落叶阔叶林内的光合作用和气孔传导率特征(英文)

本文根据Wang和BMdocchi(1989)最近提出的冠层辐射模型,进一步给出了一个模拟冠层光合作用速率和气孔传导率的模式.模式将冠层中每一层的叶面积分为向光叶、半影叶、和全遮荫叶三种,并分别计算其光合作用速率和气孔传导率。计算得到的光合速率廓线表明,在落叶阔叶林内,冠层下部的叶片常处于光照不足状态;半影效应使得透过林冠达于底部的辐射量增大,这对于林下植物的光合作用是有利的。 模式计算值与实测值之间的微弱差别应归因于纯辐射模型无法考虑湍流输送机制造成的CO_2传输和冠层底部耐荫性叶对于低光照的适应能力。     

Stomatal oscillations in adult forest trees in natural environment

Conductance of abaxial epidermis was measured in leaves ofAcer campeatre, Carpinus betulus, Quercus cerris andQ. petraea sample trees growing in an oak-hornbeam forest. Measurements were performed on several summer days during morning and midday hours. Stomata tended to oscillations only inQ. cerris. The occurrence of stomatal oscillations was controlled by environmental conditions,e.g. weather.     

Scaling carbon dioxide and water vapour exchange from leaf to canopy in a deciduous forest. I. Leaf model parametrization

In order to parametrize a leaf submodel of a canopy level gas-exchange model, a series of photosynthesis and stomatal conductance measurements were made on leaves of white oak (Quercus alba L.) and red maple (Acer rubrum L.) in a mature deciduous forest near Oak Ridge, TN. Gas-exchange characteristics of sun leaves growing at the top of a 30 m canopy and of shade leaves growing at a depth of 3–4 m from the top of the canopy were determined. Measured rates of net photosynthesis at a leaf temperature of 30°C and saturating photosynthetic photon flux density, expressed on a leaf area basis, were significantly lower (P = 0.01; n = 8) in shade leaves (7.9μmol m^?2 s^?1) than in sun leaves (11–5μmol m^?2 s^?1). Specific leaf area increased significantly with depth in the canopy, and when photosynthesis rates were expressed on a dry mass basis, they were not significantly different for shade and sun leaves. The percentage leaf nitrogen did not vary significantly with height in the canopy; thus, rates expressed on a per unit nitrogen basis were also not significantly different in shade and sun leaves. A widely used model integrating photosynthesis and stomatal conductance was parametrized independently for sun and shade leaves, enabling us to model successfully diurnal variations in photosynthesis and evapotranspiration of both classes of leaves. Key photosynthesis model parameters were found to scale with leaf nitrogen levels. The leaf model parametrizations were then incorporated into a canopy-scale gas-exchange model that is discussed and tested in a companion paper (Baldocchi & Harley 1995, Plant, Cell and Environment 18, 1157–1173).     

Leaf diffusion resistance pattern in an oak-hornbeam forest

Four tree, five shrub, and ten herbaceous species growing naturally in an oak-hornbeam forest were used for simultaneous study of the leaf diffusive resistances in the course of several summer days. Absolute minima of the stomatal resistance in the sun tree, the shrub, and the herbaceous species leaves were 1.7 to 6.2 s cm^-1, 6.1 to 10.8 s cm^-1, and 4.8 to 9.7 (17.3 inConvallaria majalis leaves) s cm^-1, respectively. Minimum daily leaf resistances in the course of a day were noted earlier in the morning in sun leaves of large trees than in shade leaves of other species. Stomata were fully opened later in the morning and they began to close sooner in the afternoon in usual shade leaves of the plants in the interior of the forest canopy than those in sun leaves in active surfaces of the canopy (tops of tree crowns). The relatively large differences in leaf resistances found among investigated species may be explained by differences in leaf anatomy (stomata frequency and size) and in ambient leaf or plant environment caused by leaf (plant) position in different vertical layers.     

