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Atmospheric conditions are expected to become warmer and drier in the future, but little is known about how evaporative demand influences forest structure and function independently from soil moisture availability, and how fast-response variables (such as canopy water potential and stomatal conductance) may mediate longer-term changes in forest structure and function in response to climate change. We used two tropical rainforest sites with different temperatures and vapour pressure deficits (VPD), but nonlimiting soil water supply, to assess the impact of evaporative demand on ecophysiological function and forest structure. Common species between sites allowed us to test the extent to which species composition, relative abundance and intraspecific variability contributed to site-level differences. The highest VPD site had lower midday canopy water potentials, canopy conductance (g c ), annual transpiration, forest stature, and biomass, while the transpiration rate was less sensitive to changes in VPD; it also had different height-diameter allometry (accounting for 51% of the difference in biomass between sites) and higher plot-level wood density. Our findings suggest that increases in VPD, even in the absence of soil water limitation, influence fast-response variables, such as canopy water potentials and g c , potentially leading to longer-term changes in forest stature resulting in reductions in biomass., (© 2023 The Authors. New Phytologist © 2023 New Phytologist Foundation.)

"Least-cost theory" posits that C 3 plants should balance rates of photosynthetic water loss and carboxylation in relation to the relative acquisition and maintenance costs of resources required for these activities. Here we investigated the dependency of photosynthetic traits on climate and soil properties using a new Australia-wide trait dataset spanning 528 species from 67 sites. We tested the hypotheses that plants on relatively cold or dry sites, or on relatively more fertile sites, would typically operate at greater CO 2 drawdown (lower ratio of leaf internal to ambient CO 2 , C i :C a ) during light-saturated photosynthesis, and at higher leaf N per area (N area ) and higher carboxylation capacity (V cmax 25 ) for a given rate of stomatal conductance to water vapour, g sw . These results would be indicative of plants having relatively higher water costs than nutrient costs. In general, our hypotheses were supported. Soil total phosphorus (P) concentration and (more weakly) soil pH exerted positive effects on the N area -g sw and V cmax 25 -g sw slopes, and negative effects on C i :C a . The P effect strengthened when the effect of climate was removed via partial regression. We observed similar trends with increasing soil cation exchange capacity and clay content, which affect soil nutrient availability, and found that soil properties explained similar amounts of variation in the focal traits as climate did. Although climate typically explained more trait variation than soil did, together they explained up to 52% of variation in the slope relationships and soil properties explained up to 30% of the variation in individual traits. Soils influenced photosynthetic traits as well as their coordination. In particular, the influence of soil P likely reflects the Australia's geologically ancient low-relief landscapes with highly leached soils. Least-cost theory provides a valuable framework for understanding trade-offs between resource costs and use in plants, including limiting soil nutrients., (© 2022 The Authors. Global Change Biology published by John Wiley & Sons Ltd.)

Soil-leaf hydraulic conductance determines canopy-atmosphere coupling in vegetation models, but it is typically derived from ex-situ measurements of stem segments and soil samples. Using a novel approach, we derive robust in-situ estimates for whole-tree conductance (k tree ), 'functional' soil conductance (k soil ), and 'system' conductance (k system , water table to canopy), at two climatically different tropical rainforest sites. Hydraulic 'functional rooting depth', determined for each tree using profiles of soil water potential (Ψ soil ) and sap flux data, enabled a robust determination of k tree and k soil . k tree was compared across species, size classes, seasons, height above nearest drainage (HAND), two field sites, and to alternative representations of k tree ; k soil was analysed with respect to variations in site, season and HAND. k tree was lower and changed seasonally at the site with higher vapour pressure deficit (VPD) and rainfall; k tree differed little across species but scaled with tree circumference; r soil (1/k soil ) ranged from 0 in the wet season to 10× less than r tree (1/k tree ) in the dry season. VPD and not rainfall may influence plot-level k; leaf water potentials and sap flux can be used to determine k tree , k soil and k system ; Ψ soil profiles can provide mechanistic insights into ecosystem-level water fluxes., (© 2021 The Authors. New Phytologist © 2021 New Phytologist Foundation.)

Rationale: Traditionally, stable isotope analysis of plant and soil water has been a technically challenging, labour-intensive and time-consuming process. Here we describe a rapid single-step technique which combines Microwave Extraction with Isotope Ratio Infrared Spectroscopy (ME-IRIS)., Methods: Plant, soil and insect water is extracted into a dry air stream by microwave irradiation within a sealed vessel. The water vapor thus produced is carried to a cooled condensation chamber, which controls the water vapor concentration and flow rate to the spectrometer. Integration of the isotope signals over the whole analytical cycle provides quantitative δ(18)O and δ(2) H values for the initial liquid water contained in the sample. Calibration is carried out by the analysis of water standards using the same apparatus. Analysis of leaf and soil water by cryogenic vacuum distillation and IRMS was used to validate the ME-IRIS data., Results: Comparison with data obtained by cryogenic distillation and IRMS shows that the new technique provides accurate water isotope data for leaves from a range of field-grown tropical plant species. However, two exotic nursery plants were found to suffer from spectral interferences from co-extracted organic compounds. The precision for extracted leaf, stem, soil and insect water was typically better than ±0.3 ‰ for δ(18)O and ±2 ‰ for δ(2) H values, and better than ±0.1 ‰ for δ(18)O and ±1 ‰ for δ(2) H values when analyzing water standards. The effects of sample size, microwave power and duration and sample-to-sample memory on isotope values were assessed., Conclusions: ME-IRIS provides rapid and low-cost extraction and analysis of δ(18)O and δ(2) H values in plant, soil and insect water (≈10-15 min for samples yielding ≈ 0.3 mL of water). The technique can accommodate whole leaves of many plant species., (Copyright © 2014 John Wiley & Sons, Ltd.)

