Leaf Biomechanics and Biomass Investment in Support in Relation to Long-Term Irradiance in Fagus

Abstract: We investigated biomass investment in support and assimilative leaf biomass in Fagus orientalis Lipsky and F. sylvatica L., and foliar biomechanical characteristics in F. orientalis to gain mechanistic insight into the determinants of leaf inclination in Fagus along the canopy light gradient. Because the leaf laminas of Fagus are elliptical, with petioles comprising only ca. 8 % of total leaf length, a leaf was approximated as a continuous sine load. Lamina load increased with increasing seasonal integrated quantum flux density in the canopy (Q_int), but leaf length was independent of irradiance. Despite greater load, leaf deflection under leaf own weight was lower for leaves at higher Q_int, indicating that foliage flexural stiffness (EI), that is a variable characterizing the resistance of beam‐like structures to bending, scaled positively with irradiance. The components of EI ‐ the leaf apparent Young's modulus of elasticity (E), which is a measure of leaf material properties, and lamina second moment of area (I), which characterizes the distribution of mass around the axis of bending ‐ were also related to irradiance, with E decreasing, but I increasing with Q_int. The positive scaling of I with Q_int was associated with increases in leaf thickness and, in particular, with increases in the degree of leaf rolling, allowing the distribution of leaf mass further away from the neutral axis. Decreases in E were correlated with decreased leaf biomass investments in the midrib at higher irradiance. Both lamina and midrib nitrogen concentrations decreased with increasing Q_int, suggesting that foliage dry mass based physiological activity was lower at higher irradiance, possibly because of an interaction of Q_int with water stress in the canopy. Given that the veins also provide a pathway for water and nutrient transport to the leaf cells, as well as for carbon translocation from the leaf, lower leaf physiological activity in high light may provide an explanation for the lower biomass investment in major veins in high light. We conclude that foliage biomechanical characteristics and leaf inclination in the canopy are significantly affected by irradiance, and that the light effects may be modified by the reverse correlation between light and water availabilities in the canopy.     

Within-canopy variation in the rate of development of photosynthetic capacity is proportional to integrated quantum flux density in temperate deciduous trees

The present study was undertaken to test for the hypothesis that the rate of development in the capacity for photosynthetic electron transport per unit area (J_max;A), and maximum carboxylase activity of Rubisco (V_cmax;A) is proportional to average integrated daily quantum flux density (Q_int) in a mixed deciduous forest dominated by the shade‐intolerant species Populus tremula L., and the shade‐tolerant species Tilia cordata Mill. We distinguished between the age‐dependent changes in net assimilation rates due to modifications in leaf dry mass per unit area (M_A), foliar nitrogen content per unit dry mass (N_M), and fractional partitioning of foliar nitrogen in the proteins of photosynthetic electron transport (F_B), Rubisco (F_R) and in light‐harvesting chlorophyll‐protein complexes (V_cmax;A ∝ M_AN_MF_R; J_max;A ∝ M_AN_MF_B). In both species, increases in J_max;A and V_cmax;A during leaf development were primarily determined by nitrogen allocation to growing leaves, increases in leaf nitrogen partitioning in photosynthetic machinery, and increases in M_A. Canopy differences in the rate of development of leaf photosynthetic capacity were mainly controlled by the rate of change in M_A. There was only small within‐canopy variation in the initial rate of biomass accumulation per unit Q_int (slope of M_A versus leaf age relationship per unit Q_int), suggesting that canopy differences in the rate of development of J_max;A and V_cmax;A are directly proportional to Q_int. Nevertheless, M_A, nitrogen, J_max;A and V_cmax;A of mature leaves were not proportional to Q_int because of a finite M_A in leaves immediately after bud‐burst (light‐independent component of M_A). M_A, leaf chlorophyll contents and chlorophyll : N ratio of mature leaves were best correlated with the integrated average quantum flux density during leaf development, suggesting that foliar photosynthetic apparatus, once developed, is not affected by day‐to‐day fluctuations in Q_int. However, for the upper canopy leaves of P. tremula and for the entire canopy of T. cordata, there was a continuous decline in N contents per unit dry mass in mature non‐senescent leaves on the order of 15–20% for a change of leaf age from 40 to 120 d, possibly manifesting nitrogen reallocation to bud formation. The decline in N contents led to similar decreases in leaf photosynthetic capacity and foliar chlorophyll contents. These data demonstrate that light‐dependent variation in the rate of developmental changes in M_A determines canopy differences in photosynthetic capacity, whereas foliar photosynthetic apparatus is essentially constant in fully developed leaves.     

