker-Blom and Smolander 1988).
Acknowledgments
The study was supported by a scholarship from the German Academic Exchange Service (DAAD) and by the German Federal Minister of Research and Technology (BMFT) under Grant BEO 51-0339476A (BITÖK, University of Bayreuth). I am grateful to Riho Kôiveer (Department of Plant Biochemistry, University of Tartu, Estonia) for providing facilities for carbohydrate analysis.
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