Trait-mediated community assembly: distinguishing the signatures of biotic and abiotic filters

Conflicting hypotheses predict how traits mediate species establishment and community assembly. Traits of newly establishing individuals are predicted to converge, or be more similar to the resident, preexisting community, when the biotic or abiotic environment favors a single best phenotype, but are predicted to diverge when trait differences reduce competitive interactions. We tested these competing hypotheses using transplant seedlings in an old-field environment, and assessed the contribution of inter- and intra-specific transplant trait variation to community-level patterns. Using a soil moisture gradient and resident plant removals, we determined when traits of newly-establishing plants converge or diverge from the resident community by calculating community weighted mean traits for transplant and resident communities. We saw evidence of environmentally- and competitively-driven trait shifts that resulted in both trait convergence and divergence from the resident community, whose traits reflect the combined effects of both drivers. Leaf dry matter content (LDMC) of transplants diverged in the presence of competition, whereas plant height and stem-specific density (SSD) showed the opposite pattern, converging with the resident community in their presence. Specific leaf area (SLA) shifted with competition but did not reflect resident community SLA. All transplant traits were influenced by soil moisture, often in an interaction with competition, indicating that the strength of convergence or divergence is contingent on the abiotic environment. Intraspecific differences in transplant traits among treatments were evident in three of four traits; intraspecific height and SLA trends mirrored transplant community-level trends, whereas intraspecific shifts in SSD were distinct from community-level trends. Our study shows competition between plant species may cause traits of newly establishing plants to converge with the resident community, as frequently as it selects for trait divergence. These opposing effects of competition suggest that it plays a pervasive role in both intraspecific and species-level trait differences among communities.

相互矛盾的假说针对性状如何介导物种定植与群落组装提出了不同预测。当生物或非生物环境倾向于单一最优表型时，新定植个体的性状被预测会与已存在的本地群落趋同（即更为相似）；而当性状差异能够降低种间竞争相互作用时，新定植个体的性状则被预测会发生趋异。我们通过在弃耕地生境中开展移栽幼苗实验对这些相互对立的假说进行了验证，并评估了种间与种内移栽性状变异对群落水平格局的贡献。我们设置了土壤湿度梯度与移除本地植株两种处理，通过计算移栽群落与本地群落的群落加权平均性状（community weighted mean, CWM），明确了新定植植物的性状何时会与本地群落趋同或趋异。研究结果显示，受环境与竞争共同驱动的性状偏移同时存在性状趋同与趋异现象，而本地群落的性状则反映了这两类驱动因子的综合效应。在存在竞争的条件下，移栽植株的叶片干物质含量（LDMC）发生了趋异；而植株高度与茎比密度（SSD）则呈现出相反的模式：在竞争存在时，它们的性状与本地群落发生趋同。比叶面积（SLA）随竞争条件发生了偏移，但并未反映出本地群落的SLA特征。所有移栽性状均受到土壤湿度的影响，且该影响常与竞争存在交互作用，这表明性状趋同或趋异的强度取决于非生物环境条件。不同处理下移栽植株的种内性状差异在4个性状中出现了3个；种内的株高与SLA变化趋势与移栽群落的整体趋势一致，而SSD的种内偏移则与群落水平趋势存在差异。本研究表明，植物物种间的竞争既可能促使新定植植物的性状与本地群落趋同，也可能同样频繁地选择性状趋异。竞争的这两种对立效应表明，其在群落内的种间与种内性状差异中均发挥着广泛作用。