Species-specific variation in germination rates contributes to spatial coexistence more than adult plant water use in four closely-related annual flowering plants

1. Spatial partitioning is a classic hypothesis to explain plant species coexistence, but evidence linking local environmental variation to spatial sorting, demography, and species’ traits is sparse. If co-occurring species’ performance is optimized differently along environmental gradients because of trait variation, then spatial variation might facilitate coexistence.
2. We used a system of four naturally co-occurring species of Clarkia (Onagraceae) to ask if distribution patchiness corresponds to variation in two environmental variables that contribute to hydrological variation. We then reciprocally sowed Clarkia into each patch type and measured demographic rates in the absence of congeneric competition. Species sorted in patches along one or both gradients, and in three of the four species, germination rate in the “home” patch was higher than all other patches.
3. Spatially variable germination resulted in the same three species exhibiting the highest population growth rates in their home patches.
4. Species’ trait values related to plant water use, as well as indicators of water stress in home patches, differed among species and corresponded to home patch attributes. However, post-germination survival did not vary among species or between patch types, and fecundity did not vary spatially.
5. Synthesis Our research demonstrates the likelihood that within-community spatial heterogeneity affects plant species coexistence, and presents novel evidence that differential performance in space is explained by what happens in the germination stage. Despite the seemingly obvious link between adult plant water-use and variation in the environment, our results distinguish the germination stage as important for spatially variable population performance.

1. 空间分隔（spatial partitioning）是解释植物物种共存的经典假说，但将局域环境变异与空间分异、种群动态及物种性状相关联的实证证据仍较为匮乏。若共存物种因性状变异，沿环境梯度的生长表现优化路径存在差异，则空间变异或可促进物种共存。
2. 我们以4种自然共存的克拉克草属（Clarkia，柳叶菜科Onagraceae）物种为研究对象，旨在探究物种分布的斑块格局是否与两类驱动水文变异的环境因子的变异相关联。随后我们将各克拉克草属物种交互播种至各斑块生境，并在排除同属竞争的条件下测定其种群动态率。结果显示，物种沿一条或两条环境梯度在斑块中出现分异；且4个物种中有3个，其"原生"斑块内的萌发率均高于其余所有斑块。
3. 空间异质性的萌发过程使得上述3个物种在其原生斑块内均展现出最高的种群增长率。
4. 与植物水分利用相关的物种性状值，以及原生斑块内的水分胁迫指标，均存在物种间差异，且与原生斑块的环境特征相匹配。但萌发后的存活率在物种间及斑块类型间均无显著差异，且繁殖力也未呈现空间变异。
5. 综合与结论 本研究证实，群落内部的空间异质性大概率会影响植物物种共存，并提供了新的实证证据，表明物种在空间中的表现差异可由萌发阶段的过程所解释。尽管成年植物的水分利用与环境变异之间的关联看似显而易见，但本研究结果明确了萌发阶段对于空间异质性驱动的种群表现差异的重要性。