PARENT SESSION
Oral Session #69: Elevated CO₂ and Global Change. Presiding: L. Gough.
Thursday, August 9, 2001. 1:00 PM to 4:45 PM. Hall of Ideas H.

Interacting global changes influence diversity and species composition in a California annual grassland .

Zavaleta, Erika¹, Field, Christopher², Shaw, Rebecca², Mooney, Harold¹, Chiariello, Nona¹, Thayer, Susan², ^{1 2}

ABSTRACT- Interacting climate and atmospheric changes are expected to alter the diversity and composition of natural plant communities. However, the mechanisms that govern species loss and gain under global change are poorly understood. We exposed 0.79 m² field plots with mean richness of 10.3 species to eight replicates of all 16 possible combinations of ambient and elevated CO₂ (700 ppm), warming (+1-2°C), precipitation (+50%), and N deposition (+7g/ m²/yr) for two years beginning fall 1998. From 1998 to 2000, warming increased species richness by 1.1 species, primarily by increasing the presence of native annuals. N deposition reduced richness by 0.5 species over the study period by eliminating some annual forbs, but only at ambient precipitation and only in the second year. Elevated CO₂ reduced richness by 0.9 species from 98-00 by eliminating some forbs, but only under warming. In all three years (pre-treatment and two experimental years), species richness across treatments was negatively correlated with litter biomass (p=.002-.01) but was not consistently related to live biomass, live stem density, or N or P availability. Change in species richness was positively but less strongly associated with soil moisture availability in both treatment years. The effects of global changes on species richness and composition in this model grassland system appear to be mediated primarily by changes in surface litter biomass and secondarily by seasonal soil moisture availability.

KEY WORDS: global change, species diversity, annual grassland, elevated CO₂