Genotypic and within canopy variation in leaf carbon isotope discrimination and its relation to short-term leaf gas exchange characteristics in cassava grown under rain-fed conditions in the tropics

In a field rain-fed trial with 15 cassava cultivars, leaf gas exchanges and carbon isotope discrimination (Δ) of the same leaves were determined to evaluate genotypic and within-canopy variations in these parameters. From 3 to 7 months after planting leaf gas exchange was measured on attached leaves from upper, middle, and lower canopy layers. All gas exchange parameters varied significantly among cultivars as well as canopy layers. Net photosynthetic rate (P _N) decreased from top canopy to bottom indicating both shade and leaf age effects. The same trend, but in reverse, was found with respect to Δ, with the highest values in low canopy level and the lowest in upper canopy. There were very significant correlations, with moderate and low values, among almost all these parameters, with P _N negatively associated with intercellular CO₂ concentration (C _i), ratio of C _i to ambient CO₂ concentration C _i/C _a, and Δ. Across all measured leaves, Δ correlated negatively with leaf water use efficiency (WUE = photosynthesis/stomatal conductance, g _s) and with g _s, but positively with C _i and C _i/C _a. The later parameters negatively correlated with leaf WUE. Across cultivars, both P _N and correlated positively with storage root yield. These results are in agreement with trends predicted by the carbon isotope discrimination model.     

The effect of tree height on crown level stomatal conductance

Variation in stomatal conductance is typically explained in relation to environmental conditions. However, tree height may also contribute to the variability in mean stomatal conductance. Mean canopy stomatal conductance of individual tree crowns (G_Si) was estimated using sap flux measurements in Fagus sylvatica L., and the hypothesis that G_Si decreases with tree height was tested. Over 13 d of the growing season during which soil moisture was not limiting, G_Si decreased linearly with the natural logarithm of vapour pressure deficit (D), and increased exponentially to saturation with photosynthetic photon flux density (Q_o). Under conditions of D = 1 kPa and saturating Q_o, G_Si decreased by approximately 60% with 30 m increase in tree height. Over the same range in height, sapwood‐to‐leaf area ratio (A_S:A_L) doubled. A simple hydraulic model explained the variation in G_Si based on an inverse relationship with height, and a linear relationship with A_S:A_L. Thus, in F. sylvatica, adjustments in A_S:A_L partially compensate for the negative effect of increased flow‐path length on leaf conductance. Furthermore, because stomata with low conductance are less sensitive to D, gas exchange of tall trees is reduced less by high D. Despite these compensations, decreasing hydraulic conductance with tree height in F. sylvatica reduces carbon uptake through a corresponding decrease in stomatal conductance.     

Three years of free-air CO2 enrichment (POPFACE) only slightly affect profiles of light and leaf characteristics in closed canopies of Populus

