Isotopic analyses of tree-rings and historical herbarium specimens have shown large increases in water use efficiency (WUE) and foliar C and reductions in N and 15N across major terrestrial biomes over the Anthropocene. These observations offer critical chemical and isotopic constraints to coupled C-N land model simulations of terrestrial C uptake. However, while observed changes are mostly attributed to CO2 fertilization, the degree to which water, C and N cycle responses have interacted over time has not been resolved. In particular, there remains large uncertainty about how changes in interactions among WUE, N availability, acquisition or assimilation have constrained C assimilation. Here, we analyze foliar C and N contents and stable isotope distributions of native grassland perennial plant species collected from 390 sites spanning broad climate and topographic gradients across Montana over the last 130 years. To control for changing local climate and resource conditions we resampled a fraction of the original herbarium collections sites in 2016. Our analyses focus on understanding the dynamics of ecosystem-level coordination of water, C and N cycles and their dependency on anthropogenic CO2 fertilization.
Results/Conclusions
Across sites and species, leaf-level WUE increased by ~40% and C content increased by ~10% while N and 15N declined by ~10% over the last 130 years. Interestingly, large declines in foliar N and 15N preceded increases in WUE by ~30 years. While internal CO2 and C content have both increased, the ratio of leaf C to internal CO2 has declined over time, suggesting a decrease in C assimilation efficiency. This inefficiency rate declined linearly as a function of increasing WUE but proportional declines were not related to foliar N or 15N. While declining foliar N and 15N and simultaneous increases in foliar C:N are consistent with a long-term decline in soil N availability, acquisition and/or assimilation, and thus with hypothesized drivers of progressive N limitation, our analysis suggests that the degree to which N has constrained C uptake is far from straightforward. We evaluate alternative hypotheses for the coordination, or lack thereof, of water, C, and N cycles by coupling physiological mechanisms of C and N assimilation operating at the leaf level with biogeochemical cycling at the ecosystem level.