Similar patterns of leaf temperatures and thermal acclimation to warming in temperate and tropical tree canopies

As the global climate warms, a key question is how increased leaf temperatures will affect tree physiology and the coupling between leaf and air temperatures in forests. To explore the impact of increasing temperatures on plant performance in open air, we warmed leaves in the canopy of two mature evergreen forests, a temperate Eucalyptus woodland and a tropical rainforest. The leaf heaters consistently maintained leaves at a target of 4 °C above ambient leaf temperatures. Ambient leaf temperatures (Tleaf) were mostly coupled to air temperatures (Tair), but at times, leaves could be 8-10 °C warmer than ambient air temperatures, especially in full sun. At both sites, Tleaf was warmer at higher air temperatures (Tair > 25 °C), but was cooler at lower Tair, contrary to the 'leaf homeothermy hypothesis'. Warmed leaves showed significantly lower stomatal conductance (-0.05 mol m-2 s-1 or -43% across species) and net photosynthesis (-3.91 μmol m-2 s-1 or -39%), with similar rates in leaf respiration rates at a common temperature (no acclimation). Increased canopy leaf temperatures due to future warming could reduce carbon assimilation via reduced photosynthesis in these forests, potentially weakening the land carbon sink in tropical and temperate forests.

Is photosynthetic enhancement sustained through three years of elevated CO2 exposure in 175-year-old Quercus robur?

Current carbon cycle models attribute rising atmospheric CO2 as the major driver of the increased terrestrial carbon sink, but with substantial uncertainties. The photosynthetic response of trees to elevated atmospheric CO2 is a necessary step, but not the only one, for sustaining the terrestrial carbon uptake, but can vary diurnally, seasonally and with duration of CO2 exposure. Hence, we sought to quantify the photosynthetic response of the canopy-dominant species, Quercus robur, in a mature deciduous forest to elevated CO2 (eCO2) (+150 μmol mol-1 CO2) over the first 3 years of a long-term free air CO2 enrichment facility at the Birmingham Institute of Forest Research in central England (BIFoR FACE). Over 3000 measurements of leaf gas exchange and related biochemical parameters were conducted in the upper canopy to assess the diurnal and seasonal responses of photosynthesis during the 2nd and 3rd year of eCO2 exposure. Measurements of photosynthetic capacity via biochemical parameters, derived from CO2 response curves, (Vcmax and Jmax) together with leaf nitrogen concentrations from the pre-treatment year to the 3rd year of eCO2 exposure, were examined. We hypothesized an initial enhancement in light-saturated net photosynthetic rates (Asat) with CO2 enrichment of ≈37% based on theory but also expected photosynthetic capacity would fall over the duration of the study. Over the 3-year period, Asat of upper-canopy leaves was 33 ± 8% higher (mean and standard error) in trees grown in eCO2 compared with ambient CO2 (aCO2), and photosynthetic enhancement decreased with decreasing light. There were no significant effects of CO2 treatment on Vcmax or Jmax, nor leaf nitrogen. Our results suggest that mature Q. robur may exhibit a sustained, positive response to eCO2 without photosynthetic downregulation, suggesting that, with adequate nutrients, there will be sustained enhancement in C assimilated by these mature trees. Further research will be required to understand the location and role of the additionally assimilated carbon.

Canopy position affects photosynthesis and anatomy in mature Eucalyptus trees in elevated CO2

Leaves are exposed to different light conditions according to their canopy position, resulting in structural and anatomical differences with consequences for carbon uptake. While these structure-function relationships have been thoroughly explored in dense forest canopies, such gradients may be diminished in open canopies, and they are often ignored in ecosystem models. We tested within-canopy differences in photosynthetic properties and structural traits in leaves in a mature Eucalyptus tereticornis canopy exposed to long-term elevated CO2 for up to three years. We explored these traits in relation to anatomical variation and diffusive processes for CO2 (i.e., stomatal conductance, gs and mesophyll conductance, gm) in both upper and lower portions of the canopy receiving ambient and elevated CO2. While shade resulted in 13% lower leaf mass per area ratio (MA) in lower versus upper canopy leaves, there was no relationship between leaf Nmass and canopy gap fraction. Both maximum carboxylation capacity (Vcmax) and maximum electron transport (Jmax) were ~ 18% lower in shaded leaves and were also reduced by ~ 22% with leaf aging. In mature leaves, we found no canopy differences for gm or gs, despite anatomical differences in MA, leaf thickness and mean mesophyll thickness between canopy positions. There was a positive relationship between net photosynthesis and gm or gs in mature leaves. Mesophyll conductance was negatively correlated with mean parenchyma length, suggesting that long palisade cells may contribute to a longer CO2 diffusional pathway and more resistance to CO2 transfer to chloroplasts. Few other relationships between gm and anatomical variables were found in mature leaves, which may be due to the open crown of Eucalyptus. Consideration of shade effects and leaf-age dependent responses to photosynthetic capacity and mesophyll conductance are critical to improve canopy photosynthesis models and will improve understanding of long-term responses to elevated CO2 in tree canopies.

