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

Abstract

AimHow 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.LocationWestern Pacific Northwest (Oregon and Washington), USA.MethodsUtilizing a 3‐year experiment in three prairie sites across a 520‐km latitudinal climate gradient, we manipulated temperature and precipitation and recorded plant cover at the peak of each growing season. 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.ResultsWarming 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 conclusionsA shift towards 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.

Quantifying climate change impacts on plant functional composition and soil nitrogen fixation in Mediterranean grasslands

“…Understanding changes in plant functional diversity and grassland function due to climate change matters regionally because rangeland, pasture, and grazed federal lands represent the largest land areas in the PNW (Neibergs et al, 2018). A previous study in the same sites showed that warming increased the cover of introduced annual species, causing subse-quent declines in other functional groups and diversity (Reed et al, 2021). The authors reported that competition for moisture and light or space, rather than nitro-gen, were critical mechanisms of community change in these seasonally water-limited Mediterranean grasslands.…”

“…We conducted this study in three experimental grassland sites: Northern (central-western Washington; characterized as cool and wet), Central (central-western Oregon; characterized as warm and wet), and Southern (southwestern Oregon; characterized as warm and dry) (Figure 1; Table 1). Details on the experiment are described elsewhere in greater detail (Reed et al, 2021), so we will only summarize them here. Each site had 25 plots divided into 15 manipulated and 10 unmanipulated plots (vegetation manipulation explained below).…”

“…Overall, biodiversity of all plots declined over three seasons. Native species showed an exacerbated decline in warming plots (Reed et al, 2021).…”

“…We used a point intercept technique to survey plot-level plant community composition (see Reed et al 2021 for further details). In Spring 2017-2019, we measured plant cover at peak standing biomass (mid-May, late-May, and mid-June at the southern, central, and northern sites, respectively) using the point intercept method.…”

Leaf traits predict water-use efficiency in U.S. Pacific Northwest grasslands under rain exclusion treatment

“…Rainout shelters were 3.7 m x 3.7 m squares and stood 1.5 m above the vegetation plots, sloped to 1 m above the plot on the far side to promote drainage, providing ~30 cm buffer around the vegetation plots. Phenology and demography information from these experimental plots have been previously published in (Peterson et al, 2021; Reed, Bridgham, et al, 2021; Reed et al, 2019; Reed, Pfeifer-Meister, et al, 2021).…”