Seasonal distribution of net primary production by functional groups in Chihuahuan Desert, and the role of seasonal precipitation

Background/Question/Methods

In hot deserts, precipitation is the principal driver for net primary production.  This study tested two hypotheses regarding aboveground net primary production (ANPP) and the effects of precipitation on ANPP in the Chihuahuan Desert, with emphasis on differences among seasons and among functional groups of species. First, we hypothesized that the majority of annual ANPP occurs in the monsoon season.  Second, we hypothesized that ANPP is positively correlated to precipitation. 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.  The ANPP and precipitation data were collected at 15 locations in 5 ecosystem types: 2 grassland types and 3 shrubland types (N=3 for each type) during 1990-2010.  Three seasons were defined: winter (Oct – Feb), spring (Mar – May), and fall (Jun- Sep).  ANPP was separated into three plant functional groups: C3 shrubs, C4 perennial grasses, and other plants (C3 and C4 annual forbs and grasses, and perennial forbs).

Results/Conclusions

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%).  Our second hypothesis was supported for some, but not all, functional groups and seasons. For example, in grasslands in general, fall perennial grass production was not related to fall precipitation. In shrublands winter and spring shrub production were high (winter: 5-21% of annual ANPP, spring: 14-42%, fall: 20-33%), but they were not related to precipitation.  In short, there were significant differences between desert grasslands and desert shrublands in terms of seasonal distribution of ANPP and response of ANPP to seasonal precipitation.  This provides important information for understanding and projecting the impact of global climate change (e.g., change in seasonality, intensity, and frequency of precipitation) on structure and function of desert ecosystems.