How does precipitation control primary production at Jornada Basin LTER?

This study tested two hypotheses regarding pattern and control of aboveground net primary production (ANPP) in the Chihuahuan Desert, with emphases on seasonality and plant functional group.  First, we hypothesized that the majority of annual ANPP occurs in the monsoon season.  Second, we hypothesized that seasonal precipitation controls concurrent seasonal ANPP. The study site, Jornada Basin USDA-LTER site, is located in the northern Chihuahuan Desert where Jul-Sep is the monsoon season during which 53% of annual rainfall occurs.  Since 1990 the ANPP and precipitation data have been collected at 15 locations in 5 ecosystem types: upland and playa grasslands, creosotebush, mesquite, and tarbush shrublands.  Three seasons were defined: winter (Oct – Feb), spring (Mar – May), and fall (Jun- Sep).  ANPP was separated into three plant functional groups: perennial grasses, shrubs, and other plants.  Our results supported the first hypothesis for grasslands, but not for shrublands. On average, 67% of annual ANPP in grasslands occurred in fall (range: 62-73%); however, only 49% of annual ANPP in shrublands occurred in fall (range: 41-58%).  The second hypothesis was supported in less than half of the cases (locations × ecosystem types × functional groups × seasons). The critical precipitation period for seasonal ANPP varied among ecosystem types and among locations within each type, and between functional groups. These results suggest that other factors, such as temperature and soil water content, may also drive seasonal variation in ANPP in deserts.  Our study provides important information for understanding and projecting the impact of global climate change on structure and function of desert ecosystems.