Abstract
The rate of CO2 assimilation by plants is directly influenced by the concentration of CO2 in the atmosphere, ca. In response to a short-term change in ca, plants adjust stomatal conductance to CO2 and water vapour to maximise carbon gain in terms of the amount of water lost. This is one of several fundamental feedback processes between plants and their environment that govern the exchange of water for carbon. As an environmental variable, ca further has a unique global and historic significance. Although relatively stable and uniform in the short term, global ca has varied substantially on the timescale of thousands to millions of years, and currently is increasing at seemingly an unprecedented rate. This may exert profound impacts on both climate and plant function. Here we utilise extensive data sets and numerous models to develop an integrated, multi-scale assessment of the impact of changing ca on plant carbon dioxide uptake and water use. We find that, overall, the sensitivity of plants to rising or falling atmospheric CO2 concentration is qualitatively similar across all scales considered. It is characterised by an adaptive feedback response that moves towards maximising the rate of return, in the form of carbon, for the water and nitrogen resources invested in the process of carbon assimilation. This is achieved through predictable adjustments to stomatal anatomy and chloroplast biochemistry. Importantly, the long-term response to changing ca can be described by simple equations rooted in the formulation of more commonly studied short-term responses.
