The effect of resource addition on plant community structure in desert grassland.

Aridland ecosystems are strongly limited by water availability and potentially by nitrogen availability, as well. 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 long-term nitrogen fertilization experiment in low productivity desert grassland at the Sevilleta. We are testing the hypothesis that diversity and ANPP will increase with N addition in high rainfall years, but ANPP and diversity will be similar in fertilized and control plots in years of average or below average rainfall. To test this hypothesis we measure species composition and ANPP annually starting in 2004 in four 1-m2 subplots located in each of twenty 5x10m2 plots ten of which were fertilized with 10gNm-2yr-1. Fertilization treatments started in 1995. Results since 2004 show that seasonal and annual rainfall differ considerably among years: non-monsoon precipitation was 155, 178, 29 and 162 mm and monsoon precipitation was 149, 143, 196 and 104 mm in 2004-2007, respectively. In 2004, ANPP was significantly higher on fertilized (108.1 gm-2) compared to control plots (77.3gm-2), yet there were no significant differences in species diversity in fertilized (12.6m-2) or control (13.0m-2) plots. There were no differences in ANPP (48.0 vs. 49.5 gm-2) or species diversity (10.1 vs. 10.2m-2) 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 (245 gm-2) compared to control plots (115 gm-2) and diversity was 13.1 vs 14.6 species m-2. Much of the increase in ANPP in 2006 was in forb biomass. In 2007, richness declined somewhat from 2006 levels, averaging 12.2 and 13.2 species m-2 in control and fertilized plots, respectively. Biomass for 2007 is still being calculated. 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.