Modelling is used to predict long‐term forest responses to increased atmospheric CO₂ concentrations. Although productivity models are based on light intercepted by the canopy, very little experimental data are available for closed forest stands. Nevertheless, the relationships between light inside a canopy, leaf area, canopy structure, and individual leaf characteristics may be affected by elevated CO₂, affecting in turn carbon gain. Using a free‐air CO₂ enrichment (FACE) design in a high‐density plantation of Populus spp., we studied the effects of increased CO₂ concentrations on transmittance (τ) of photosynthetic photon flux density (Q_p), on ratios of red/far‐red light (R/FR), on leaf area index (LAI), on leaf inclination, on leaf chlorophyll (chl) and nitrogen (N) concentrations, and on specific leaf area (SLA) in the 2nd and 3rd years of treatment. Continuous measurements of τ were made in addition to canopy height profiles of light and leaf characteristics. Two years of Q_p measurements showed an average decrease of canopy transmittance in the FACE treatment, with very small differences at canopy closure. Results were explained by an unaffected LAI in closed canopies, without a FACE‐induced stimulation of relative crown depth. In agreement, leaf inclination and extinction coefficients for light were similar in control and FACE conditions. Ratios of R/FR were not significantly affected by the FACE treatment, neither were leaf characteristics, with the exception of leaf N, which allows speculation about N limitation. In general, treatment differences in canopy profiles resulted from an initial stimulation of height growth in the FACE treatment. P. × euramericana differed from P. alba and P. nigra, but species did not differ significantly in their response to the FACE treatment. By the time fast‐growing high‐density forest plantations have passed the exponential growth phase and reached canopy closure, the likely effects of elevated atmospheric CO₂ concentration on canopy architecture and absorption of Q_p are minor.     

Direct and indirect effects of elevated CO2 on transpiration from Quercus myrtifolia in a scrub-oak ecosystem

Elevated atmospheric carbon dioxide (C_a) usually reduces stomatal conductance, but the effects on plant transpiration in the field are not well understood. Using constant‐power sap flow gauges, we measured transpiration from Quercus myrtifolia Willd., the dominant species of the Florida scrub‐oak ecosystem, which had been exposed in situ to elevated C_a (350 µmol mol ^{? 1} above ambient) in open‐top chambers since May 1996. Elevated C_a reduced average transpiration per unit leaf area by 37%, 48% and 49% in March, May and October 2000, respectively. Temporarily reversing the C_a treatments showed that at least part of the reduction in transpiration was an immediate, reversible response to elevated C_a. However, there was also an apparent indirect effect of C_a on transpiration: when transpiration in all plants was measured under common C_a, transpiration in elevated C_a‐grown plants was lower than that in plants grown in normal ambient C_a. Results from measurements of stomatal conductance (g_s), leaf area index (LAI), canopy light interception and correlation between light and g_s indicated that the direct, reversible C_a effect on transpiration was due to changes in g_s caused by C_a, and the indirect effect was caused mainly by greater self‐shading resulting from enhanced LAI, not from stomatal acclimation. By reducing light penetration through the canopy, the enhanced self‐shading at elevated C_a decreased stomatal conductance and transpiration of leaves at the middle and bottom of canopy. This self‐shading mechanism is likely to be important in ecosystems where LAI increases in response to elevated C_a.     

Seasonal variability in the effect of elevated CO2 on ecosystem leaf area index in a scrub-oak ecosystem

We report effects of elevated atmospheric CO₂ concentration (C_a) on leaf area index (LAI) of a Florida scrub‐oak ecosystem, which had regenerated after fire for between three and five years in open‐top chambers (OTCs) and was yet to reach canopy closure. LAI was measured using four nondestructive methods, calibrated and tested in experiments performed in calibration plots near the OTCs. The four methods were: PAR transmission through the canopy, normalized difference vegetation index (NDVI), hemispherical photography, and allometric relationships between plant stem diameter and plant leaf area. Calibration experiments showed: (1) Leaf area index could be accurately determined from either PAR transmission through the canopy or hemispherical photography. For LAI determined from PAR transmission through the canopy, ecosystem light extinction coefficient (k) varied with season and was best described as a function of PAR transmission through the canopy. (2) A negative exponential function described the relationship between NDVI and LAI; (3) Allometric relationships overestimated LAI. Throughout the two years of this study, LAI was always higher in elevated C_a, rising from, 20% during winter, to 55% during summer. This seasonality was driven by a more rapid development of leaf area during the spring and a relatively greater loss of leaf area during the winter, in elevated C_a. For this scrub‐oak ecosystem prior to canopy closure, increased leaf area was an indirect mechanism by which ecosystem C uptake and canopy N content were increased in elevated C_a. In addition, increased LAI decreased potential reductions in canopy transpiration from decreases in stomatal conductance in elevated C_a. These findings have important implications for biogeochemical cycles of C, N and H₂O in woody ecosystems regenerating from disturbance in elevated C_a.     

