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.