Incorporating non-stomatal limitation improves the performance of leaf and canopy models at high vapour pressure deficit

Vapour pressure deficit (D) is projected to increase in the future as temperature rises. In response to increased D, stomatal conductance (gs) and photosynthesis (A) are reduced, which may result in significant reductions in terrestrial carbon, water and energy fluxes. It is thus important for gas exchange models to capture the observed responses of gs and A with increasing D. We tested a series of coupled A-gs models against leaf gas exchange measurements from the Cumberland Plain Woodland (Australia), where D regularly exceeds 2 kPa and can reach 8 kPa in summer. Two commonly used A-gs models were not able to capture the observed decrease in A and gs with increasing D at the leaf scale. To explain this decrease in A and gs, two alternative hypotheses were tested: hydraulic limitation (i.e., plants reduce gs and/or A due to insufficient water supply) and non-stomatal limitation (i.e., downregulation of photosynthetic capacity). We found that the model that incorporated a non-stomatal limitation captured the observations with high fidelity and required the fewest number of parameters. Whilst the model incorporating hydraulic limitation captured the observed A and gs, it did so via a physical mechanism that is incorrect. We then incorporated a non-stomatal limitation into the stand model, MAESPA, to examine its impact on canopy transpiration and gross primary production. Accounting for a non-stomatal limitation reduced the predicted transpiration by ~19%, improving the correspondence with sap flow measurements, and gross primary production by ~14%. Given the projected global increases in D associated with future warming, these findings suggest that models may need to incorporate non-stomatal limitation to accurately simulate A and gs in the future with high D. Further data on non-stomatal limitation at high D should be a priority, in order to determine the generality of our results and develop a widely applicable model.

Photosynthetic enhancement by elevated CO₂ depends on seasonal temperatures for warmed and non-warmed Eucalyptus globulus trees

Arguments based on the biochemistry of photosynthesis predict a positive interaction between elevated atmospheric [CO2] and temperature on photosynthesis as well as growth. In contrast, few long-term studies on trees find greater stimulation of photosynthesis in response to elevated [CO2] at warmer compared with cooler temperatures. To test for CO2 × temperature interactions on leaf photosynthesis and whole-plant growth, we planted Eucalyptus globulus Labill. in climate-controlled chambers in the field at the Hawkesbury Forest Experiment research site, and investigated how photosynthetic enhancement changed across a range of seasonal temperatures. Trees were grown in a complete two-way factorial design with two CO2 concentrations (ambient and ambient + 240 ppm) and two temperatures (ambient and ambient + 3 °C) for 15 months until they reached ∼10 m height, after which they were harvested for biomass. There was significant enhancement of photosynthesis and growth with elevated [CO2], with the photosynthetic stimulation varying with season, but there was no significant effect of warming. Photosynthetic enhancement was higher in summer (+46% at 28 °C) than in winter (+14% at 20 °C). Photosynthetic enhancement as a function of leaf temperature was consistent with theoretical expectations, but was strongly mediated by the intercellular [CO2]/ambient [CO2] (Ci/Ca) ratio across seasons. Total tree biomass after 15 months was 66% larger in elevated CO2 (P = 0.017) with no significant warming effect detected. The fraction of biomass in coarse roots was reduced in warmed trees compared with ambient temperature controls, but there was no evidence of changed biomass allocation patterns in elevated CO2. We conclude that there are strong and consistent elevated CO2 effects on photosynthesis and biomass of E. globulus. It is crucial to consider stomatal conductance under a range of conditions to appraise the interactive effect of [CO2] and temperature on photosynthetic enhancement and subsequent implications for tree growth and forest productivity in future climates.