Canopy‐scale relationships between stomatal conductance and photosynthesis in irrigated rice

Modeling stomatal behavior is critical in research on land–atmosphere interactions and climate change. The most common model uses an existing relationship between photosynthesis and stomatal conductance. However, its parameters have been determined using infrequent and leaf‐scale gas‐exchange measurements and may not be representative of the whole canopy in time and space. In this study, we used a top‐down approach based on a double‐source canopy model and eddy flux measurements throughout the growing season. Using this approach, we quantified the canopy‐scale relationship between gross photosynthesis and stomatal conductance for 3 years and their relationships with leaf nitrogen content throughout each growing season above a paddy rice canopy in Japan. The canopy‐averaged stomatal conductance (g_sc) increased with increasing gross photosynthesis per unit green leaf area (A_g), as was the case with leaf‐scale measurements, and 41–90% of its variation was explained by variations in A_g adjusted to account for the leaf‐to‐air vapor‐pressure deficit and CO₂ concentration using the Leuning model. The slope (m) in this model (g_sc versus the adjusted A_g) was almost constant within a 15‐day period, but changed seasonally. The m values determined using an ensemble dataset for two mid‐growing‐season 15‐day periods were 30.8 (SE = 0.5), 29.9 (SE = 0.7), and 29.9 (SE = 0.6) in 2004, 2005, and 2006, respectively; the overall mid‐season value was 30.3 and did not greatly differ among the 3 years. However, m appeared to be higher during the early and late growing seasons. The ontogenic changes in leaf nitrogen content strongly affected A_g and thus g_sc. In addition, we have discussed the agronomic impacts of the interactions between leaf nitrogen content and g_sc. Despite limitations in the observations and modeling, our canopy‐scale results emphasize the importance of continuous, season‐long estimates of stomatal model parameters for crops using top‐down approaches.     

Shape of leaf photosynthetic electron transport versus temperature response curve is not constant along canopy light gradients in temperate deciduous trees

Responses of foliar light-saturated net assimilation rate (A_max), capacity for photosynthetic electron transport (J_max) and mitochondrial respiration rate (R_d) to long-term canopy light and temperature environment were investigated in a temperate deciduous canopy composed of Populus tremula L. in the upper (17–28 m) and of Tilia cordata Mill. in the lower canopy layer (4–17 m). Climatic measurements indicated that seasonal average daily maximum air temperature (T_max) was 5·5 °C (range 0·7–10·5 °C) higher in the top than in the bottom of the canopy, and strong positive correlations were observed between T_max and seasonal average integrated quantum flux density (Q_int), as well as between seasonal average daily mean temperature and Q_int. Because of changes in leaf dry mass and nitrogen per unit area, A_max, J_max, and R_d scaled positively with Q_int in both species at a common leaf temperature (T). According to J_max versus T response curves and dark chlorophyll fluorescence transients, photosynthetic electron transport was less heat resistant in P. tremula with optimum temperature of J_max, T_opt, of 33·5 ± 0·6 °C than in T. cordata with T_opt of 40·7 ± 0·6 °C. This difference was suggested to manifest evolutionary adaptation of photosynthetic electron transport to cooler environments in P. tremula, the range of which extends farther north than that in T. cordata. Possibly because of acclimation to long-term canopy temperature environment, T_opt was positively related to Q_int in P. tremula, foliage of which was also exposed to higher irradiances and temperatures, but not in T. cordata, in the canopy of which quantum flux densities and temperatures were lower, and gradients in the environmental factors less pronounced. Parallel to changes in T_opt, the activation energy for photosynthetic electron transport decreased with increasing Q_int in P. tremula, indicating that J_max of leaves acclimated to colder environment was more responsive to T in lower temperatures than that of high T acclimated leaves. Similar alterations in the activation energy for mitochondrial respiration rate were also observed, indicating that acclimation to temperature of mitochondrial and chloroplastic electron transport proceeds in a co-ordinated manner, and possibly involves long-term changes in membrane fluidity properties. We conclude that, because of correlations between temperature and light, the shapes of J_max versus T, and R_d versus T response curves vary within tree canopies, and this needs to be taken account in modelling whole canopy photosynthesis.     

