Sensitivity of plants to changing atmospheric CO 2 concentration: from the geological past to the next century

The rate of CO 2 assimilation by plants is directly influenced by the concentration of CO 2 in the atmosphere, c a . As an environmental variable, c a also has a unique global and historic significance. Although relatively stable and uniform in the short term, global c a 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 datasets and models to develop an integrated, multi‐scale assessment of the impact of changing c a on plant carbon dioxide uptake and water use. We find that, overall, the sensitivity of plants to rising or falling c a is qualitatively similar across all scales considered. It is characterised by an adaptive feedback response that tends to maintain 1 − c i / c a , the relative gradient for CO 2 diffusion into the leaf, relatively constant. This is achieved through predictable adjustments to stomatal anatomy and chloroplast biochemistry. Importantly, the long‐term response to changing c a can be described by simple equations rooted in the formulation of more commonly studied short‐term responses. Contents Summary 1077 I. Introduction 1078 II. Atmospheric CO 2 concentrations through time 1079 III. Plant sensitivity to CO 2 at geological timescales 1080 IV. Plant sensitivity to CO 2 over the last 200 yr 1081 V. Increased water‐use efficiency? 1084 VI. Simple formulation of stomatal conductance in land surface models for simulating long‐term CO 2 response 1087 VII. Conclusions 1088 Acknowledgements 1089 References 1089