Models of stomatal conductance (gs) are based on coupling between gs and CO2 assimilation (Anet), and it is often assumed that the slope of this relationship ('g1') is constant across species. However, if different plant species have adapted to different access costs of water, then there will be differences in g1 among species. We hypothesized that g1 should vary among species adapted to different climates, and tested the theory and its linkage to plant hydraulics using four Eucalyptus species from different climatic origins in a common garden.Optimal stomatal theory predicts that species from subhumid zones have a lower marginal water cost of C gain, hence lower g1 than humid-zone species. In agreement with the theory that g1 is related to tissue carbon costs for water supply, we found a relationship between wood density and g1 across Eucalyptus species of contrasting climatic origins. There were significant reductions in the parameter g1 during drought in humid but not sub-humid species, with the latter group maintaining g1 in drought. There are strong differences in stomatal behaviour among related tree species in agreement with optimal stomatal theory, and these differences are consistent with the economics involved in water uptake and transport for carbon gain.Key-words: drought; leaf gas exchange models; net photosynthesis; plant hydraulic conductance; stomatal optimization theory.Abbreviations: G, CO2 compensation point of Anet; G*, CO2 compensation point of Anet in the absence of dark respiration; l, unit marginal water cost of plant carbon assimilation; Yl, leaf water potential; Anet, rate of net photosynthetic CO2 assimilation; Ca, CO2 mole fraction in air at the leaf surface; Cst, CO2 mole fraction in the substomatal cavity; E, transpiration rate; D, leaf-air vapour pressure difference. D0, parameter for base level of the D response; g0, stomatal conductance when net photosynthesis is zero; g1, slope of the unified stomatal optimisation model; gs, stomatal conductance; KL, soil-to-leaf hydraulic conductance; kstem, stem specific hydraulic conductivity measured on branches; Q, quantum flux density inside the leaf cuvette.

Background and Aims Plant hydraulic traits influence the capacity of species to grow and survive in waterlimited environments, but their comparative study at a common site has been limited. The primary aim of this study was to determine whether selective pressures on species originating in drought-prone environments constrain hydraulic traits among related species grown under common conditions. Methods Leaf tissue water relations, xylem anatomy, stomatal behaviour and vulnerability to drought-induced embolism were measured on six Eucalyptus species growing in a common garden to determine whether these traits were related to current species climate range and to understand linkages between the traits. Key Results Hydraulically weighted xylem vessel diameter, leaf turgor loss point, the water potential at stomatal closure and vulnerability to drought-induced embolism were significantly (P < 0Á05) correlated with climate parameters from the species range. There was a coordination between stem and leaf parameters with the water potential at turgor loss, 12 % loss of conductivity and the point of stomatal closure significantly correlated. Conclusions The correlation of hydraulic, stomatal and anatomical traits with climate variables from the species' original ranges suggests that these traits are genetically constrained. The conservative nature of xylem traits in Eucalyptus trees has important implications for the limits of species responses to changing environmental conditions and thus for species survival and distribution into the future, and yields new information for physiological models.

Selecting plantation species to balance water use and production requires accurate models for predicting how species will tolerate and respond to environmental conditions. Although interspecific variation in water use occurs, species-specific parameters are rarely incorporated into physiologically based models because often the appropriate species parameters are lacking. To determine the physiological control over water use in Eucalyptus, five stands of Eucalyptus species growing in a common garden were measured for sap flux rates and their stomatal response to vapour pressure deficit (D) was assessed. Maximal canopy conductance and whole-canopy stomatal sensitivity to D and reduced water availability were lower in species originating from more arid climates of origin than those from humid climates. Species from humid climates showed a larger decline in maximal sap flux density (JSmax) with reduced water availability, and a lower D at which stomatal closure occurred than species from more arid climates, implying larger sensitivity to water availability and D in these species. We observed significant (P < 0.05) correlations of species climate of origin with mean vessel diameter (R(2) = 0.90), stomatal sensitivity to D (R(2) = 0.83) and the size of the decline in JSmax to restricted water availability (R(2) = 0.94). Thus aridity of climate of origin appears to have a selective role in constraining water-use response among the five Eucalyptus plantation species. These relationships emphasize that within this congeneric group of species, climate aridity constrains water use. These relationships have implications for species choices for tree plantation success against drought-induced losses and the ability to manage Eucalyptus plantations against projected changes in water availability and evaporation in the future.

While trees can acclimate to warming, there is concern that tropical rainforest species may be less able to acclimate because they have adapted to a relatively stable thermal environment. Here we tested whether the physiological adjustments to warming differed among Australian tropical, subtropical and warm-temperate rainforest trees.Photosynthesis and respiration temperature responses were quantified in six Australian rainforest seedlings of tropical, subtropical and warm-temperate climates grown across four growth temperatures in a glasshouse. Temperature-response models were fitted to identify mechanisms underpinning the response to warming.Tropical and subtropical species had higher temperature optima for photosynthesis (T optA ) than temperate species. There was acclimation of T optA to warmer growth temperatures. The rate of acclimation (0.35-0.78°C °C-1 ) was higher in tropical and subtropical than in warm-temperate trees and attributed to differences in underlying biochemical parameters, particularly increased temperature optima of V cmax25 and J max25 . The temperature sensitivity of respiration (Q 10 ) was 24% lower in tropical and subtropical compared with warmtemperate species.Overall, tropical and subtropical species had a similar capacity to acclimate to changes in growth temperature as warm-temperate species, despite being grown at higher temperatures. Quantifying the physiological acclimation in rainforests can improve accuracy of future climate predictions and assess their potential vulnerability to warming.