Cool-dry season depression in gas exchange of canopy leaves and water flux of tropical trees at the northern limit of Asian tropics

Trees on the northern boundary of Asian tropics experience hot-humid and cool-dry seasons, but little is known about their seasonal dynamics in canopy physiology. We used a canopy crane to reach the canopy of nine tropical tree species and measured canopy leaf gas exchange, water status, and trunk sap flux during the hot-humid and cool-dry seasons in Xishuangbanna, China. We found that most tree species exhibited significant reductions in maximum photosynthetic rate (A_max), stomatal conductance (g_smax), predawn and midday leaf water potentials, and maximum sap flux density in the cool-dry season. Compared to the hot-humid season, A_max declined by 19–60%, and maximum water flux declined by ?14% (an increase) to 42%. The cool-dry season decline in A_max of four species can be partly explained by an increased stomatal limitation (decreased g_smax and intercellular CO₂ concentrations). Therefore, a predicted increase in drought in this region may decrease the carbon sequestration and productivity of these forests. We did not find a tradeoff between performance (A_max in the hot-humid season) and persistence through the cool-dry season; species with higher A_max in the hot-humid season did not show higher percent seasonal declines in the cool-dry season. A_max was significantly and positively associated with the trunk sap flux for both seasons, but the association was weaker in the cool-dry season. Thus, our results suggest that some tradeoffs and trait associations are environment dependent. Our results are important for understanding carbon and water fluxes of seasonal tropical forests and their responses to environmental changes.

     

Variation in the carbon and oxygen isotope composition of plant biomass and its relationship to water‐use efficiency at the leaf‐ and ecosystem‐scales in a northern Great Plains grassland

Measurements of the carbon (δ¹³C_m) and oxygen (δ¹⁸O_m) isotope composition of C₃ plant tissue provide important insights into controls on water‐use efficiency. We investigated the causes of seasonal and inter‐annual variability in water‐use efficiency in a grassland near Lethbridge, Canada using stable isotope (leaf‐scale) and eddy covariance measurements (ecosystem‐scale). The positive relationship between δ¹³C_m and δ¹⁸O_m values for samples collected during 1998–2001 indicated that variation in stomatal conductance and water stress‐induced changes in the degree of stomatal limitation of net photosynthesis were the major controls on variation in δ¹³C_m and biomass production during this time. By comparison, the lack of a significant relationship between δ¹³C_m and δ¹⁸O_m values during 2002, 2003 and 2006 demonstrated that water stress was not a significant limitation on photosynthesis and biomass production in these years. Water‐use efficiency was higher in 2000 than 1999, consistent with expectations because of greater stomatal limitation of photosynthesis and lower leaf c_i/ca during the drier conditions of 2000. Calculated values of leaf‐scale water‐use efficiency were 2–3 times higher than ecosystem‐scale water‐use efficiency, a difference that was likely due to carbon lost in root respiration and water lost during soil evaporation that was not accounted for by the stable isotope measurements.     