Differential leaf traits of a neotropical tree Cariniana legalis (Mart.) Kuntze (Lecythidaceae): comparing saplings and emergent trees

Cariniana legalis is an emergent tree that reaches the upper canopy in Brazilian Semideciduous Forest. Spatial contrasts in microclimatic conditions between the upper canopy and understorey in a forest may affect morpho-physiological leaf traits. In order to test the hypothesis that the upper canopy is more stressful to leaves than a gap environment we compared emergent trees of C. legalis, 28–29 m in height to gap saplings, 6–9 m in height, for the following parameters: leaf area, leaf mass area (LMA the dry weight:leaf area ratio), leaf thickness, leaf anatomical parameters, stomata conductance, and chlorophyll a fluorescence. Leaves from emergent trees had smaller leaf areas but greater LMA compared to saplings. Leaf thickness, palisade layer thickness, and stomatal density were higher for emergent trees than for saplings. The opposite pattern was observed for spongy layer thickness and spongy/palisade ratio. Stomatal conductance was also higher for emergent tree leaves than for sapling leaves, but the magnitude of depression on stomatal conductance near midday was more pronounced in emergent trees. The potential quantum yield of photosystem II, as determined by the F _v/F _m ratio was lower for leaves from saplings. The lower values of stomatal conductance, indicating restriction in CO₂ diffusion into the mesophyll can be related to higher photoinhibition observed in the saplings. Leaves from emergent trees and saplings exhibited similar values for apparent electron transport rates and non-photochemical quenching. Our results suggest that changes in leaf traits could be associated to dry conditions at the upper canopy as well as to the ontogenetic transition between sapling/emergent tree life stages.     

Is distribution of hydraulic constraints within tree crowns reflected in photosynthetic water-use efficiency? An example of <Emphasis Type="Italic">Betula pendula</Emphasis>

Spatial and daily variation in photosynthetic water-use efficiency was examined in leaves of Betula pendula Roth with respect to distribution of hydraulic conductance within the crown, morphological properties of stomata, and water availability. Intrinsic water-use efficiency (A _n/g _s) was determined from gas-exchange measurements performed both in situ in a natural forest stand and on detached shoots under laboratory conditions. In intact foliage, sun leaves demonstrated significantly higher (P < 0.001) A _n/g _s than shade leaves, as photosynthesis in the lower canopy was chronically limited by low light availability. However, this difference reversed in the mid-day period under sufficient irradiance (I > 800 μmol m⁻² s⁻¹): A _n/g _s averaged 28.8 and 24.0 μmol mol⁻¹ (P < 0.01) for shade and sun leaves, respectively. This last finding coincided with the data obtained in laboratory conditions: under equivalent leaf water supply and light, A _n/g _s in shade foliage was greater (P < 0.001) than in sun foliage across a wide range of irradiance. Thus, shade foliage of B. pendula is characterized by inherently higher A _n/g _s than sun foliage, associated with more conservative stomatal behavior, and lower soil-to-leaf (K _T) and leaf hydraulic conductances. Under unlimited light conditions, a within-crown trade-off between A _n/g _s and K _T becomes apparent. Differences in stomatal conductance between the detached shoots from sunlit and shaded canopy layers were largely attributable to the variation in stomatal morphology; significant relationships were established with characteristics combining stomatal size and density (relative stomatal surface, stomatal pore area index). Stomatal morphology is very likely involved in long-term adjustment of photosynthetic WUE.     

不同生境栓皮栎天然更新幼苗植冠构型分析

栓皮栎存在于秦岭南坡的多种林分中,生活在不同生境中的个体往往形成不同的树冠形态和构型特征。为了说明不同生境条件下栓皮栎幼苗的植冠构型变化,采用典型抽样法,对秦岭南坡3种生境中(林冠下、林隙、林缘)的栓皮栎天然更新幼苗的侧枝、叶片特征及其空间分布进行了调查分析,结果表明:3种不同生境中栓皮栎幼苗植冠形态发生了明显的可塑性变化,(1)林冠下的幼苗明显为开阔型树冠,林隙和林缘处的幼苗树冠相对紧密;(2)幼苗的1级侧枝密度与分枝角度在3种生境下均差异显著(P0.05);从Ⅰ到Ⅳ层,林冠下幼苗的分枝角度在冠层内变化幅度不到5°,而林缘处幼苗的分枝角度变化高达40°;发生5个以上1级侧枝的概率以林冠下最大,为0.6;(3)从林缘、林隙到林冠下,幼苗的叶长、叶宽、单叶面积、叶面积指数逐渐降低,数量叶密度和比叶面积则逐渐增大,与其它两种生境相比,林冠下幼苗的叶片逐渐向树冠上层集中,且以更高序的侧枝为主要着生枝条;(4)林隙中栓皮栎幼苗的树高、地径明显优于林缘和林冠下,缩短了苗木进入主林层的时间,林隙对栓皮栎种群更新有利。在今后栓皮栎林的经营中,可以通过适当间伐来增加林隙数量,为森林更新和结构的优化的提供有利条件。     

