Significance

Changes in tree physiology driven by environmental change can alter the balance of forest ecosystem carbon and water fluxes. We performed a meta-analysis of published tree ring literature, comprising 36 different species across 84 sites globally, to show stimulated leaf photosynthesis, not reduced stomatal conductance, is primarily responsible for recently increasing tree intrinsic water use efficiency (iWUE), which integrates the balance between carbon and water fluxes. Furthermore, we show trends in tree iWUE are similar in magnitude to the increase in atmospheric CO₂ over the 20th century and that climate interacts with CO₂ to modulate tree iWUE. These findings will help to guide efforts of refining the role of forests in process-based models under future environmental change.

Abstract

We conducted a meta-analysis of carbon and oxygen isotopes from tree ring chronologies representing 34 species across 10 biomes to better understand the environmental drivers and physiological mechanisms leading to historical changes in tree intrinsic water use efficiency (iWUE), or the ratio of net photosynthesis (A_net) to stomatal conductance (g_s), over the last century. We show a ∼40% increase in tree iWUE globally since 1901, coinciding with a ∼34% increase in atmospheric CO₂ (C_a), although mean iWUE, and the rates of increase, varied across biomes and leaf and wood functional types. While C_a was a dominant environmental driver of iWUE, the effects of increasing C_a were modulated either positively or negatively by climate, including vapor pressure deficit (VPD), temperature, and precipitation, and by leaf and wood functional types. A dual carbon–oxygen isotope approach revealed that increases in A_net dominated the observed increased iWUE in ∼83% of examined cases, supporting recent reports of global increases in A_net, whereas reductions in g_s occurred in the remaining ∼17%. This meta-analysis provides a strong process-based framework for predicting changes in tree carbon gain and water loss across biomes and across wood and leaf functional types, and the interactions between C_a and other environmental factors have important implications for the coupled carbon–hydrologic cycles under future climate. Our results furthermore challenge the idea of widespread reductions in g_s as the major driver of increasing tree iWUE and will better inform Earth system models regarding the role of trees in the global carbon and water cycles.

The full (paywalled) article appeared on the PNAS website at https://www.pnas.org/content/118/7/e2014286118