COS 241-5 - Air pollution exerts stronger controls than climate on intrinsic water use efficiency in two broadleaf deciduous tree species in the eastern US

Background/Question/Methods

Trees continuously regulate leaf physiology to maximize carbon gains while simultaneously minimizing unavoidable water loss. The balance between these two processes, or water use efficiency (WUE), is fundamentally important to understanding changes in carbon uptake and transpiration from the leaf to the globe under increasing atmospheric CO₂ (iCO₂). Yet, additional environmental factors can modify the response of WUE to iCO₂ through independent effects on carbon assimilation or stomatal conductance. Here, we couple annually-resolved long-term records of tree ring carbon isotope signatures with contemporary leaf physiological measurements of Quercus rubra and Liriodendron tulipifera in the eastern United States to reconstruct historical leaf physiology. We use a combination of linear mixed-effects models, generalized additive models, and hierarchical partitioning to ask the following questions: 1) Has the iWUE of Quercus rubra and Liriodendron tulipifera in the Central Appalachian Mountains changed since the mid-20th century? 2) If so, are changes in iWUE attributable to changes in A_net or g_s? and 3) Which environmental factors are most important in driving trends and interannual variability in tree iWUE?

Results/Conclusions

We first show 16 – 25% increases in tree intrinsic water use efficiency (iWUE), or the ratio of net photosynthesis (A_net) to stomatal conductance to water (g_s), across both species since the mid-20th century. We find iCO₂ to be one of the primary drivers of increasing iWUE, but also highlight the individual and interactive effects of nitrogen (NO_x) and sulfur (SO₂) air pollution overwhelming climate at four study locations spanning nearly 100 km. Reconstructions of seasonally-integrated A_net and g_s revealed a ca. 43 – 50% stimulation of A_net was responsible for increasing iWUE in both tree species throughout ca. 79 – 86% of the chronologies with reductions in g_s attributable to the remaining 14 – 21%. Our findings place into context the importance of considering local environmental conditions, including air pollution, on leaf physiology, and build upon a growing body of literature documenting stimulated A_net overwhelming reductions in g_s as a primary mechanism of increasing iWUE of trees.