Effect of canopy gap light environment on evaporative load and stomatal conductance in the temperate forest understory herb Aster macrophyllus (Asteraceae)

Aster macrophyllus, a temperate forest understory species of the northeastern United States, inhabits a broad range of light habitats. Plants receiving several minutes of direct sun in canopy gap and forest edge habitats occasionally wilt, a response indicative of water stress. We compared two alterative scenarios for patterns of evaporative load and stomatal conductance for plants in large (0.15 ha) tree canopy gaps and small (3 m²) herbaceous subcanopy gaps: 1) evaporative loads are typically moderate and stomatal conductance is largely governed by light intensity; or 2) evaporative loads are often substantial, mandating stomatal closure to prevent excessive transpiration. In all cases evaporative loads were elevated by light intensity above 25% of full sun. This was accompanied by substantial stomatal closure. Transitions from low to moderate light intensity (<13% full sun) caused little increase in leaf evaporative load, and stimulated increases in stomatal conductance. Very brief periods of high light also stimulated stomatal opening. Light environments in the small herbaceous subcanopy gaps differ greatly in their patterns of evaporative load from day to day.     

Photosynthesis in an Australian rainforest tree,Argyrodendron peralatum,during the rapid development and relief of water deficits in the dry season

Summary Rates of apparent photosynthesis were measured in situ at five positions between the upper crown and a lower branch of a 34 m tall Argyrodendron peralatum (F.M. Bailey) H.L. Edlin ex I.H. Boas tree, and on an understorey sapling of the same species growing in a northern Australian rainforest. At the end of the dry season, rapid reductions in photosynthetic rates occurred in the upper crown within three days after a rain event, but changes in the lower crown and the sapling were less marked. Complete recovery of photosynthesis followed a second rain event. At high photon flux densities, stomatal conductance to water vapour decreased in a curvilinear fashion as the vapour pressure difference between leaf and air increased. Apparent photosynthesis was linearly related to stomatal conductance on the first clear day after each rain event, but there was no relationship between these parameters at the end of a brief natural drying cycle. Under conditions of adequate water supply, stomatal conductances of both upper crown and understorey leaves increased linearly with increasing photon flux density up to about 300 mol m^-2 s^-1. During water deficits, stomatal conductances in leaves from the understorey increased much more rapidly at very low photon flux densities than did conductances in leaves from the upper canopy.     

Seasonal variation in stomatal conductance and physiological factors observed in a secondary warm-temperate forest

This study quantified stomatal conductance in a CO₂-fertilized warm-temperate forest. The study considered five items: (1) the characteristics of the diurnal and seasonal variation, (2) simultaneous measurements of canopy-scale fluxes of heat and CO₂ and the normalized difference vegetation index (NDVI), (3) the stomatal conductance of sunlit and shaded leaves, (4) a stomatal conductance model, and (5) the effects of leaf age on stomatal conductance. Sampled plants included evergreen and deciduous species. Stomatal conductance, SPAD, and leaf nitrogen content were measured between March and December 2001. Sunlit leaves had the largest diurnal and seasonal variation in conductance in terms of both magnitude and variability. In contrast, shaded leaves had only low conductance and slight variation. Stomatal conductance increased sharply in new shooting leaves of Quercus serrata until reaching a maximum 2 months after full leaf expansion. The seasonal changes in the canopy-scale heat and CO₂ fluxes were similar to the change in the canopy-scale NDVI of the upper-canopy plants. These seasonal changes were correlated with the leaf-level H₂O/CO₂ exchanges of upper-canopy plants, although these did not represent the stomatal conductance in fall completely. Seasonal variations in the leaf nitrogen content and SPAD were similar, except leaf foliation, until day 130 of the year, when the behaviors were completely the opposite. A Jarvis-type model was used to estimate the stomatal conductance. We modified it to include SPAD as a measure of leaf age. The seasonal variation in stomatal conductance was not as sensitive to SPAD, although estimates for evergreen species showed improvements.     

