Summary Recent decades have been characterized by increasing temperatures worldwide, resulting in an exponential climb in vapor pressure deficit (VPD). VPD has been identified as an increasingly important driver of plant functioning in terrestrial biomes and has been established as a major contributor in recent drought‐induced plant mortality independent of other drivers associated with climate change. Despite this, few studies have isolated the physiological response of plant functioning to high VPD, thus limiting our understanding and ability to predict future impacts on terrestrial ecosystems. An abundance of evidence suggests that stomatal conductance declines under high VPD and transpiration increases in most species up until a given VPD threshold, leading to a cascade of subsequent impacts including reduced photosynthesis and growth, and higher risks of carbon starvation and hydraulic failure. Incorporation of photosynthetic and hydraulic traits in ‘next‐generation’ land‐surface models has the greatest potential for improved prediction of VPD responses at the plant‐ and global‐scale, and will yield more mechanistic simulations of plant responses to a changing climate. By providing a fully integrated framework and evaluation of the impacts of high VPD on plant function, improvements in forecasting and long‐term projections of climate impacts can be made.

Based on measurements of δO and δC in organic matter of C-plants, we have developed a conceptual model that gives insight into the relationship between stomatal conductance (g ) and photosynthetic capacity (A) resulting from differing environmental constraints and plant-internal factors. This is a semi-quantitative approach to describing the long-term effects of environmental factors on CO and HO gas exchange, whereby we estimate the intercellular CO concentration (c ) from δC and the air humidity from δO. Assuming that air humidity is an important factor influencing g , the model allows us to distinguish whether differences in c are caused by a response of g or of A. As an application of the model we evaluated the isotope data from three species in plots differing in intensity of land use (hay meadows and abandoned areas) at three sites along a south north transect in the Eastern Alps. We found three different δO-δC response patterns in native and planted grassland species (cultivated in the greenhouse). After preliminary confirmation by gas-exchange measurements we conclude that the proposed model is a promising tool for deriving carbon water relations in different functional groups from δO and δC isotope data.

We investigated the response of conifer trees in northern Eurasia to climate change and increasing CO 2 over the last century by measuring the carbon isotope ratio in tree rings. Samples from Larix, Pinus and Picea trees growing at 26 high-latitude sites (59-711N) from Norway to Eastern Siberia were analysed. When comparing the periods 1861-1890 and 1961-1990, the isotope discrimination and the ratio of the intercellular to ambient CO 2 concentration (c i /c a ) remained constant for trees growing in mild oceanic climate and under extremely cold and dry continental conditions. This shows a strong coordination of gas-exchange processes, consisting in a biochemical acclimation and a reduction of the stomatal conductance. The correlation for c i /c a between the two investigated periods was particularly strong for Larix (r 2 5 0.90) and Pinus (r 2 5 0.94), but less pronounced for Picea (r 2 5 0.47). Constant c i /c a under increasing CO 2 in the atmosphere resulted in improved intrinsic water-use efficiency (W i ), the amount of water loss at the leaf level per unit carbon gain. We found that 125 out of 126 trees showed increasing W i from 1861 to 1890 to 1961 to 1990, with an average improvement of 19.2 AE 0.9% (mean AE SE). The adaptation in gas exchange and reduced transpiration of trees growing in this region must have had a strong impact on the water and energy budget, resulting in a drier and warmer surface air layer today than would exist without this vegetation-climate feedback.

Whether rising atmospheric carbon dioxide (CO2) concentrations will cause forests to grow faster and store more carbon is an open question. Using free air CO2 release in combination with a canopy crane, we found an immediate and sustained enhancement of carbon flux through 35-meter-tall temperate forest trees when exposed to elevated CO2. However, there was no overall stimulation in stem growth and leaf litter production after 4 years. Photosynthetic capacity was not reduced, leaf chemistry changes were minor, and tree species differed in their responses. Although growing vigorously, these trees did not accrete more biomass carbon in stems in response to elevated CO2, thus challenging projections of growth responses derived from tests with smaller trees.

Soil respiration constitutes the second largest flux of carbon (C) between terrestrial ecosystems and the atmosphere. This study provides a synthesis of soil respiration (R s ) in 20 European grasslands across a climatic transect, including ten meadows, eight pastures and two unmanaged grasslands. Maximum rates of R s (R s max ), R s at a reference soil temperature (10°C; R s 10 ) and annual R s (estimated for 13 sites) ranged from 1.9 to 15.9 μmol CO 2 m −2 s −1 , 0.3 to 5.5 μmol CO 2 m −2 s −1 and 58 to 1988 g C m −2 y −1 , respectively. Values obtained for Central European mountain meadows are amongst the highest so far reported for any type of ecosystem. Across all sites R s max was closely related to R s 10 .Assimilate supply affected R s at timescales from daily (but not necessarily diurnal) to annual.Reductions of assimilate supply by removal of aboveground biomass through grazing and cutting resulted in a rapid and a significant decrease of R s . Temperature-independent seasonal fluctuations of R s of an intensively managed pasture were closely related to changes in leaf area index (LAI). Across sites R s 10 increased with mean annual soil temperature (MAT), LAI and gross primary productivity (GPP), indicating that assimilate supply overrides potential acclimation to prevailing temperatures. Also annual R s was closely related to LAI and GPP. Because the latter two parameters were coupled to MAT, temperature was a suitable surrogate for deriving estimates of annual R s across the grasslands studied. These findings contribute to our understanding of regional patterns of soil C fluxes and highlight the importance of assimilate supply for soil CO 2 emissions at various timescales.