Vapor Pressure Deficit, Soil Moisture, and Stream Flow Relationships to Land-Atmosphere Fluxes of Water and Carbon.

Abstract:

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 demands (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 of carbon (NEP), and water use efficiency (WUE_NEP/ET) respond to changing WC and VPD, (2) how annual NEP is influenced by annual precipitation and growing season length (GSL), and (3) the relationship of stream flow (Q), a catchment-wide indicator of water availability, to NEP and WUE_NEP/ET. 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.