It is commonly assumed that the nitrogen to phosphorus (N:P) ratio of a terrestrial plant reflects the relative availability of N and P in the soil in which the plant grows. Here, this was assessed for a tropical pioneer tree, Ficus insipida. Seedlings were grown in sand and irrigated with nutrient solutions containing N:P ratios ranging from <1 to >100. The experimental design further allowed investigation of physiological responses to N and P availability. Homeostatic control over N:P ratios was stronger in leaves than in stems or roots, suggesting that N:P ratios of stems and roots are more sensitive indicators of the relative availability of N and P at a site than N:P ratios of leaves. The leaf N:P ratio at which the largest plant dry mass and highest photosynthetic rates were achieved was approximately 11, whereas the corresponding whole-plant N:P ratio was approximately 6. Plant P concentration varied as a function of transpiration rate at constant nutrient solution P concentration, possibly due to transpiration-induced variation in the mass flow of P to root surfaces. The transpiration rate varied in response to nutrient solution N concentration, but not to nutrient solution P concentration, demonstrating nutritional control over transpiration by N but not P. Water-use efficiency varied as a function of N availability, but not as a function of P availability.

Based upon considerations of a theoretical model of (15)N/(14)N fractionation during steady-state nitrate uptake from soil, we hypothesized that, for plants grown in a common soil environment, whole-plant delta(15)N (deltaP) should vary as a function of the transpiration efficiency of nitrogen acquisition (F(N)/v) and the difference between deltaP and root delta(15)N (deltaP - deltaR). We tested these hypotheses with measurements of several tropical tree and liana species. Consistent with theoretical expectations, both F(N)/v and deltaP - deltaR were significant sources of variation in deltaP, and the relationship between deltaP and F(N)/v differed between non-N(2)-fixing and N(2)-fixing species. We interpret the correlation between deltaP and F(N)/v as resulting from variation in mineral nitrogen efflux-to-influx ratios across plasma membranes of root cells. These results provide a simple explanation of variation in delta(15)N of terrestrial plants and have implications for understanding nitrogen cycling in ecosystems.

The response of whole-plant water-use efficiency, termed transpiration efficiency (TE), to variation in soil fertility was assessed in a tropical pioneer tree, Ficus insipida Willd. Measurements of stable isotope ratios (delta(13)C, delta(18)O, delta(15)N), elemental concentrations (C, N, P), plant growth, instantaneous leaf gas exchange, and whole-plant water use were used to analyse the mechanisms controlling TE. Plants were grown individually in 19 l pots with non-limiting soil moisture. Soil fertility was altered by mixing soil with varying proportions of rice husks, and applying a slow release fertilizer. A large variation was observed in leaf photosynthetic rate, mean relative growth rate (RGR), and TE in response to experimental treatments; these traits were well correlated with variation in leaf N concentration. Variation in TE showed a strong dependence on the ratio of intercellular to ambient CO(2) mole fractions (c(i)/c(a)); both for instantaneous measurements of c(i)/c(a) (R(2)=0.69, P <0.0001, n=30), and integrated estimates based on C isotope discrimination (R(2)=0.88, P <0.0001, n=30). On the other hand, variations in the leaf-to-air humidity gradient, unproductive water loss, and respiratory C use probably played only minor roles in modulating TE in the face of variable soil fertility. The pronounced variation in TE resulted from a combination of the strong response of c(i)/c(a) to leaf N, and inherently high values of c(i)/c(a) for this tropical tree species; these two factors conspired to cause a 4-fold variation among treatments in (1-c(i)/c(a)), the term that actually modifies TE. Results suggest that variation in plant N status could have important implications for the coupling between C and water exchange in tropical forest trees.

Aim: Within C3 plants, photosynthesis is a balance between CO2 supply from the atmosphere via stomata and demand by enzymes within chloroplasts. This process is dynamic and a complex but crucial aspect of photosynthesis. We sought to understand the spatial pattern in CO2 supply–demand balance on a global scale, via analysis of stable isotopes of carbon within leaves (Δ13C), which provide an integrative record of CO2 drawdown during photosynthesis. LocationGlobal. Time period1951–2011. Major taxa studiedVascular plants. Methods: We assembled a database of leaf carbon isotope ratios containing 3,979 species–site combinations from across the globe, including 3,645 for C3 species. We examined a wide array of potential climate and soil drivers of variation in Δ13C. Results: The strongest drivers of carbon isotope discrimination at the global scale included atmospheric pressure, potential evapotranspiration and soil pH, which explained 44% of the variation in Δ13C. Addition of eight more climate and soil variables (each explaining small but highly significant amounts of variation) increased the explained variation to 60%. On top of this, the largest plant trait effect was leaf nitrogen per area, which explained 11% of Δ13C variation. Main conclusions: By considering variation in Δ13C at a considerably larger scale than previously, we were able to identify and quantify key drivers in CO2 supply–demand balance previously unacknowledged. Of special note is the key role of soil properties, with greater discrimination on low-pH and high-silt soils. Unlike other plant traits, which show typically wide variation within sets of coexisting species, the global pattern in carbon stable isotope ratios is much more conservative; there is relatively narrow variation in time-integrated CO2 concentrations at the site of carboxylation among plants in a given soil and climate.