Atmospheric and soil drought effects on California native perennial grass community composition

Background/Question/Methods

Ongoing anthropogenic activities have resulted in an increase in the occurrence of drought. This change in drought frequency is already driving a shift in grassland vegetation productivity and plant functional composition. However, experimental drought studies have historically focused on precipitation inputs but largely ignored the role of atmospheric relative humidity. A recent study found that reduced precipitation in the soil limited aboveground biomass of a grassland plant community but only when the atmosphere was also dry. We manipulated atmospheric relative humidity and soil moisture in an outdoor pot experiment growing eight species of native CA grasses at California State University, Los Angeles. We measured biomass and composition of the community after two years of growth.

Results/Conclusions

We found significant difference in grass community composition between treatments experiencing dry soil and humid atmosphere (typical of rainout shelter-based experiments) vs. those experiencing dry soil and dry air. In particular, pots experiencing atmospheric drought and soil drought conditions favored E. glaucus, H. californicum, and S. cernua. The role of atmospheric drought is capable of altering grasses’ community composition under drought and non-drought conditions. Without manipulating atmospheric drought, these changes would have been overlooked.