Effects of drought on mesophyll conductance and photosynthetic limitations at different tree canopy layers

In recent years, many studies have focused on the limiting role of mesophyll conductance (g_m) to photosynthesis (A_n) under water stress, but no studies have examined the effect of drought on g_m through the forest canopy. We investigated limitations to A_n on leaves at different heights in a mixed adult stand of sessile oak (Quercus petraea) and beech (Fagus sylvatica) trees during a moderately dry summer. Moderate drought decreased A_n of top and lowest beech canopy leaves much more than in leaves located in the mid canopy; whereas in oak, A_n of the lower canopy was decreased more than in sunlit leaves. The decrease of A_n was probably not due to leaf‐level biochemistry given that V_Cmax was generally unaffected by drought. The reduction in A_n was instead associated with reduction in stomatal and mesophyll conductances. Drought‐induced increases in stomatal limitations were largest in leaves from the top canopy, whereas drought‐induced increases in mesophyll limitations were largest in leaves from the lowest canopy. Sensitivity analysis highlighted the need to decompose the canopy into different leaf layers and to incorporate the limitation imposed by g_m when assessing the impact of drought on the gas exchange of tree canopies.     

Photosynthetic water use efficiency in tree crowns of <Emphasis Type="Italic">Shorea beccariana</Emphasis> and <Emphasis Type="Italic">Dryobalanops aromatica</Emphasis> in a tropical rain forest in Sarawak,East Malaysia

Photosynthetic water use efficiency (PWUE), stomatal conductance (g _s), and water potential were measured at two different positions in the tree crown of two emergent tropical tree species (Shorea beccariana Burck, Dryobalanops aromatica Gaertn. f.). The trees were about 50 m high, in a tropical rain forest in Sarawak, East Malaysia. In both species, g _s at the upper crown position at midday was lower than at the lower crown position, even though both positions were exposed to full sunlight; the difference was greater in S. beccariana. Hydraulic limitation occurs in the upper crown position in both species. A midday depression was observed in the photon saturated photosynthetic rate in both species, especially at the upper crown. However, PWUE was markedly higher in the upper crown than the lower crown at midday, even though no morphological adjustment was observed in the leaves; this difference was greater in S. beccariana.     

The phenology of leaf quality and its within‐canopy variation is essential for accurate modeling of photosynthesis in tropical evergreen forests

Leaf quantity (i.e., canopy leaf area index, LAI), quality (i.e., per‐area photosynthetic capacity), and longevity all influence the photosynthetic seasonality of tropical evergreen forests. However, these components of tropical leaf phenology are poorly represented in most terrestrial biosphere models (TBMs). Here, we explored alternative options for the representation of leaf phenology effects in TBMs that employ the Farquahar, von Caemmerer & Berry (FvCB) representation of CO₂ assimilation. We developed a two‐fraction leaf (sun and shade), two‐layer canopy (upper and lower) photosynthesis model to evaluate different modeling approaches and assessed three components of phenological variations (i.e., leaf quantity, quality, and within‐canopy variation in leaf longevity). Our model was driven by the prescribed seasonality of leaf quantity and quality derived from ground‐based measurements within an Amazonian evergreen forest. Modeled photosynthetic seasonality was not sensitive to leaf quantity, but was highly sensitive to leaf quality and its vertical distribution within the canopy, with markedly more sensitivity to upper canopy leaf quality. This is because light absorption in tropical canopies is near maximal for the entire year, implying that seasonal changes in LAI have little impact on total canopy light absorption; and because leaf quality has a greater effect on photosynthesis of sunlit leaves than light limited, shade leaves and sunlit foliage are more abundant in the upper canopy. Our two‐fraction leaf, two‐layer canopy model, which accounted for all three phenological components, was able to simulate photosynthetic seasonality, explaining ~90% of the average seasonal variation in eddy covariance‐derived CO₂ assimilation. This work identifies a parsimonious approach for representing tropical evergreen forest photosynthetic seasonality in TBMs that utilize the FvCB model of CO₂ assimilation and highlights the importance of incorporating more realistic phenological mechanisms in models that seek to improve the projection of future carbon dynamics in tropical evergreen forests.     

Coupled response of stomatal and mesophyll conductance to light enhances photosynthesis of shade leaves under sunflecks

Light gradients within tree canopies play a major role in the distribution of plant resources that define the photosynthetic capacity of sun and shade leaves. However, the biochemical and diffusional constraints on gas exchange in sun and shade leaves in response to light remain poorly quantified, but critical for predicting canopy carbon and water exchange. To investigate the CO₂ diffusion pathway of sun and shade leaves, leaf gas exchange was coupled with concurrent measurements of carbon isotope discrimination to measure net leaf photosynthesis (A_n), stomatal conductance (g_s) and mesophyll conductance (g_m) in Eucalyptus tereticornis trees grown in climate controlled whole‐tree chambers. Compared to sun leaves, shade leaves had lower A_n, g_m, leaf nitrogen and photosynthetic capacity (A_max) but g_s was similar. When light intensity was temporarily increased for shade leaves to match that of sun leaves, both g_s and g_m increased, and A_n increased to values greater than sun leaves. We show that dynamic physiological responses of shade leaves to altered light environments have implications for up‐scaling leaf level measurements and predicting whole canopy carbon gain. Despite exhibiting reduced photosynthetic capacity, the rapid up‐regulation of g_m with increased light enables shade leaves to respond quickly to sunflecks.     