A test of the hydraulic limitation hypothesis in fast-growing Eucalyptus saligna

The hydraulic limitation hypothesis proposes that (1) reduced growth in taller trees is caused by decreased photosynthesis resulting from a decrease in hydraulic conductance promoted by a longer root‐to‐leaf flow path, and (2) this mechanism reduces stand productivity after canopy closure. This hypothesis was tested by comparing the physiology of 7 m (1 year) and 26 m (5 year) Eucalyptus saligna plantations where above‐ground productivity for the 26 m trees was approximately 69% of that for the 7 m trees, and water and nutrients were not limiting. The study compared whole tree physiology [water flux (Q_l), average crown conductance (G_T), crown hydraulic conductance per unit leaf area (K_L), carbon isotope discrimination (δ¹³C)] and leaf physiology under light saturation (leaf water potential at the canopy top (Ψ_LEAF), photosynthetic capacity (A_max), and photosynthesis (A) and stomatal conductance (g_s). K_L was 50% lower in the taller trees, but whole tree Q_l and G_T were the same for the 7 m and 26 m trees. Photosynthetic capacity was the same for leaves at the canopy top, but δ¹³C was ?1.8‰ lower for the 26 m trees. A and g_s were either lower in the taller trees or equal, depending on sampling date. The taller trees maintained 0.8 MPa lower Ψ_LEAF during the day and had 2.6‐times higher sapwood area per unit leaf area; these factors compensated for the effects of increased height and gravitational potential in the taller trees to maintain higher G_T. The hydraulic limitation hypothesis (as originally stated) failed to explain the sharp decline in net primary productivity after canopy closure in this study. The effects of increased height appear to be a universal hydraulic problem for trees, but compensation mitigated these effects and maintained Q_l and G_T in the present study. Compensation may induce other problems (such as lower Ψ_LEAF or higher respiratory costs) that could reduce carbon gain or shift carbon allocation, and future studies of hydraulic limitation should consider compensation and associated carbon costs. In this study, the combination of similar G_T and lower δ¹³C for the 26 m trees suggests that total crown photosynthesis was lower for the 26 m trees, perhaps a result of the lower Ψ_LEAF.     

Stomatal conductance and not stomatal density determines the long-term reduction in leaf transpiration of poplar in elevated CO<Subscript>2</Subscript>

Using a free-air CO₂ enrichment (FACE) experiment, poplar trees (Populus × euramericana clone I214) were exposed to either ambient or elevated [CO₂] from planting, for a 5-year period during canopy development, closure, coppice and re-growth. In each year, measurements were taken of stomatal density (SD, number mm⁻²) and stomatal index (SI, the proportion of epidermal cells forming stomata). In year 5, measurements were also taken of leaf stomatal conductance (g _s, μmol m⁻² s⁻¹), photosynthetic CO₂ fixation (A, mmol m⁻² s⁻¹), instantaneous water-use efficiency (A/E) and the ratio of intercellular to atmospheric CO₂ (C_i:C_a). Elevated [CO₂] caused reductions in SI in the first year, and in SD in the first 2 years, when the canopy was largely open. In following years, when the canopy had closed, elevated [CO₂] had no detectable effects on stomatal numbers or index. In contrast, even after 5 years of exposure to elevated [CO₂], g _s was reduced, A/E was stimulated, and C_i:C_a was reduced relative to ambient [CO₂]. These outcomes from the long-term realistic field conditions of this forest FACE experiment suggest that stomatal numbers (SD and SI) had no role in determining the improved instantaneous leaf-level efficiency of water use under elevated [CO₂]. We propose that altered cuticular development during canopy closure may partially explain the changing response of stomata to elevated [CO₂], although the mechanism for this remains obscure.     

Do the capacity and kinetics for modification of xanthophyll cycle pool size depend on growth irradiance in temperate trees?

