Data from: A test of priority effect persistence in semi-natural grasslands through the removal of plant functional groups during community assembly

Background: It is known that during plant community assembly, the early colonizing species can affect the establishment, growth or reproductive success of later arriving species, often resulting in unpredictable assembly outcomes. These so called ‘priority effects’ have recently been hypothesized to work through niche-based processes, with early colonizing species either inhibiting the colonization of other species of the same niche through niche preemption, or affecting the colonization success of species of different niches through niche modification. With most work on priority effects performed in controlled, short-term mesocosm experiments, we have little insight in how niche preemption and niche modification processes interact to shape the community composition of natural vegetations. In this study, we used a functional trait approach to identify potential niche-based priority effects in restored semi-natural grasslands. More specifically, we imposed two treatments that strongly altered the community’s functional trait composition; removal of all graminoid species and removal of all legume species, and we compared progressing assembly with unaltered control plots. Results: Our results showed that niche preemption effects can be, to a limited extent, relieved by species removal. This relief was observed for competitive grasses and herbs, but not for smaller grassland species. Although competition effects acting within functional groups (niche preemption) occurred for graminoids, there were no such effects for legumes. The removal of legumes mainly affected functionally unrelated competitive species, likely through niche modification effects of nitrogen fixation. On the other hand, and contrary to our expectations, species removal was after 4 years almost completely compensated by recolonization of the same species set, suggesting that priority effects persist after species removal, possibly through soil legacy effects. Conclusions: Our results show that both niche modification and niche preemption priority effects can act together in shaping community composition in a natural grassland system. Although small changes in species composition occurred, the removal of specific functional groups was almost completely compensated by recolonization of the same species. This suggests that once certain species get established, it might prove difficult to neutralize their effect on assembly outcome, since their imposed priority effects might act long after their removal.研究背景：众所周知，在植物群落构建过程中，早期定植物种会影响后续到达物种的定植、生长或繁殖成功率，往往导致难以预测的群落构建结果。这类被称为‘优先效应（priority effects）’的现象，近期被认为通过生态位相关过程发挥作用：早期定植物种可通过生态位抢占（niche preemption）抑制同生态位其他物种的定植，或通过生态位修饰（niche modification）影响不同生态位物种的定植成功率。目前绝大多数优先效应相关研究均基于受控短期中宇宙实验（mesocosm experiment），我们对生态位抢占与生态位修饰过程如何共同作用以塑造自然植被群落组成的认知仍十分有限。本研究采用功能性状（functional trait）分析法，在修复后的半自然草原中识别潜在的生态位型优先效应。具体而言，我们设置了两种显著改变群落功能性状组成的处理：移除所有禾本类物种（graminoid species），以及移除所有豆科物种（legume species），并将正在进行的群落构建过程与未处理的对照样地进行对比。 研究结果：结果显示，物种移除可在一定程度上缓解生态位抢占效应。该缓解效应仅在竞争力较强的禾草和草本植物中观测到，而在小型草原物种中未出现此类现象。尽管功能类群内部的竞争效应（即生态位抢占）在禾本类物种中存在，但豆科物种中并未出现该效应。移除豆科物种主要影响了功能类群无关的竞争力较强的物种，这一现象大概率由固氮作用（nitrogen fixation）带来的生态位修饰效应所导致。另一方面，与我们的预期相反，经过4年后，物种移除的影响几乎完全被相同物种集合的重新定植所抵消，这表明优先效应在物种被移除后仍会持续存在，可能通过土壤遗留效应（soil legacy effects）发挥作用。 研究结论：本研究结果表明，生态位修饰与生态位抢占两种优先效应可共同作用，以塑造自然草原系统的群落组成。尽管群落组成出现了小幅变化，但特定功能类群的移除几乎完全被对应物种的重新定植所补偿。这意味着，一旦特定物种完成定植，想要抵消其对群落构建结果的影响可能极为困难，因为它们所引发的优先效应可能在其被移除后仍会长期存在。