Interactive direct and plant-mediated effects of elevated atmospheric [CO2 ] and temperature on a eucalypt-feeding insect herbivore

Understanding the direct and indirect effects of elevated [CO2 ] and temperature on insect herbivores and how these factors interact are essential to predict ecosystem-level responses to climate change scenarios. In three concurrent glasshouse experiments, we measured both the individual and interactive effects of elevated [CO2 ] and temperature on foliar quality. We also assessed the interactions between their direct and plant-mediated effects on the development of an insect herbivore of eucalypts. Eucalyptus tereticornis saplings were grown at ambient or elevated [CO2 ] (400 and 650 μmol mol(-1) respectively) and ambient or elevated ( + 4 °C) temperature for 10 months. Doratifera quadriguttata (Lepidoptera: Limacodidae) larvae were feeding directly on these trees, on their excised leaves in a separate glasshouse, or on excised field-grown leaves within the temperature and [CO2 ] controlled glasshouse. To allow insect gender to be determined and to ensure that any sex-specific developmental differences could be distinguished from treatment effects, insect development time and consumption were measured from egg hatch to pupation. No direct [CO2 ] effects on insects were observed. Elevated temperature accelerated larval development, but did not affect leaf consumption. Elevated [CO2 ] and temperature independently reduced foliar quality, slowing larval development and increasing consumption. Simultaneously increasing both [CO2 ] and temperature reduced these shifts in foliar quality, and negative effects on larval performance were subsequently ameliorated. Negative nutritional effects of elevated [CO2 ] and temperature were also independently outweighed by the direct positive effect of elevated temperature on larvae. Rising [CO2 ] and temperature are thus predicted to have interactive effects on foliar quality that affect eucalypt-feeding insects. However, the ecological consequences of these interactions will depend on the magnitude of concurrent temperature rise and its direct effects on insect physiology and feeding behaviour.

Do thick leaves avoid thermal damage in critically low wind speeds?

Transient lulls in air movement are rarely measured, but can cause leaf temperature to rise rapidly to critical levels. The high heat capacity of thick leaves can damp this rapid change in temperature. However, little is known about the extent to which increased leaf thickness can reduce thermal damage, or how thick leaves would need to be to have biological significance. We evaluated quantitatively the contribution of small increases in leaf thickness to the reduction in thermal damage during critically low wind speeds under desert conditions. We employed a numerical model to investigate the effect of thickness relative to transpiration, absorptance and leaf size on damage avoidance. We used measured traits and thermotolerance thresholds of real leaves to calculate the leaf temperature response to naturally occurring variable low wind speed. Our results demonstrated that an increase in thickness of only fractions of a millimetre can prevent excursions to damaging high temperatures. This damping effect of increased thickness was greatest when other means of reducing leaf temperature (transpiration, reflectance or reduced size) were lacking. For perennial desert flora, we propose that increased leaf thickness is important in decreasing the incidence of extreme heat stress and, in some species, in enhancing long-term survival.

Light interception efficiency explained by two simple variables: a test using a diversity of small- to medium-sized woody plants

• Plant light interception efficiency is a crucial determinant of carbon uptake by individual plants and by vegetation. Our aim was to identify whole-plant variables that summarize complex crown architecture, which can be used to predict light interception efficiency. • We gathered the largest database of digitized plants to date (1831 plants of 124 species), and estimated a measure of light interception efficiency with a detailed three-dimensional model. Light interception efficiency was defined as the ratio of the hemispherically averaged displayed to total leaf area. A simple model was developed that uses only two variables, crown density (the ratio of leaf area to total crown surface area) and leaf dispersion (a measure of the degree of aggregation of leaves). • The model explained 85% of variation in the observed light interception efficiency across the digitized plants. Both whole-plant variables varied across species, with differences in leaf dispersion related to leaf size. Within species, light interception efficiency decreased with total leaf number. This was a result of changes in leaf dispersion, while crown density remained constant. • These results provide the basis for a more general understanding of the role of plant architecture in determining the efficiency of light harvesting.

Carbon dioxide and water vapor exchange in a warm temperate grassland

Grasslands cover about 40% of the ice-free global terrestrial surface, but their contribution to local and regional water and carbon fluxes and sensitivity to climatic perturbations such as drought remains uncertain. Here, we assess the direction and magnitude of net ecosystem carbon exchange (NEE) and its components, ecosystem carbon assimilation ( A(c)) and ecosystem respiration ( R(E)), in a southeastern United States grassland ecosystem subject to periodic drought and harvest using a combination of eddy-covariance measurements and model calculations. We modeled A(c) and evapotranspiration (ET) using a big-leaf canopy scheme in conjunction with ecophysiological and radiative transfer principles, and applied the model to assess the sensitivity of NEE and ET to soil moisture dynamics and rapid excursions in leaf area index (LAI) following grass harvesting. Model results closely match eddy-covariance flux estimations on daily, and longer, time steps. Both model calculations and eddy-covariance estimates suggest that the grassland became a net source of carbon to the atmosphere immediately following the harvest, but a rapid recovery in LAI maintained a marginal carbon sink during summer. However, when integrated over the year, this grassland ecosystem was a net C source (97 g C m(-2) a(-1)) due to a minor imbalance between large A(c) (-1,202 g C m(-2) a(-1)) and R(E) (1,299 g C m(-2) a(-1)) fluxes. Mild drought conditions during the measurement period resulted in many instances of low soil moisture (theta<0.2 m(3)m(-3)), which influenced A(c) and thereby NEE by decreasing stomatal conductance. For this experiment, low theta had minor impact on R(E). Thus, stomatal limitations to A(c) were the primary reason that this grassland was a net C source. In the absence of soil moisture limitations, model calculations suggest a net C sink of -65 g C m(-2) a(-1) assuming the LAI dynamics and physiological properties are unaltered. These results, and the results of other studies, suggest that perturbations to the hydrologic cycle are key determinants of C cycling in grassland ecosystems.

