Soil moisture limitation of high-elevation aboveground primary productivity mediated by nitrogen availability

Background/Question/Methods Moisture availability is highly variable in high-elevation ecosystems, and is recognized as an influential factor in their structure and function. But, moisture constraints on high-elevation primary productivity have never been demonstrated across a broad range of sites. To address this issue, we compiled data from a global set of nine high-elevation studies to examine if and when aboveground primary productivity is limited by soil moisture.

Results/Conclusions Our results show that soil moisture limits high-elevation aboveground primary productivity consistently across space, time, and vegetative communities. There is a high probability (99%) that this relationship is represented best with a nonlinear, asymptotic Michaelis-Menten function. A linear regression also fit the data and was highly significant (P < 0.001), but had a much lower probability of being the best model (1%). We tested two competing hypotheses for the mechanisms behind this response – a direct "biophysical" limitation and an indirect "biogeochemical" limitation due to nitrogen availability – along a soil moisture gradient at Niwot Ridge, CO. We found that soil inorganic nitrogen availability increased significantly with soil moisture (P = 0.02). The response was significantly reflected in aboveground vegetative nitrogen (P = 0.029). This suggests that soil moisture limits high-elevation aboveground primary productivity indirectly through its influence on nitrogen cycling. Our findings have consequences for ecosystem responses to interannual climate variability and directional climate change. Predicted decreases in soil moisture are likely to substantially decrease the aboveground primary productivity of high-elevation ecosystems.