Multiple lines of evidence point to increasing tree intrinsic water use efficiency (iWUE), or the ratio of net photosynthesis (Anet) to stomatal conductance (gs), over the twentieth century. Rising atmospheric CO2 (Ca) is often directly linked to increasing iWUE, although whether this predominantly occurs through stimulated Anet or reduced gs has remained elusive. Moreover, whether Ca interacts with climate, and if tree physiological responses differ among functional types remains an open question. An emerging technique of coupling isotopically derived estimates of iWUE with oxygen isotope leaf water enrichment above source water (∆18Olw; derived from tree ring δ18O) allows a qualitative attribution of changes in iWUE to underlying Anet and gs, as ∆18Olw and gs are inversely related. In this study we synthesized published data from tree ring carbon and oxygen isotope chronologies to examine global trends in tree ring derived iWUE, used linear mixed effects (LME) models to identify those factors that best explain the multi-decadal to centurial trends in iWUE, and investigate the potential underlying changes in Anet and gs through an analysis of coupled tree ring derived iWUE and ∆18Olw over time.
Results/Conclusions
Using 113 unique tree ring carbon and oxygen isotope chronologies, comprising 36 unique species across 84 sites globally, we show tree-level iWUE increased, on average, by ca. 40% over the twentieth century. We identified a breakpoint in the combined iWUE chronology at 1963, after which iWUE increased linearly at a rate of 1.67% year-1, approximately 3.9 times faster than the previous 63 years. Across all species and study locations since 1963, LME model results indicated increasing iWUE was strongly, positively related to Ca and growing season vapor pressure deficit and was negatively related to growing season precipitation. Of all chronologies examined since the 1963 breakpoint, we found 18.6% showed decreasing ∆18Olw, 71.7% showed constant ∆18Olw, and 9.7% showed increasing ∆18Olw, despite consistent increases in iWUE. This points to enhanced Anet overwhelming increases in gs where ∆18Olw declined, as well as driving increasing iWUE under constant gs where ∆18Olw remained constant, such that increasing iWUE was driven by stimulated Anet in ca. 90% of chronologies examined. These data highlight the complexity of tree responses to environmental change and reinforce the importance of increased photosynthesis, and not reductions in leaf gs, as the primary diver in global increases in iWUE.