Site fertility and the morphological and photosynthetic acclimation of Pinus sylvestris needles to light

Morphological and photosynthetic acclimation of current-year needles to canopy gradients in light availability (seasonal mean integrated quantum flux density, Q(int)) was studied in the temperate conifer, Pinus sylvestris L., at two sites of contrasting nutrient availability. The nutrient-rich site supported a monospecific P. sylvestris stand on an old-field. The trees were approximately 30 years old and 19-21 m tall. Mean foliar N and P contents (+/- SD) were 1.53 +/- 0.11% and 0.196 +/- 0.017%, respectively. The nutrient-poor site was located on a raised bog supporting a sparse stand of 50- to 100-year-old trees, with a height of 1-2 m, and mean needle N and P contents of 0.86 +/- 0.12% and 0.074 +/- 0.010%, respectively. At both sites, needle thickness (T) and width (W) increased with increasing Qint, and leaf dry mass per unit leaf area (MA) was also greater at higher irradiance. The light effects on MA-the product of needle density (D) and volume to total area ratio (V/AT)-resulted primarily from large increases in V/AT with Qint rather than from modifications of D, which was relatively insensitive to light. Although needle morphology versus light relationships were qualitatively similar at both sites, needles were shorter, and the slopes of W, T, MA and V/AT versus light relationships were lower, at the nutrient-poor than at the nutrient-rich site, indicating that the plasticity of foliar morphological characteristics was affected by nutrient availability. As a result of lower plasticity, needles at the nutrient-poor site were narrower, thinner, and had lower MA at high irradiance than needles at the nutrient-rich site. The maximum carboxylase activity of ribulose-1,5-bisphosphate carboxylase/oxygenase (Vcmax) and the maximum photosynthetic electron transport rate (Jmax) scaled positively with foliar N and P contents. The correlations were generally stronger with P than with N, suggesting that needle photosynthetic capacity was more heavily limited by the availability of P than of N. The Jmax/Vcmax ratio was positively related to the foliar P/N ratio, indicating that Jmax was more strongly suppressed than Vcmax under conditions of low P availability. Phosphorus and N deficiency also limited the plasticity of foliar photosynthetic characteristics. There was a moderate increase in needle photosynthetic capacity of up to 1.6-fold from the bottom to the top of the canopy at the nutrient-rich site, but net assimilation rates were essentially independent of canopy position at the nutrient-poor site. Stomatal constraints on photosynthesis were similar between the sites, indicating that photosynthetic acclimation was curtailed at the biochemical level. We conclude that the foliar capacity for morphological and physiological acclimation to high light significantly decreases with decreasing nutrient availability in P. sylvestris, and that both N and P availability are potentially important determinants of foliar carbon gain capacities.

Sensitivity of photosynthetic electron transport to photoinhibition in a temperate deciduous forest canopy: Photosystem II center openness, non-radiative energy dissipation and excess irradiance under field conditions

