Land–atmosphere carbon and water flux relationships to vapor pressure deficit, soil moisture, and stream flow

• We analyzed H2O and CO2 fluxes from a forest in the northern Rocky Mountains.
• Weekly, growing season NEP was correlated with soil moisture and stream flow.
• Weekly, growing season WUE was correlated with soil moisture and VPD.
• Despite water limitations, annual NEP was positively correlated with GSL.
• A warmer, drier climate could shift the GSL–NEP relationship.

Climatic change is exerting considerable influence on the hydrologic and biogeochemical cycles of snow- dominated montane forest ecosystems. Growing season drought stress is a common occurrence after snowmelt-derived soil water content (WC) and stream flow (Q) have declined, leading to an increase in atmospheric water demand (i.e., vapor pressure deficit, VPD). Here, we analyzed a 6-year record (2006–2011) of H2O and CO2 fluxes from the Tenderfoot Creek Experimental Forest, a montane forest in the northern Rocky Mountains to examine (1) how growing season evapotranspiration (ET), net ecosystem production (NEP), and water-use efficiency (WUE, NEP/ET) respond to changing WC and VPD, (2) how stream flow (Q), an integrated measure of catchment-level water availability, relates to NEP, and (3) how annual NEP is related to annual precipitation and the temperature-defined growing season length (GSL). Growing season NEP exhibited a linear relationship with WC and a log-linear relationship with Q, indicative of persistent water limitations when streamflow and soil moisture reach their annual minima late in the growing season. Nevertheless, years with long GSLs had relatively higher NEP, with a small net carbon sink maintained even at low levels of WC and Q, suggesting that trees are able to obtain water from deeper portions of the soil profile (>30 cm) during droughts. However, the warmer, drier climate projected for this region could bring this system closer to a critical threshold of GSL, WC, and VPD, introducing vegetation water stress that could alter the current relationship between GSL and annual NEP.