Size-dependent variability of leaf and shoot hydraulic conductance in silver birch

Variation in leaf and shoot hydraulic conductance was examined on detached shoots of silver birch (Betula pendula Roth), cut from the lower third (shade leaves) and upper third of the crown (sun leaves) of large trees growing in a natural temperate forest stand. Hydraulic conductances of whole shoots (K _S), leaf blades (K _lb), petioles (K _P) and branches (i.e. leafless stem; K _B) were determined by water perfusion using a high-pressure flow meter in quasi-steady state mode. The shoots were exposed to irradiance of photosynthetic photon flux density of 200–250 μmol m⁻² s⁻¹, using different light sources. K _lb depended significantly (P < 0.001) on light quality, canopy position and leaf blade area (A _L). K _lb increased from crown base to tree top, in parallel with vertical patterns of A _L. However, the analysis of data on shade and sun leaves separately revealed an opposite trend: the bigger the A _L the higher K _lb. Leaf anatomical study of birch saplings revealed that this trend is attributable to enhanced vascular development with increasing leaf area. Hydraulic traits (K _S, K _B, K _lb) of sun shoots were well co-ordinated and more strongly correlated with characteristics of shoot size than those of shade shoots, reflecting their greater evaporative load and need for stricter adjustment of hydraulic capacity with shoot size. K _S increased with increasing xylem cross-sectional area to leaf area ratio (Huber value; P < 0.01), suggesting a preferential investment in water-conducting tissue (sapwood) relative to transpiring tissue (leaves), and most likely contributing to the functional stability of the hydraulic system, essential for fast-growing pioneer species.     

Responses of sap flux and stomatal conductance of Pinus taeda L. trees to stepwise reductions in leaf area

Herbivory or artificial foliage removal has been shown to affect gas exchange and canopy water relations. In this study, canopy architecture and water relations in response to progressive defoliation were examined in a stand of 8-year-old loblolly pine (Pinus taeda L.) trees, a shade-intolerant, pioneer species common in the south-eastern USA. Sap flux was measured with constant heat sap flow gauges in order to estimate canopy stomatal conductance (Gs) while foliage in the 6 m high stand was harvested in 1 m increments from the bottom up. Leaf-level stomatal conductance and water potential data were also collected. Profiles of silhouette area ratio and specific leaf area showed no trends with crown height, reflecting an open canopy (leaf area index = 1.55). Therefore, short-term changes in Gs with foliage removal were attributed to hydraulic effects rather than influences of changes in mean microclimate conditions on Gs of remaining foliage. A large increase in Gs was observed during the 6 h pruning period which fully compensated for the reductions in foliage area down to 45%. Canopy stomatal conductance and whole plant liquid phase conductance as calculated from sap flux were both influenced by the rate of growth as indicated by the annual basal area increment.     

Photosynthetic activity and water relations of sprouts ofPasania edulis

In order to investigate the factors causing fast growth of sprouts ofPasania edulis, photosynthetic activity and water relation characteristics of lower (mature) leaves and upper (expanding) leaves of the sprouts were compared with those of seedlings and adult trees ofP. edulis. Apparent quantum yield was generally low. Maximum photosynthetic rate was highest in the lower leaves of sprouts. Stomatal frequency was higher in sprout leaves than in seedling leaves. Osmotic potential at the water saturation point and water potential at the turgor loss point, in leaves, were higher in sprouts than in seedlings and adult trees. Symplasmic water content per unit leaf area was higher in sprouts than in seedlings. These water relation parameters in leaves indicated that sprout leaves are superior in maintaining cell turgor against water loss, but are not tolerant to water stress. In field measurements, sprout leaves showed higher stomatal conductance and transpiration rates. These results indicated that sprout leaves fully realized their high potential productivity even under field conditions. The leaf specific conductance, from the soil to the leaf, was higher in sprouts than in seedlings. Large and deep root systems of the original stumps of the sprouts may be attributed to the high leaf specific conductance.     