We used chlorophyll fluorescence techniques to investigate responses of Photosystem II (PSII) quantum yield to light availability in the short term (quantum flux density integrated over the measurement day, Qd) and in the long term (Qd averaged over the season, Qs) in a mixed deciduous forest comprising shade-tolerant and water-stress-sensitive Tilia cordata Mill. in the lower canopy and shade-intolerant and water-stress-resistant Populus tremula L. in the upper canopy. In both species, intrinsic efficiency of PSII in the dark-adapted state (Fv/Fm) was lower during the day than during the night, and the difference in Fv/Fm between day and night increased with increasing Qs. Although the capacity for photosynthetic electron transport increased with increasing Qs in both species, maximum quantum efficiency of PSII in the light-adapted state (alpha) decreased with increasing Qs. At a common Qs, alpha was lower in T. cordata than in P. tremula primarily because of a higher fraction of closed PSII centers, and to a smaller extent because of limited, non-radiative, excitation energy dissipation in the pigment bed in T. cordata. Across both species, photochemical quenching (qP), which measures the openness of PSII centers, varied more than fivefold, but the efficiency of excitation energy capture by open PSII centers (Fv'/Fm'), which is an estimate of non-radiative excitation energy dissipation in PSII antennae, varied by only 50%. Chlorophyll turnover rates increased with increasing irradiance, especially in T. cordata, possibly because of increased photodestruction. Diurnal measurements of PSII quantum yields (PhiPSII) indicated that, under similar environmental conditions, PhiPSII was always lower in the afternoon than in the morning, and the fraction of daily integrated photosynthetic electron transport lost because of diurnal declines in PhiPSII (Delta) increased with increasing Qd. At a common Qd, mean daily PSII center reduction state, the fraction of light in excess (1 - fractions of light used in photochemistry and dissipated as heat) and Delta were higher in T. cordata than in P. tremula. This was attributed to greater stomatal closure during the day, which led to a greater reduction in the requirement for assimilative electron flow in T. cordata. Across both species, Delta scaled negatively with the fraction of light utilized photochemically, demonstrating the leading role of PSII center openness in maintaining high PSII efficiency. Because photosynthesis (A) at current ambient carbon dioxide concentration is limited by CO2 availability in high light and mainly by photosynthetic electron transport rates in low light, overall daily down-regulation of PhiPSII primarily influences A in low light. Given that foliar water stress scales positively with Qs in both species, we conclude that the inverse patterns of variation in water and light availabilities in the canopy result in a greater decline in A than is predicted by decreases in stomatal conductance alone.

Possible explanation of the disparity between the in vitro and in vivo measurements of Rubisco activity: a study in loblolly pine grown in elevated pCO2

Rubisco activity can be measured using gas exchange (in vivo) or using in vitro methods. Commonly in vitro methods yield activities that are less than those obtained in vivo. Rubisco activity was measured both in vivo and in vitro using a spectrophotometric technique in mature Pinus taeda L. (loblolly pine) trees grown using free-air CO2 enrichment in elevated (56 Pa) and current (36 Pa) pCO2. In addition, for studies where both in vivo and in vitro values of Rubisco activity were reported net CO2 uptake rate (A) was modelled based on the in vivo and in vitro values of Rubisco activity reported in the literature. Both the modelling exercise and the experimental data showed that the in vitro values of Rubisco activity were insufficient to account for the observed values of A. A trichloroacetic acid (TCA) precipitation of the protein from samples taken in parallel with those used for activity analysis was co-electrophoresed with the extract used for determining in vitro Rubisco activity. There was significantly more Rubisco present in the TCA precipitated samples, suggesting that the underestimation of Rubisco activity in vitro was attributable to an insufficient extraction of Rubisco protein prior to activity analysis. Correction of in vitro values to account for the under-represented Rubisco yielded mechanistically valid values for Rubisco activity. However, despite the low absolute values for Rubisco activity determined in vitro, the trends reported with CO2 treatment concurred with, and were of equal magnitude to, those observed in Rubisco activity measured in vivo.

Soil fertility limits carbon sequestration by forest ecosystems in a CO2-enriched atmosphere

Northern mid-latitude forests are a large terrestrial carbon sink. Ignoring nutrient limitations, large increases in carbon sequestration from carbon dioxide (CO2) fertilization are expected in these forests. Yet, forests are usually relegated to sites of moderate to poor fertility, where tree growth is often limited by nutrient supply, in particular nitrogen. Here we present evidence that estimates of increases in carbon sequestration of forests, which is expected to partially compensate for increasing CO2 in the atmosphere, are unduly optimistic. In two forest experiments on maturing pines exposed to elevated atmospheric CO2, the CO2-induced biomass carbon increment without added nutrients was undetectable at a nutritionally poor site, and the stimulation at a nutritionally moderate site was transient, stabilizing at a marginal gain after three years. However, a large synergistic gain from higher CO2 and nutrients was detected with nutrients added. This gain was even larger at the poor site (threefold higher than the expected additive effect) than at the moderate site (twofold higher). Thus, fertility can restrain the response of wood carbon sequestration to increased atmospheric CO2. Assessment of future carbon sequestration should consider the limitations imposed by soil fertility, as well as interactions with nitrogen deposition.

