Additive effects of simulated climate changes, elevated CO2 and nitrogen deposition on grassland diversity.

Biodiversity responses to ongoing climate and atmospheric
changes will affect both ecosystem processes and the delivery of
ecosystem goods and services. Combined effects of co-occurring
global changes on diversity, however, are poorly understood. We
examined plant diversity responses in a California annual grassland
to manipulations of four global environmental changes, singly and
in combination: elevated CO2, warming, precipitation, and nitrogen
deposition. After 3 years, elevated CO2 and nitrogen deposition
each reduced plant diversity, whereas elevated precipitation
increased it and warming had no significant effect. Diversity
responses to both single and combined global change treatments
were driven overwhelmingly by gains and losses of forb species,
which make up most of the native plant diversity in California
grasslands. Diversity responses across treatments also showed no
consistent relationship to net primary production responses, illustrating
that the diversity effects of these environmental changes
could not be explained simply by changes in productivity. In two to
four-way combinations, simulated global changes did not interact
in any of their effects on diversity. Our results show that
climate and atmospheric changes can rapidly alter biological diversity,
with combined effects that, at least in some settings, are
simple, additive combinations of single-factor effects.