Effects of climate variability and N limitation in a desert grassland

Aridland ecosystems are strongly limited by water availability. The unimodal model of productivity and species diversity predicts that diversity will increase with productivity in low productivity systems such as desert grasslands. We analyzed the relationship between aboveground net primary production (ANPP), species diversity and climate variability in a 10-yr long nitrogen fertilization experiment in low productivity desert grassland at the Sevilleta LTER site in central New Mexico. We hypothesized that diversity and ANPP would increase in high rainfall years, but ANPP and diversity would be similar in fertilized and control plots in years of average or below average rainfall. To test this hypothesis we measured species composition and ANPP during 2004-2006 in four 1m2 subplots located in each of twenty 5x10m2 plots ten of which were fertilized with 10gNm-2yr-1. Fertilization treatments started in 1995. Seasonal and annual rainfall differed among years: non-monsoon precipitation was 155, 178, and 29 mm and monsoon precipitation was 149, 143, and 196 mm in 2004-2006, respectively. In 2004, ANPP was significantly higher on fertilized (107.9 gm-2) compared to control plots (77.3gm-2), yet there were no significant differences in species diversity in fertilized (12.8m-2) or control (12.4m-2) plots. There were no differences in ANPP (49.6 vs. 48.2 gm2) or species diversity (10.2 vs. 10.5) in fertilized and control plots in 2005. The 2006 growing season experienced very low winter/spring rainfall and above average monsoon rainfall. In response, ANPP was more than double on fertilized (253 g/m2) compared to control plots (122 g/m2) and diversity was 11.4 vs 12.5 per m2. These results show that (1) slight changes in the amount and seasonality of precipitation have strong impacts on ANPP, (2) ANPP is strongly N limited in periods with high soil water availability, but (3) species diversity and ANPP are generally decoupled in this aridland ecosystem.