The physical environment has changed rapidly in the last 100 years, and ecologists are working to understand the response of biotic communities. Past studies primarily focus on range and phenological shifts of individual species due to warming, although the response of entire communities is of increasing interest. Other aspects of global change, such as the amount and timing of precipitation and N-deposition, also have important effects, especially in arid regions. We performed nitrogen and precipitation manipulations for several years in a Southern California grassland-shrubland ecotone to determine long-term community response to global change. Plots were manipulated to receive approximately 50% more than ambient precipitation, ambient precipitation, or 50% less than ambient precipitation and had nitrogen added or not in a fully factorial design with eight replicates for each habitat type (grassland and coastal sage scrub). Species composition data were collected in all plots by point-intercept method during the peak of the growing season in each year. Shifts in community composition were analyzed with non-metric multidimensional scaling and general linear models.
Results/Conclusions
The cover of shrubs decreased dramatically in Coastal Sage Scrub plots with reduced precipitation, while the cover of grasses increased in these plots, especially with Nitrogen deposition. These water-reduction plots effectively experienced a type conversion from a shrub-dominated system to a system dominated by non-native, invasive annual grasses. The grassland plots with decreased precipitation shifted from grass-dominated to forb-dominated systems. Non-native annual grasses, especially Bromus diandrus, increased dramatically with added water. Nitrogen addition decreased the cover of N-fixing forbs (in the grassland plots) and N-fixing shrubs (in the shrub plots), and increased the cover of grasses in both systems. Our results indicate that N addition and drought can cause type conversions, both of Coastal Sage Scrub to non-native grasslands and of grasslands to forb-dominated systems.