Understanding the Associated Effects of Temperature and Soil Moisture on Summer Carbon Fluxes of Forest Ecosystems in the Contiguous United States

Individual effects of temperature and soil moisture on plants are well studied. Higher temperature up to an optimal value increases Gross Primary Productivity (GPP). Ecosystem Respiration (ER) follows increases with warming following a Q10 functionality With increasing soil moisture, both GPP and ER increase until the soil gets saturated. However, the associated effects of temperature and soil moisture on forest ecosystems are more complex. More than 100 years of gridded temperature, precipitation and evapotranspiration data are used to produce a soil water deficit index (P-E index). The eddy covariance flux measurements in forest ecosystems are used to provide summer carbon fluxes data (GPP, ER and NEP). The correlations between carbon fluxes and temperature or soil moisture are categorized as: 1) negative, negative; 2) negative, positive; 3) positive, negative; and 4) positive, positive. Based on the locations and description of each forest, we propose the following mechanisms to explain the respective correlations: 1) soil moisture limits GPP or ER; 2) temperature surpasses the optimal GPP or ER; 3) GPP or ER change as expected; and 4) temperature dominates GPP or ER. The NEP is determined by the direction and magnitude of changes in GPP relative to ER.  Based on 7 eddy covariance sites, we have found that eastern sites tend to be either as expected or temperature limited, while western sites are more moisture limited or above optimal temperature.  In the majority of sites, NEP decreases with both warming and drought.  This study will help us better understand the effect of future extremes on carbon fluxes.