Effects of light availability versus hydraulic constraints on stomatal responses within a crown of silver birch

Responses of leaf conductance (g_L) to variation in photosynthetic photon flux density (Q_P), leaf-to-air vapour pressure difference (VPD), bulk leaf water potential (_x), and total hydraulic conductance (G_T) were examined in silver birch (Betula pendula Roth) with respect to leaf position in the crown. To reduce limitations caused by insufficient water supply or low light availability, experiments were also performed with branchlets cut from two different canopy layers. The intact upper-canopy leaves demonstrated 1.8–2.0 times higher (P<0.001) daily maxima of g_L compared with the lower-canopy leaves growing in the shadow of upper branches. In the morning, g_L in the shade foliage was primarily constrained by low light availability, in the afternoon, by limited water supply. Leaf conductance decreased when _x fell below certain values around midday, while the sun foliage experienced greater negative water potentials than the shade foliage. Midday stomatal openness was controlled by leaf water status and temperature, rather than by transpiration rate (E) via the feedforward mechanism. Mean G_T was 1.7 times higher (P<0.001) for the upper-canopy foliage compared to that of the lower canopy. At least 34–39% of the total resistance to the water flow from soil up to the shade foliage, and 54% up to the sun foliage, resided in 30-cm distal parts of the branches. Artificial reduction of hydraulic constraints raised _x and made g_L less sensitive to changes in both atmospheric and plant factors. Improved water supply increased g_L and E in the lower-canopy foliage, but not in the upper-canopy foliage. The results support the idea that leaves in the lower canopy are hydraulically more constrained than in the upper canopy.     

Late-stage canopy tree species with extremely low δ13C and high stomatal sensitivity to seasonal soil drought in the tropical rainforest of French Guiana

We assessed the daily time‐courses of CO₂ assimilation rate (A), leaf transpiration rate (E), stomatal conductance for water vapour (g_s), leaf water potential ( Ψ _w) and tree transpiration in a wet and a dry season for three late‐stage canopy rainforest tree species in French Guiana differing in leaf carbon isotope composition ( δ ¹³C). The lower sunlit leaf δ ¹³C values found in Virola surinamensis ( ? 29·9‰) and in Diplotropis purpurea ( ? 30·9‰), two light‐demanding species, as compared to Eperua falcata ( ? 28·6‰), a shade‐semi‐tolerant species, were clearly associated with higher maximum g_s values of sunlit leaves in the two former species. These two species were also characterized by a high sensitivity of g_s, sap flow density (Ju) and canopy conductance (g_c) to seasonal soil drought, allowing maintenance of high midday Ψ _w values in the dry season. The data for Diplotropis provided an original picture of increasing midday Ψ _w with increasing soil drought. In Virola, stomata were extremely sensitive to seasonal soil drought, leading to a dramatic decrease in leaf and tree transpiration in the dry season, whereas midday Ψ _w remained close to ? 0·3 MPa. The mechanisms underlying such an extremely high sensitivity of stomata to soil drought remain unknown. In Eperua, g_s of sunlit leaves was non‐responsive to seasonal drought, whereas Ju and g_c were lower in the dry season. This suggests a higher stomatal sensitivity to seasonal drought in shaded leaves than in sunlit ones in this species.     

DIFFERENCES IN LEAF STRUCTURE,CHLOROPHYLL, AND NUTRIENTS FOR THE UNDERSTORY TREE ASIMINA TRILOBA

To investigate differences in leaf structure, chlorophyll and nutrients on terminal branches of the understory tree Asimina triloba, the first (proximal) and the last (distal) leaves to develop in the spring were compared. Proximal leaf expansion was completed before the overstory canopy was fully closed but distal leaf expansion occurred during and after the development of the overstory canopy. Fully expanded proximal leaves were 76% smaller in area, were 18% thicker and had 36% more stomates per m of leaf area when compared to distal leaves. In addition, maximum stomatal conductance to water vapor was greater (150 vs. 120 mmol m^−-2s^−-1) and the minimum PPFD required for maximum conductance was higher (200 vs. 150 μmol m^−-2s^−-1) for the proximal leaves. Chlorophyll content was also greater for proximal leaves, but nitrogen and phosphorus contents were lower throughout the entire summer. Seasonal measurements indicated an increase in chlorophyll a content and reductions in nitrogen content throughout the summer growth period for leaves from both positions. The results suggest that distal and proximal leaves differed physiologically and that the measured differences were related to the changing irradiance environment during leaf development. The time of leaf expansion, as indicated by leaf position on the branch, may be an important consideration when examining the water and photosynthetic relations of understory trees.