Direct versus indirect effects of warming on annual competition and coexistence between an introduced annual (Lolium multiflorum) and a native perennial (Festuca roemeri) grass in the Willamette Valley, Oregon

Alejandro Brambila and Lauren M. Hallett, Environmental Studies Program and Biology Department, University of Oregon, Eugene, OR

Background/Question/Methods
Global temperature increases are expected to be a major driver of ecosystem change. Temperature plays a large role in structuring grassland plant communities along the US West coast, with California dominated by annual grasses, and perennial grasses dominant in the cooler Pacific Northwest. As the Pacific Northwest climate becomes more similar to California’s, warming experiments suggest that perennial grasses may be replaced by annuals. The mechanisms of this conversion, however, are not well understood. Warming may negatively affect perennials directly, or indirectly through altered competitive effects on seedlings or adults from conspecifics or annual grasses. Here we disentangle the direct and indirect effects of warming on a perennial (Festuca roemeri) and annual (Lolium multiflorum) grass species. We manipulated temperature using passive warming chambers, within which we manipulated the relative densities of annual seeds, perennial seeds and perennial adults. To test effectiveness of warming chambers we measured soil and air temperatures throughout the year. To evaluate warming response, we measured seedling establishment, growth, survival, seed production and overall cover. We compared the perennial response to warming with and without annual competitors at each life stage to isolate the mechanisms by which warming drives perennial to annual grass conversion.
Results/Conclusions
Our passive warming chambers successfully increased soil and air temperatures from February until peak biomass in late May, at which point mean chamber air and soil temperatures were approximately 2°C and 1.5°C warmer respectively. We expected warming would negatively affect perennials by directly reducing establishment and seed production. In the absence of competition, warming instead increased perennial seedling establishment and adult perennial height, while reducing adult circumference and seed production. However, warming did not increase perennial seedling establishment when grown with adult perennials. Adult perennials grown at higher densities had lower seed production when warmed. We expected negative indirect effects on perennials via increased competition with annuals. Instead, annual competition did not affect perennial seedling establishment, adult survival or seed production, and warming reduced annual establishment, height and per-capita seed production. However, annual grasses reached peak biomass earlier under warming, and warming did not reduce per capita seed production when grown with adult perennials. Overall, these results surprisingly suggest that the direct effect of warming may in fact benefit perennial grasses. However, annuals may still be successful thanks to a shift in growth timing, and relatively strong competitive effects of adult perennials on conspecifics.