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.

研究目的：鉴于植物功能群（plant functional group）对生态系统服务具有已被广泛认可的调控效应，气候变化将如何改变植物功能群组成，是一个至关重要的核心科学问题。气候可对不同功能群产生直接影响，而经由生物相互作用变化介导的间接效应，则可能放大或抵消气候的直接作用。因此，明晰气候对植物群落产生影响的内在机制，对生态保护与修复实践具有重要指导意义。

研究区域：美国西太平洋西北区域（俄勒冈州与华盛顿州）。

研究方法：本研究在横跨520公里纬度气候梯度的3个草原样地中开展了为期3年的控制实验，通过人工调控温度与降水量，并于每年春季记录植物盖度。我们采用结构方程模型（structural equation model），探究非生物驱动因子（即温度、土壤水分与土壤氮素）如何调控植物功能群盖度，以及这些功能群又如何进一步决定群落整体的植物多样性。

研究结果：升温显著提升了外来一年生植物的盖度，进而引发其他功能群盖度与群落多样性的下降。尽管观测到温度与水分对现存植被（包括本土一年生植物、本土多年生植物与外来多年生植物）存在直接影响，但此类影响通常会因外来一年生植物的存在而被放大。在这个季节性水分受限的地中海气候系统中，群落结构变化的关键驱动机制是水分、光照或空间的竞争，而非氮素竞争。群落多样性的下降主要源于本土一年生植物盖度的减少，以及外来一年生植物优势度的持续提升。

主要结论：该系统中朝向外来一年生植物优势度不断增加的群落转变，或与此前加利福尼亚草原所经历的群落变化相似，可能引发太平洋西北草原的「加州化（Californication）」现象。此类现象或将对当地土地管理者未来维持物种丰富、功能多样的草原生态系统的工作带来挑战。