Abstract
Plant transpiration is a key component of the terrestrial water cycle, and it is important to understand whether rates are likely to increase or decrease in the future. Plant transpiration rates are affected by biophysical factors, such as air temperature, vapour pressure deficits and net radiation, and by plant factors, such as canopy leaf area and stomatal conductance. Under future climate change, global temperature increases, and associated increases in vapour pressure deficits, will act to increase canopy transpiration rates. Increasing atmospheric CO2 concentrations, however, is likely to lead to some reduction in stomatal conductance, which will reduce canopy transpiration rates. The objective of the present paper was to quantitatively compare the importance of these opposing driving forces. First, we reviewed the existing literature and list a large range of observations of the extent of decreasing stomatal conductance with increasing CO2 concentrations. We considered observations ranging from short-term laboratory-based experiments with plants grown under different CO2 concentrations to studies of plants exposed to the naturally increasing atmospheric CO2 concentrations. Using these empirical observations of plant responses, and a set of well-tested biophysical relationships, we then estimated the net effect of the opposing influences of warming and CO2 concentration on transpiration rates. As specific cases studies, we explored expected changes in greater detail for six specific representative locations, covering the range from tropical to boreal forests. For most locations investigated, we calculated reductions in daily transpiration rates over the twenty-first century that became stronger under higher atmospheric CO2 concentrations. It showed that the effect of CO2-induced reduction of stomatal conductance would have a stronger transpiration-depressing effect than the stimulatory effect of future warming. For currently cold regions, global warming would, however, lengthen the growing seasons so that annual sums of transpiration could increase in those regions despite reductions in daily transpiration rates over the summer months.
Highlights
Various data compilations have shown that stomatal conductance is typically reduced by about 40% under doubled [CO2] [32–36]
Our analysis indicated that for most sites, reductions in stomatal conductance driven by increases in atmospheric [CO2] are likely to be of greater importance in determining changes in transpiration rates than increases in temperature and associated vapour pressure deficits (VPDs)
They point to reduced transpiration rates into the future for most parts of the world because for most locations and under most representative concentration pathways (RCPs), stomatal closure under elevated [CO2] has a quantitatively larger effect in reducing transpiration rates than the effect of increasing temperature in increasing transpiration rates
Summary
Climate change is well recognised as an important environmental change that will shape our future. While most attention has been focused on the radiative consequences of increasing [CO2] in the atmosphere [1], This article is part of the Topical Collection on Modelling Productivity and Function [CO2] has direct effects on plant growth and function [2–4]. Under water-limited conditions, relative plant growth responses to elevated [CO2] can potentially be even greater because increases in photosynthesis and decreases in stomatal conductance can together enhance water use efficiency to a numerically greater extent than the photosynthetic enhancement alone This has led to the theoretical consideration that water plants grown with a limited water supply should respond more strongly to elevated [CO2] than plants grown with adequate soil moisture [10]
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