Seasonal precipitation and soil moisture recharge as competing drivers of carbon and water fluxes across a gradient of semi-arid ecosystems

Abstract:

Ecosystems of the southwestern U.S. receive precipitation in two prevailing weather patterns, the convective monsoon storms prevalent in late summer months, and synoptic-scale storm systems that are most frequent in late fall and winter. We examined a seven year record of eddy covariance data from six ecosystem types along an elevation/aridity gradient in the southwestern U.S. to determine the degree to which CO₂ and H₂O fluxes correlate with seasonal and interannual patterns in precipitation and soil water availability. Our results show that the majority of total annual primary production (GPP), ecosystem respiration (R_eco), and evapotranspiration (ET) occurred during the monsoon season at most sites and in most years. During spring, typically the driest period in this region, the sites frequently had high rates of GPP and comparatively low R_eco and ET, meaning this time period is important for annual carbon sequestration. Correlation between seasonal productivity measures and precipitation are generally weak, suggesting that some productivity peaks, such as the one in spring, are supported by stored soil water. We estimated the size of available soil water pools at multiple depths using extensive measurements of soil volumetric water content. Preliminary findings show strong relationships between seasonal CO₂ and H₂O fluxes and estimated available water in deeper soil layers. Our work indicates that relatively infrequent soil water recharge events are key to understanding the temporal distribution of ecosystem CO₂ and H₂O fluxes in the southwestern U.S., and potentially in other semi-arid ecosystems.