Does free-Air carbon dioxide enrichment affect photochemical energy use by evergreen trees in different Seasons? A chlorophyll fluorescence study of mature loblolly pine

Previous studies of the effects of growth at elevated CO(2) on energy partitioning in the photosynthetic apparatus have produced conflicting results. The hypothesis was developed and tested that elevated CO(2) increases photochemical energy use when there is a high demand for assimilates and decreases usage when demand is low. Modulated chlorophyll a fluorescence and leaf gas exchange were measured on needles at the top of a mature, 12-m loblolly pine (Pinus taeda L.) forest. Trees were exposed to ambient CO(2) or ambient plus 20 Pa CO(2) using free-air CO(2) enrichment. During April and August, periods of shoot growth, light-saturated photosynthesis and linear electron transport were increased by elevated CO(2). In November, when growth had ceased but temperatures were still moderate, CO(2) treatment had no significant effect on linear electron transport. In February, when low temperatures were likely to inhibit translocation, CO(2) treatment caused a significant decrease in linear electron transport. This coincided with a slower recovery of the maximum photosystem II efficiency on transfer of needles to the shade, indicating that growth in elevated CO(2) induced a more persistent photoinhibition. Both the summer increase and the winter decrease in linear electron transport in elevated CO(2) resulted from a change in photochemical quenching, not in the efficiency of energy transfer within the photosystem II antenna. There was no evidence of any effect of CO(2) on photochemical energy sinks other than carbon metabolism. Our results suggest that elevated CO(2) may increase the effects of winter stress on evergreen foliage.

From tropics to tundra: global convergence in plant functioning

Despite striking differences in climate, soils, and evolutionary history among diverse biomes ranging from tropical and temperate forests to alpine tundra and desert, we found similar interspecific relationships among leaf structure and function and plant growth in all biomes. Our results thus demonstrate convergent evolution and global generality in plant functioning, despite the enormous diversity of plant species and biomes. For 280 plant species from two global data sets, we found that potential carbon gain (photosynthesis) and carbon loss (respiration) increase in similar proportion with decreasing leaf life-span, increasing leaf nitrogen concentration, and increasing leaf surface area-to-mass ratio. Productivity of individual plants and of leaves in vegetation canopies also changes in constant proportion to leaf life-span and surface area-to-mass ratio. These global plant functional relationships have significant implications for global scale modeling of vegetation-atmosphere CO2 exchange.

Leaf nutrition and photosynthetic performance of sugar maple (Acer saccharum) in stands with contrasting health conditions

Leaf nutrition and photosynthetic performance of sugar maple (Acer saccharum Marsh.) were compared between two sugar maple stands in northwestern Vermont with contrasting health conditions as indicated by annual basal area growth, degree of crown dieback, and foliar appearance. Observations made during the diurnal cycle of both stands showed no apparent leaf water stress. In both stands, leaves had similar concentrations of major non-structural carbohydrates (starch and sucrose). Over two consecutive growing seasons (1991 and 1992), we consistently observed lower leaf Ca and Mg concentrations in the declining stand than in the healthy stand. Compared with the healthy stand, lower leaf chlorophyll concentrations and apparent leaf chlorosis were observed in the declining stand, and some trees had very low foliar Ca and Mg concentrations (0.31 +/- 0.03% and 0.09 +/- 0.01%, respectively). Trees in the declining stand had lower light-saturated net photosynthetic rates on a dry mass basis at both ambient CO(2) (P(n,amb)) and saturating CO(2) (P(n,sat)) than trees in the healthy stand. There were significant linear correlations between P(n,amb) and leaf mass per unit area (LMA) and between P(n,sat) per unit leaf area and LMA. There were also linear correlations between both P(n,amb) and P(n,sat) and leaf N when expressed on an area basis in both stands, indicating that variation in LMA may have been largely responsible for the observed photosynthesis-nitrogen relationship. The values of P(n,amb) and P(n,sat) were not significantly correlated with leaf N on a mass basis but were weakly correlated with leaf Ca and Mg on a mass basis. We conclude that low leaf Ca or Mg concentrations may limit leaf CO(2) assimilation and tree carbohydrate status in the declining stand.

Different photosynthesis-nitrogen relations in deciduous hardwood and evergreen coniferous tree species

