Increased efficiency of water use does not stimulate tree productivity

Rising concentrations of atmospheric CO<inf>2</inf> (c<inf>a</inf>) increase plant photosynthesis (A<inf>n</inf>) and reduce stomatal conductance (g<inf>s</inf>). This increases the intrinsic water-use efficiency (iWUE = A<inf>n</inf>/ g<inf>s</inf>), a major proxy of tree adaptation to climate change. However, whether an increase in iWUE leads to a concomitant increase in tree growth remains in dispute, prompting interest in theoretical links between iWUE and tree productivity. Here using an optimality theory for kinetics of stomatal aperture, we establish an envelope delineating maximal relative increases in tree productivity that can be inferred/expected from relative increases in iWUE. The resulting expressions are used to interpret relations between iWUE (an observable proxy) and tree growth (the target variable), using available experimental data from manipulation experiments and tree-ring isotopes. While rising c<inf>a</inf> increases iWUE, proportional increases in tree growth are unlikely given ameliorating environmental (for example, rising atmospheric dryness) and anatomical/physiological (for example, tree height) influences.

Nature Climate Change

Nature Climate Change

16

1758-6798

N/A

N/A

18