Document: LES-3-34-73

Plant-plant interactions in a Mojave Desert plant community exposed to elevated atmospheric CO₂ during a year with above-average precipitation.

DEFALCO, L.A.* ^1,2 and R.S.NOWAK ¹

University of Nevada, Reno, Reno, NV 89557 USA ¹
US Geological Survey, Las Vegas Field Station, Las Vegas, NV USA ²

Abstract:
Desert ecosystems are expected to have large increases in plant productivity as atmospheric CO₂ rises because greater plant water use efficiency should increase soil moisture. The growth responses of plants to elevated CO₂ are complex, especially in natural plant communities where different growth forms interact to acquire limited soil moisture. We examined plant-plant interactions in an intact Mojave Desert ecosystem using FACE (Free Air CO₂ Enrichment) technology. We measured the absolute growth rate (AGR) and reproductive effort (number of flowers/maximum biomass) of target individuals of two C₃ species: Eriogonum trichopes (annual forb) and Achnatherum hymenoides (perennial grass). We also quantified the change in plant cover surrounding targets for analysis in a multiple linear regression to determine whether these neighbors influenced target plant performance under elevated (550 mol mol^-1) and ambient CO₂ concentrations. Eriogonum AGR and reproductive effort were not significantly different between CO₂ treatments because this annual species completed its life cycle during the spring when above-average precipitation maintained high soil moisture. While the four dominant annual species surrounding Eriogonum targets increased in canopy cover under elevated CO₂, the total cover of all annual plant species was not significantly different between CO₂ treatments. Thus, species composition and growth response differed among neighbor annual species under elevated CO₂. Achnatherum AGR was significantly greater under elevated CO₂, and this enhanced response occurred in summer when soil moisture became more limited. Although neighbor grasses and forbs did not have an effect on Achnatherum performance, the cover of neighbor perennial shrubs and grasses was associated with a reduction in AGR, and this effect was greater under elevated CO₂. As CO₂ continues to rise, differences in the responses of species in mixed communities to available moisture will likely influence future plant communities of arid systems.

Keywords: Global change, Elevated CO₂, Invasive species, Mojave Desert

This abstract is being presented at: 1:45 PM in session:
Oral Session #30: Effects of Elevated Carbon Dioxide.