No abstract

The terrestrial carbon and water cycles are intimately linked: the carbon cycle is driven by photosynthesis, while the water balance is dominated by transpiration, and both fluxes are controlled by plant stomatal conductance. The ratio between these fluxes, the plant water-use efficiency (WUE), is a useful indicator of vegetation function. WUE can be estimated using several techniques, including leaf gas exchange, stable isotope discrimination, and eddy covariance. Here we compare global compilations of data for each of these three techniques. We show that patterns of variation in WUE across plant functional types (PFTs) are not consistent among the three datasets. Key discrepancies include the following: leaf-scale data indicate differences between needleleaf and broadleaf forests, but ecosystem-scale data do not; leaf-scale data indicate differences between C and C species, whereas at ecosystem scale there is a difference between C and C crops but not grasslands; and isotope-based estimates of WUE are higher than estimates based on gas exchange for most PFTs. Our study quantifies the uncertainty associated with different methods of measuring WUE, indicates potential for bias when using WUE measures to parameterize or validate models, and indicates key research directions needed to reconcile alternative measures of WUE.

The response of photosynthesis to temperature is a central facet of plant response to climate. Such responses have been found to be highly variable among species and among studies. Understanding this variability is key when trying to predict the effects of rising global temperatures on plant productivity. There are three major factors affecting the response of leaf net photosynthesis to temperature (A(n)-T): (i) photosynthetic biochemistry, (ii) respiration and (iii) vapour pressure deficit (D) and stomatal sensitivity to vapour pressure deficit during measurements. The overall goal of our study was to quantify the relative contribution of each of these factors in determining the response of A(n) to temperature. We first conducted a sensitivity analysis with a coupled photosynthesis-stomatal (A(n)-g(s)) model, using ranges for parameters of each factor taken from the literature, and quantified how these parameters affected the A(n)-T response. Second, we applied the A(n)-g(s) model to two example sets of field data, which had different optimum temperatures (T(opt)) of A(n), to analyse which factors were most important in causing the difference. We found that each of the three factors could have an equally large effect on T(opt) of A(n). In our comparison between two field datasets, the major cause for the difference in T(opt) was not the biochemical component, but rather the differences in respiratory components and in D conditions during measurements. We concluded that shifts in A(n)-T responses are not always driven by acclimation of photosynthetic biochemistry, but can result from other factors. The D conditions during measurements and stomatal responses to D also need to be quantified if we are to better understand and predict shifts in A(n)-T with climate.

Leaf transpiration rate (E) frequently shows a peaked response to increasing vapour pressure deficit (D). The mechanisms for the decrease in E at high D, known as the 'apparent feedforward response', are strongly debated but explanations to date have exclusively focused on hydraulic processes. However, stomata also respond to signals related to photosynthesis. We investigated whether the apparent feed-forward response of E to D in the field can be explained by the response of photosynthesis to temperature (T), which normally co-varies with D in field conditions. As photosynthesis decreases with increasing T past its optimum, it may drive a decrease in g s that is additional to the response of g s to increasing D alone. If this additional decrease is sufficiently steep and coupling between A and g s occurs, it could cause an overall decrease in E with increasing D. We tested this mechanism using a gas exchange model applied to leaf-scale and whole-tree CO 2 and H 2 O fluxes measured on Eucalyptus saligna growing in whole-tree chambers. A peaked response of E to D was observed at both leaf and whole-tree scales. We found that this peaked response was matched by a gas exchange model only when T effects on photosynthesis were incorporated. Furthermore, at elevated [CO 2 ], E peaked at higher D. We hypothesize thatcould be explained by an increase in the T optimum for A, as frequently observed, however we found no support for a higher T optimum for A in elevated [CO 2 ] in this study. We conclude that field-based studies of the relationship between E and D need to consider signals related to changing photosynthesis in addition to purely hydraulic mechanisms.