Introduced annuals mediate climate-driven community change in Mediterranean prairies of the Pacific Northwest, USA

Aim: How climate change will alter plant functional group composition is a critical question given the well-recognized effects of plant functional groups on ecosystem services. While climate can have direct effects on different functional groups, indirect effects mediated through changes in biotic interactions have the potential to amplify or counteract direct climatic effects. As a result, identifying the underlying causes for climate effects on plant communities is important to conservation and restoration initiatives.

Location: Western Pacific Northwest (Oregon and Washington), USA.

Methods: Utilizing a three year experiment in three prairie sites across a 520 km latitudinal climate gradient, we manipulated temperature and precipitation and recorded plant cover each spring. We used structural equation models to examine how abiotic drivers (i.e., temperature, moisture, and soil nitrogen) controlled functional group cover, and how these groups in turn determined overall plant diversity.

Results: Warming increased the cover of introduced annual species, causing subsequent declines in other functional groups and diversity. While we found direct effects of temperature and moisture on extant vegetation (i.e., native annuals, native perennials, and introduced perennials), these effects were typically amplified by introduced annuals. Competition for moisture and light or space, rather than nitrogen, were critical mechanisms of community change in this seasonally water-limited Mediterranean-climate system. Diversity declines were driven by reductions in native annual cover and increasing dominance by introduced annuals.

Main Conclusions: A shift toward increasing introduced annual dominance in this system may be akin to that previously experienced in California grasslands, resulting in the "Californication" of Pacific Northwest prairies. Such a phenomenon may challenge local land managers in their efforts to maintain species-rich and functionally diverse prairie ecosystems in the future.