The present study investigated the interaction of growth irradiance (Q_int) with leaf capacity for and kinetics of adjustment of the pool size of xanthophyll cycle carotenoids (sum of violaxanthin, antheraxanthin and zeaxanthin; VAZ) and photosynthetic electron transport rate (J_max) after changes in leaf light environment. Individual leaves of lower‐canopy/lower photosynthetic capacity species Tilia cordata Mill. and upper canopy/higher photosynthetic capacity species Populus tremula L. were either illuminated by additional light of 500–800 µmol m^?2 s^?1 for 12 h photoperiod or enclosed in shade bags. The extra irradiance increased the total amount of light intercepted by two‐fold for the upper and 10–15‐fold for the lower canopy leaves, whereas the shade bags transmitted 45% of incident irradiance. In control leaves, VAZ/area, VAZ/Chl and J_max were positively associated with leaf growth irradiance (Q_int). After 11 d extra illumination, VAZ/Chl increased in all cases due to a strong reduction in foliar chlorophyll, but VAZ/area increased in the upper canopy leaves of both species, and remained constant or decreased in the lower canopy leaves of T. cordata. The slope for VAZ/area changes with cumulative extra irradiance was positively associated with Q_int only in T. cordata, but not in P. tremula. Nevertheless, all leaves of P. tremula increased VAZ/area more than the most responsive leaves of T. cordata. Shading reduced VAZ content only in P. tremula, but not in T. cordata, again demonstrating that P. tremula is a more responsive species. Compatible with the hypothesis of the role of VAZ in photoprotection, the rates of photosynthetic electron transport declined less in P. tremula than in T. cordata after the extra irradiance treatment. However, foliar chlorophyll contents of the exposed leaves declined significantly more in the upper canopy of P. tremula, which is not consistent with the suggestion that the leaves with the highest VAZ content are more resistant to photoinhibition. This study demonstrates that previous leaf light environment may significantly affect the adaptation capacity of foliage to altered light environment, and also that species differences in photosynthetic capacity and acclimation potentials importantly alter this interaction.     

Desert dogma revisited: coupling of stomatal conductance and photosynthesis in the desert shrub, Larrea tridentata

The success of the desert shrub Larrea tridentata (creosotebush) has been largely attributed to temperature acclimation and stomatal control of photosynthesis (A) under drought stress. However, there is a paucity of field data on these relationships. To address this void, we conducted a joint field and modelling study that encompassed a diverse set of environmental conditions. At a Larrea‐dominated site in southern New Mexico we manipulated soil moisture during the growing season over a 2‐year period and measured plant pre‐dawn water potential (Ψ_pd), stomatal conductance (g) and A of individual shrubs. We used these data to develop a semi‐mechanistic photosynthesis model (A–Season) that explicitly couples internal CO₂ (C_i) and g. Vapour pressure deficit (VPD) and Ψ_pd affect instantaneous g in a manner that is consistent with a biophysical model of stomatal regulation of leaf water potential. C_i is modelled as a function of g, derived from a simplification of a typical A–C_i curve. After incorporating the effects of growing temperature on stomatal behaviour, the model was able to capture the large diurnal fluctuations in A, g and C_i and the observed hysteresis in g versus C_i dynamics. Our field data and application of the A–Season model suggest that dogma attributed to Larrea's success is supported with regard to stomatal responses to VPD and Ψ_pd, but not for mechanisms of temperature acclimation and CO₂ demand.     

Adjustment of foliage structure and function to a canopy light gradient in two co-existing deciduous trees. Variability in leaf inclination angles in relation to petiole morphology

 Foliar inclination angles, petiole morphology and dry matter partitioning between assimilative and support biomass were studied in shade-intolerant Populus tremula L. and shade-tolerant Tilia cordata Mill. along a natural light gradient across the canopy. The leaves of sub-canopy species T. cordata were on average exposed to lower irradiances, and they were also more horizontal with greater blade inclination angles (ϕ_B, defined as the angle between the leaf fall-line and the horizon; ϕ_B was positive for the leaves inclined upwards, and negative for the leaves inclined downwards) than those in P. tremula. Seasonal average daily integrated quantum flux density (Q _int, mol m^–2 day^–1) and ϕ_B were not related in T. cordata, and only a weak negative effect of Q _int on ϕ_B was detected in P. tremula. Nevertheless, when both species were pooled, there was a strong negative relationship between Q _int and ϕ_B, implying that the leaves became progressively vertical with increasing height in the canopy. Interspecific differences in foliage inclination were mainly related to petiole morphology, in particular to petiole length, rather than to contrasting biomass investment patterns between assimilative and support tissues within the leaf. It was suggested that more horizontal leaves, resulting from the species-specific structure of petioles, partly explain the superior performance of shade-tolerant T. cordata in the understory and the sub-canopy. Received: 13 November 1997 / Accepted: 6 March 1998     

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.     

