Spring drought regulates summer net ecosystem CO₂ exchange in a sagebrush-steppe ecosystem

Climate, as reflected by seasonal variations in precipitation and temperature, plays a critical role in ecosystem productivity and composition in the Western US and other arid regions. This study was conducted in the sagebrush-steppe ecosystem co-dominated by mountain big sagebrush (Artemisia tridentata var. vaseyana) and perennial grasses in south-central Wyoming across two growing seasons (2004 and 2005). A dry spring in 2004 and a wet spring in 2005 provided an opportunity to evaluate the influence of precipitation timing on the magnitudes and patterns of net ecosystem CO₂ exchange (NEE) and its environmental drivers. Summer weather conditions (light, temperature, and vapor pressure deficit) for the 2 years were similar but deep soil moisture content was lower in 2004. Daily rates of NEE demonstrated a net carbon source in 2004 and a net carbon sink in 2005. Midday depression of NEE frequently occurred in both measurement years (June-August in 2004 and mid July-August in 2005) due to stomatal control restricting water loss from the system under low soil moisture and high temperature and vapor pressure deficit conditions. Across different soil moisture regimes, the controlling factors on NEE differed. Under water limitation, soil water availability (soil drought) was the main driving factor of growing season NEE regardless of weather conditions while vapor pressure deficit (atmospheric drought) was the main driver of NEE when the ecosystem was not limited by soil moisture. Nighttime NEE showed strong non-linear relationship with soil moisture but no relationship with soil temperature, demonstrating that respiratory processes in the sagebrush-steppe ecosystem were limited by soil moisture during summer. The dynamic response of NEE to two summer seasons with dissimilar spring precipitation indicates that intra-seasonal variability in precipitation and subsequent impact on deep soil moisture should be taken into consideration to explain magnitudes and patterns of NEE at diurnal to seasonal time scales.