Spatial and temporal variability in propagule limitation of California native grasses

Community composition and diversity arise from limitations in propagule supply (i.e. propagule or seed limitation) and propagule establishment after arriving at a site (i.e. establishment or microsite limitation). Recent meta-analyses suggest that the degree of propagule limitation depends on local abiotic and biotic conditions, which in turn are likely to vary spatially and temporally. Nevertheless, seed addition studies testing propagule limitation are rarely replicated in multiple locations and years and often lack experimental manipulations of critical determinants of propagule limitation, such as the density and species richness of the propagule pool and disturbance and resource supply of recipient site. The invasion of California (USA) grasslands by exotic annual species from the Mediterranean region is unique in its scope (over 9 million ha) and persistence (more than 150 years). This invasion provides an exciting context in which to test the role of spatial and temporal variability in mediating propagule limitation and ultimately the potential for restoration. Here I present the results of native-grass, seed-addition trials conducted along a 500 km gradient in California in three consecutive years spanning a wide range of environmental conditions and initial conditions (e.g. rainfall, seeding density and disturbance). While native grasses were able to establish at many locations, per capita seed survival was low suggesting that the fate of these species is governed by interplay between propagule and establishment limitation. Establishment primarily varied spatially, with the most successful establishment occurring at fertile locations with low resident species richness. While recruitment was highly variable among years initially, there was no difference among seedling trials after three years.

群落组成与多样性的形成源于繁殖体供给限制（propagule supply limitation，即繁殖体限制（propagule limitation）或种子限制（seed limitation））以及繁殖体抵达生境后的定植限制（establishment or microsite limitation，即定植限制（establishment limitation）或微生境限制（microsite limitation））。
近期的元分析（meta-analysis）表明，繁殖体限制程度取决于局地非生物与生物条件，而这些条件本身又存在空间和时间上的变异。
然而，验证繁殖体限制的种子添加实验（seed addition study）极少在多个地点和多年份中重复开展，且往往未对繁殖体限制的关键调控因子进行实验操控——这些因子包括繁殖体库（propagule pool）的密度与物种丰富度，以及受体生境（recipient site）的干扰状况与资源供给水平。
来自地中海地区的外来一年生植物（exotic annual species）入侵美国加利福尼亚草原（California grassland）的事件，在入侵规模（超900万公顷）与持续时长（超150年）上均极具代表性。
该入侵事件为探究时空变异如何调控繁殖体限制、进而解析生态修复潜力提供了绝佳的研究场景。
本研究呈现了连续三年在加利福尼亚州沿500公里的环境梯度开展的本土禾草种子添加试验（seed-addition trial）结果，实验覆盖了广泛的环境条件与初始条件（如降雨量、播种密度与干扰强度）。
尽管本土禾草可在诸多样点成功定植，但单株种子存活率（per capita seed survival）较低，这表明该类物种的存续由繁殖体限制与定植限制的共同作用决定。
定植成功率主要呈现空间变异特征，在土壤肥力较高且本地物种丰富度较低的样点，定植效果最佳。
尽管初期的幼苗招募（recruitment）在不同年份间差异显著，但三年后的幼苗定植实验未观察到年份间的显著差异。