The relationship between photosynthetic capacity (A _max) and leaf nitrogen concentration (N) among all C₃ species can be described roughly with one general equation, yet within that overall pattern species groups or individual species may have markedly different A _max-N relationships. To determine whether one or several predictive, fundamental A _max-N relationships exist for temperate trees we measured A _max, specific leaf area (SLA) and N in 22 broad-leaved deciduous and 9 needle-leaved evergreen tree species in Wisconsin, United States. For broad-leaved deciduous trees, mass-based A _max was highly correlated with leaf N (r ²=0.75, P<0.001). For evergreen conifers, mass-based A _max was also correlated with leaf N (r ²=0.59, P<0.001) and the slope of the regression (rate of increase of A _max per unit increase in N) was lower (P<0.001) by two-thirds than in the broad-leaved species (1.9 vs. 6.4 μmol CO₂ g^-1 N s^-1), consistent with predictions based on tropical rain forest trees of short vs. long leaf life-span. On an area basis, there was a strong A _max-N correlation among deciduous species (r ²=0.78, P<0.001) and no correlation (r ²=0.03, P>0.25) in the evergreen conifers. Compared to deciduous trees at a common leaf N (mass or area basis), evergreen trees had lower A _max and SLA. For all data pooled, both leaf N and A _max on a mass basis were correlated (r ²=0.6) with SLA; in contrast, area-based leaf N scaled tightly with SLA (r ²=0.81), but area-based A _max did not (r ²=0.06) because of low A _max per unit N in the evergreen conifers. Multiple regression analysis of all data pooled showed that both N (mass or area basis) and SLA were significantly (P<0.001) related to A _max on mass (r ²=0.80) and area (r ²=0.55) bases, respectively. These results provide further evidence that A _max-N relationships are fundamentally different for ecologically distinct species groups with differing suites of foliage characteristics: species with long leaf life-spans and low SLA, whether broad-leaved or needle-leaved, tend to have lower A _max per unit leaf N and a lower slope and higher intercept of the A _max-N relation than do species with shorter leaf life-span and higher SLA. A single global A _max-N equation overestimates and underestimates A _max for temperate trees at the upper and lower end of their leaf N range, respectively. Users of A _max-N relationships in modeling photosynthesis in different ecosystems should appreciate the strengths and limitations of regression equations based on different species groupings.

Photosynthesis and water-use efficiency of sugar maple (Acer saccharum) in relation to pear thrips defoliation

An experimental introduction of pear thrips (Taeniothrips inconsequens Uzel), a major defoliator in sugar maple (Acer saccharum Marsh.) forests in northeastern North America, was conducted in a field plantation to determine if compensatory gas exchange occurs in response to feeding damage by this piercing-sucking insect. Sugar maple trees were enclosed in netting (167 micro m mesh) and pear thrips adults were introduced before leaf expansion in the spring. Pear thrips reduced whole-tree leaf area by approximately 23% and reduced leaf size (both mass and area) by 20% in the upper crown. Measurements of net CO(2) assimilation rate (A(net)) and stomatal conductance (g(s)) were made on tagged foliage that was later analyzed for stable carbon isotope composition (delta(13)C) to provide estimates of short- and long-term leaf water use efficiency (WUE). Pear thrips feeding reduced A(net) for fully expanded leaves by approximately 20%, although leaf chlorophyll content and leaf mass per unit area were apparently not affected. Comparison of A(net), g(s), instantaneous WUE and leaf delta(13)C between damaged and control trees as well as visibly undamaged versus moderately damaged foliage on pear thrips-infested trees indicated that there were no effects of pear thrips feeding damage on WUE or leaf delta(13)C. Long-term WUE among sugar maple trees in the field plantation, indicated by leaf delta(13)C analysis, was related to shorter-term estimates of leaf gas exchange behavior such as g(s) and calculated leaf intercellular CO(2) concentration (C(i)). We conclude that pear thrips feeding has no effect on leaf WUE, but at the defoliation levels in our experiment, it may reduce leaf A(net), as a result of direct tissue damage or through reduced g(s). Therefore, even small reductions in leaf A(net) by pear thrips feeding damage may have an important effect on the seasonal carbon balance of sugar maple when integrated over the entire growing season.

Photosynthesis-nitrogen relations in Amazonian tree species : I. Patterns among species and communities

Among species, photosynthetic capacity (A_max) is usually related to leaf nitrogen content (N), but variation in the species-specific relationship is not well understood. To address this issue, we studied A_max-N relationships in 23 species in adjacent Amazonian communities differentially limited by nitrogen (N), phosphorus (P), and/or other mineral nutrients. Five species were studied in each of three late successional forest types (Tierra Firme, Caatinga and Bana) and eight species were studied on disturbed sites (cultivated and early secondary successional Tierra Firme plots). A_max expressed on a mass basis (A_mass) was correlated (p<0.05) with N_mass in 17 of 23 species, and A_max on an area basis (A_area) was correlated (p<0.05) with N_area in 21 of 23 species. The slopes of A_max-N relationships were greater and intercepts lower for disturbance adapted early successional species than for late successional species. On a mass basis, the A_max-N slope averaged ≈15 μmol CO₂ [g N]^-1 s^-1 for 7 early secondary successional species and ≈4 μmol CO₂ [g N]^-1 s^-1 for 15 late successional species, respectively. Species from disturbed sites had shorter leaf life-span and greater specific leaf area (SLA) than late successional species. Across all 23 species, the slope of the A_mass-N_mass relationship was related (p<0.001) positively to SLA (r²=0.70) and negatively to leaf life-span (r²=0.78) and temporal niche during secondary succession (years since cutting-and-burning, r²=0.90). Thus, disturbance adapted early successional species display a set of traits (short leaf life-span, high SLA and A_max and a steep slope of A_max-N) conducive to resource acquisition and rapid growth in their high resource regeneration niches. The significance and form of the A_max-N relationship were associated with the relative nutrient limitations in the three late successional communities. At species and community levels, A_max was more highly dependent on N in the N-limited Caatinga than in the P-and N-limited Bana and least in the P-and Ca-limited Tierra Firme on oxisol-and differences among these three communities in their massbased A_max-N slope reflects this pattern (6.0, 2.4, and 0.7 μmol CO₂ [g N]^-1 s^-1, respectively). Among all 23 species, the estimated leaf N_mass needed to reach compensation (net photosynthesis ≈ zero) was positively related to the A_mass-N_mass slope and to dark respiration rates and negatively related to leaf life-span. Variation among species in the A_max-N slope was well correlated with potential photosynthetic N use efficiency, A_max per unit leaf N. The dependence of A_max on N and the form of the relationship vary among Amazonian species and communities, consistent with both relative availabilities of N, P, and other mineral nutrients, and with intrinsic ecophysiological characteristics of species adapted to habitats of varying resource availability.

