Negative plant soil feedback influences plant community development in grassland restoration

Restoration of degraded lands is primarily guided by plant community ecology. However, many seed-based restorations fail or result in unpredictable outcomes, which indicates a need to better understand what determines community trajectory and stability. Here we surveyed two ongoing restoration projects, examined data from an eight-year-old field experiment, and conducted several greenhouse experiments to determine if an observed sudden boom-and-bust pattern of western yarrow (Achillea millefolium) was due to an accumulation of soil pathogens, and if those could spill over and influence other native species commonly used in restoration of semi-arid grasslands. We show the lower yarrow cover in the older restoration project coincided with a higher relative abundance of putative fungal pathogens, in particular a Paraphoma sp., which is known to cause crown rot. This pathogen also increased over time in the field experiment while yarrow declined. The onset of this decline was density-dependent and occurred faster in monocultures with high yarrow density than in mixed plots where yarrow density was lower. In the greenhouse, yarrow grew better with fungicides in live, but not sterile soil, suggesting suppressive effects of fungal pathogens. However, other native species grew better in yarrow-trained soil than soil conditioned by another native plant, indicating no pathogen spill-over after yarrow but instead a potential to facilitate soil conditions for succeeding natives. The rapid establishment and competitive nature of yarrow followed by pathogen-mediated declines make it an interesting potential bridge species in restoration of degraded semi-arid grasslands. We demonstrate that plant soil feedback can be a strong driver of plant community development during restoration. Incorporating it into foundational theories of restoration ecology, which are currently based largey on plant-plant interactions may improve predictability in restoration projects.

退化土地的修复工作主要以植物群落生态学（plant community ecology）为指导。然而，诸多基于种子的修复工程要么宣告失败，要么产出难以预判的结果，这凸显出深入解析调控植物群落演替轨迹与稳定性的核心因素的必要性。本研究针对两项正在推进的修复项目开展调研，分析了一项为期8年的野外试验数据，并实施多组温室实验，旨在验证西洋蓍草（Achillea millefolium）所呈现的突发兴衰模式，是否由土壤病原菌的累积所诱发，以及这类病原菌是否会扩散并影响半干旱草原修复中常用的其他本土物种。研究结果显示，修复年限更长的样地中，西洋蓍草的盖度更低，同时与疑似真菌病原菌（尤其是已知可引发冠腐病的拟茎点霉属物种（Paraphoma sp.））的相对丰度呈显著负相关。该病原菌在野外试验中随时间推移丰度持续上升，而西洋蓍草的盖度则逐步下降。这种种群衰退的发生具有密度依赖性：在西洋蓍草密度较高的单优栽培群落中，衰退进程比西洋蓍草密度更低的混合样地更快。温室实验表明，在活体土壤环境中施加杀菌剂可显著促进西洋蓍草生长，而在灭菌土壤环境中则无此效果，这证实真菌病原菌对西洋蓍草存在抑制作用。不过，其他本土物种在经西洋蓍草驯化的土壤中的生长表现，优于在经其他本土植物驯化的土壤中的表现，这说明西洋蓍草并未引发病原菌扩散，反而可能改善了后续本土植物的生长土壤环境。西洋蓍草能够快速定植且具备较强的竞争能力，随后又因病原菌介导出现种群衰退，使其成为半干旱退化草原修复中极具潜力的桥接物种。本研究证实，植物-土壤反馈（plant soil feedback）可在修复过程中成为驱动植物群落发育的关键因素。当前的修复生态学基础理论主要依托植物间相互作用构建，若将植物-土壤反馈纳入理论框架，或将显著提升修复项目的可预测性。