Gas exchange and resource-use efficiency of Leymus cinereus (Poaceae): diurnal and seasonal responses to naturally declining soil moisture

We examined factors that limit diurnal and seasonal photosynthesis in Leymus cinereus, a robust tussock grass from shrub-steppes of western North America. Data from plants in a natural stand and in experimental field plots indicate that this bunchgrass has 1) a high photosynthetic capacity, 2) high leaf nitrogen content and high nitrogen-use efficiency, 3) a steep leaf-to-air diffusion gradient for carbon dioxide, which enhances intrinsic water-use efficiency, and 4) photosynthetic tissues that tolerate severe water stress and recover quickly from moderate water stress. Midday depressions of CO₂ assimilation (A) and stomatal conductance were slight in plants with plentiful water, but marked in plants subject to moderate water stress. Midday stomatal closure in moderately stressed plants reduced intercellular carbon dioxide concentration (c_i) by ≈40 μl liter^-1. The maximum rate of A achieved during the day for severely stressed plants (predawn water potential = -4 MPa) was one-third and daily carbon gain per unit leaf area was about one-fourth that of well-watered plants. For plants in the natural stand, CO₂-saturated photosynthesis declined almost linearly with decreasing soil water availability over the growing season, whereas there was little effect on A at CO₂ ambient levels or on carboxylation efficiency until predawn water potentials reached -1.8 MPa. Nitrogen-use efficiency declined with diminishing soil moisture, but there was no seasonal change in stomatal limitation or instantaneous water-use efficiency as estimated from A vs. c_i curves at optimal leaf temperature and moderate atmospheric evaporative demand. Thus, reduced stomatal conductance in response to increased evaporative demand may increase stomatal limitation diumally, but over the growing season, stomatal limitation estimated from A vs. c_i curves is relatively constant because maximum stomatal conductance is closely tuned to the CO₂ assimilatory capacity of the mesophyll.     

Assessment of annual carbon exchange in a water-stressed Pinus radiata plantation: an analysis based on eddy covariance measurements and an integrated biophysical model

We used a combination of eddy flux, chamber and environmental measurements with an integrated suite of models to analyse the seasonality of net ecosystem carbon uptake (F_CO2) in an 8-year-old, closed canopy Pinus radiata D.Don plantation in New Zealand (42°52′ S, 172°45′ E). The analyses utilized a biochemically based, big-leaf model of tree canopy photosynthesis (A_c), coupled to multiplicative environmental-constraint functions of canopy stomatal conductance (G_c) via environmental measurements, a temperature-dependent model of ecosystem respiration (R_eco), and a soil water balance model. Available root zone water storage capacity at the measurement site is limited to about 50 mm for the very stony soil, and annual precipitation is only 660 mm, distributed evenly throughout the year. Accordingly the site is prone to soil moisture deficit throughout the summer. G _c and A_c obtained maximum rates early in the growing season when plentiful soil water supply was associated with sufficient quantum irradiance (Q_abs), and moderate air saturation deficit (D) and temperature (T). From late spring onwards, soil water deficit and D confined G_c and A_c congruously, which together with the solely temperature dependency of R_eco resulted in the pronounced seasonality in F_CO2. Reflecting a light-limitation of A_c in the closed canopy, modelled annual carbon (C) uptake was most sensitive to changes in Q_abs. However, Q_abs did not vary significantly between years, and changes in annual F_CO2 were mostly due to variability in summer rainfall and D. Annual C-uptake of the forest was 717 g C m^–2 in a near-average rainfall year, exceeding by one third the net uptake in a year with 20% less than average rainfall (515 g C m^–2).