Data from: Dispersal versus environmental filtering in a dynamic system: drivers of vegetation patterns and diversity along stream riparian gradients

1. Both environmental filtering and dispersal filtering are known to influence plant species distribution patterns and biodiversity. Particularly in dynamic habitats, however, it remains unclear whether environmental filtering (stimulated by stressful conditions) or dispersal filtering (during re-colonization events) dominates in community assembly, or how they interact. Such a fundamental understanding of community assembly is critical to the design of biodiversity conservation and restoration strategies. 2. Stream riparian zones are species-rich dynamic habitats. They are characterized by steep hydrological gradients likely to promote environmental filtering, and by spatiotemporal variation in the arrival of propagules likely to promote dispersal filtering. We quantified the contributions of both filters by monitoring natural seed arrival (dispersal filter) and experimentally assessing germination, seedling survival and growth of 17 riparian plant species (environmental filter) along riparian gradients of three lowland streams that were excavated to bare substrate for restoration. Subsequently, we related spatial patterns in each process to species distribution and diversity patterns after 1 and 2 years of succession. 3. Patterns in initial seed arrival were very clearly reflected in species distribution patterns in the developing vegetation and were more significant than environmental filtering. However, environmental filtering intensified towards the wet end of the riparian gradient, particularly through effects of flooding on survival and growth, which strongly affected community diversity and generated a gradient in the vegetation. Strikingly, patterns in seed arrival foreshadowed the gradient that developed in the vegetation; seeds of species with adult optima at wetter conditions dominated seed arrival at low elevations along the riparian gradient while seeds of species with drier optima arrived higher up. Despite previous assertions suggesting a dominance of environmental filtering, our results demonstrate that nonrandom dispersal may be an important driver of early successional riparian vegetation zonation and biodiversity patterns as well. 4. Synthesis: Our results demonstrate (and quantify) the strong roles of both environmental and dispersal filtering in determining plant community assemblies in early successional dynamic habitats. Furthermore, we demonstrate that dispersal filtering can already initiate vegetation gradients, a mechanism that may have been overlooked along many environmental gradients where interspecific interactions are (temporarily) reduced.

1. 众所周知，环境过滤（environmental filtering）与扩散过滤（dispersal filtering）均会影响植物物种分布格局与生物多样性。然而，尤其是在动态生境中，群落构建过程中究竟是受胁迫条件驱动的环境过滤占主导，还是定殖重建过程中的扩散过滤占主导，抑或是二者如何相互作用，目前仍不明确。对群落构建机制的这一基础认知，对于制定生物多样性保护与恢复策略至关重要。
2. 河流河岸带（stream riparian zones）是物种丰富的动态生境，其特征为存在陡峭的水文梯度（hydrological gradients），易催生环境过滤过程；同时繁殖体（propagules）到达过程存在时空变异，易推动扩散过滤过程。本研究针对3条因修复工程被清整为裸露底质（bare substrate）的低地溪流（lowland streams）的河岸梯度，通过监测自然种子到达情况（表征扩散过滤过程），并针对17种河岸植物物种开展实验，评估其萌发、幼苗存活与生长情况（表征环境过滤过程），量化了两种过滤过程的贡献。随后，我们将演替第1年和第2年各过程的空间格局与物种分布、多样性格局相关联。
3. 初始种子到达的格局清晰反映了发育中植被的物种分布格局，且其影响力强于环境过滤。不过，环境过滤作用会随着河岸梯度向湿润端增强，尤其通过洪水（flooding）对幼苗存活与生长的影响，这极大地影响了群落多样性，并催生了植被梯度。值得注意的是，种子到达的格局预先预示了植被后续形成的梯度：在河岸梯度海拔较低处，成年植株适宜生境更湿润的物种的种子占主导，而适宜生境更干燥的物种的种子则更多出现在海拔较高处。尽管此前有研究认为环境过滤占主导地位，但本研究结果表明，非随机扩散同样可能是驱动早期演替河岸植被分带与生物多样性格局的重要机制。
4. 总结：本研究证实并量化了环境过滤与扩散过滤在调控早期演替动态生境中植物群落构建过程中的重要作用。此外，我们证明扩散过滤即可催生植被梯度，这一机制在许多种间相互作用被（暂时）削弱的环境梯度中可能被忽视了。