Abstract

Invasion by exotic plant species can profoundly affect native plant species performance and the inferred proximate cause is typically competition. We used invaded grasslands in the semi-arid Western USA to separate resource competition from structural interference of an exotic grass on native forb performance, specifically evaluating the role of competition from living vegetation versus litter accumulation (hereafter, thatch). We simultaneously tested whether a positive thatch-feedback exists for the dominant exotic grass species, Bromus diandrus. Thatch and B. diandrus density were manipulated and coupled with native seed addition to separate the effect of B. diandrus competition relative to thatch accumulation. To determine the response of native forb species and B. diandrus to varying thatch abundance, we created a gradient of thatch densities and measured species response, soil moisture and light availability. The thatch of B. diandrus greatly reduced native forb performance (number of individuals and biomass) and resulted in near complete exclusion of many species. The effect of living B. diandrus density on native forb performance was minimal and inconsistent. Forb performance and light availability both exhibited exponential declines with thatch build-up suggesting that light reduction is a primary mechanism through which thatch affects forbs. Simultaneous with forb suppression, B. diandrus performance was positively affected by thatch, consistent with a positive feedback initiated by invasion of this species. Our results demonstrate that thatch accumulation, rather than competition for resources, is the primary cause of native species decline in grasslands invaded by this exotic annual grass. In addition, the dominance of B. diandrus in many invaded grasslands may be reinforced through a positive thatch-feedback and require active management, such as grazing or fire, to be broken.

中文翻译：

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所有的野花都去了哪里？异国草茅草的作用

摘要

外来植物物种的入侵会深刻影响本地植物物种的表现，并且推断出的直接原因通常是竞争。我们使用了半干旱美国西部的入侵草地，将资源竞争与外来草的结构干扰对本机福布性能的干扰分开，专门评估了竞争对活体植被与凋落物积累（以下称为茅草）的作用。我们同时测试了优势外来草种Bromus diandrus是否存在正的茅草反馈。操纵茅草和双歧双歧杆菌的密度，并与添加天然种子耦合以分开双歧双歧杆菌的作用竞争相对于茅草积累。为了确定原生的Forb物种和B. diandrus对不同的茅草丰度的响应，我们创建了茅草密度的梯度，并测量了物种的响应，土壤湿度和光的利用率。B. diandrus的茅草极大地降低了天然福布的性能（个体数量和生物量），并导致许多物种几乎完全被排斥。生活的双歧双歧杆菌密度对天然forb性能的影响极小且不一致。随着茅草的积累，叉草的性能和光的利用率都呈指数下降，这表明减光是茅草影响叉草的主要机制。同时与杂类草抑制，B. diandrus茅草对性能有积极影响，这与入侵该物种引起的积极反馈一致。我们的结果表明，茅草的积累而不是资源的竞争，是这种外来一年生草入侵的草原上本地物种减少的主要原因。此外，通过积极的茅草反馈，可以增强在许多入侵草地上双歧双歧杆菌的优势地位，并需要积极管理，例如放牧或火灾。