Water relations and gas exchange of Acer saccharum seedlings in contrasting natural light and water regimes

Field measurements were made of leaf photosynthesis (A), stomatal conductance (g) and leaf water relations for sugar maple (Acer saccharum Marsh.) seedlings growing in a forest understory, small gap or large clearing habitat in southwestern Wisconsin, USA. Predawn water status, leaf gas exchange and plasticity in field and laboratory water relations characteristics were compared among contrasting light environments in a wet year (1987) and a dry year (1988) to evaluate possible interactions between light and water availability in these habitats. Leaf water potentials (Psi(leaf)) at predawn and midday were lower for clearing than gap or understory seedlings. Acclimation of tissue osmotic potentials to light environment was observed among habitats but did not occur within any of the habitats in response to prolonged drought. During a summer drought in 1988, decreases in daily maximum g (g(max)) and maximum A (A(max)) in clearing seedlings were correlated with predawn Psi(leaf), which reached a seasonal minimum of -2.0 MPa. Under well-watered conditions, diurnal fluctuations in Psi(leaf) of up to 2.0 MPa in clearing seedlings occurred along with large midday depressions of A and g. In a wet year, strong stomatal responses to leaf-to-air vapor pressure difference (VPD) in sunny habitats were observed over nine diurnal courses of gas exchange measurements on seedlings in a gap and a clearing. Increasing stomatal limitations to photosynthesis appeared to be responsible for the reduction in A at high VPD for clearing seedlings. In understory seedlings, however, low water-use efficiency and development of leaf water deficits in sunflecks was related to reduced stomatal limitations to photosynthesis relative to seedlings in sunny habitats. Predawn Psi(leaf) and VPD appear to be important factors limiting carbon assimilation in sugar maple seedlings in light-saturating irradiances, primarily through stomatal closure. The overall results are consistent with the idea that sugar maple seedlings exhibit "conservative" water use patterns and have low drought tolerance. Leaf water relations and patterns of water use should be considered in studies of acclimation and species photosynthetic performance in contrasting light environments.

Vertical variation in canopy structure and CO(2) exchange of oak-maple forests: influence of ozone, nitrogen, and other factors on simulated canopy carbon gain

Stand-level and physiological measurements were made for oak and maple species common in Wisconsin forests. Scaling relationships were identified to allow the development of a model for estimating net carbon exchange at the levels of a leaf, canopy stratum, and whole canopy. Functional relationships were determined between tissue gas exchange rates and perceived controlling variables. Vertical variation in leaf properties and in the distribution of foliage by weight, area, and species were characterized for several closed canopy forests. Forest canopies were divided into four horizontal strata to develop predictive models for canopy gas exchange. Leaf and canopy layer carbon dioxide exchange rates were predicted using leaf nitrogen concentration, leaf mass per area, ozone exposure, predawn leaf water potential, photosynthetically active radiation, and vapor pressure deficit as driving variables. Direct measurements of leaf gas exchange were used to validate the components (subroutines) of the model. Net carbon dioxide exchange was simulated for canopy layers at 5-min intervals over a diurnal time course. Simulations of canopy CO(2) exchange were made for a 30-m tall, mixed oak-maple forest under hypothetical ambient and greater-than-ambient ozone pollution regimes. Daily canopy net CO(2) exchange was predicted for seven forest stands and compared with estimates of aboveground net primary production, N availability, leaf area index, and canopy N.