RAPID: Ecological memories and theory-guided recovery of post-fire steppe

随着环境变化的进展，野火变得越来越普遍，摧毁了整个景观，每年给美国公众造成数十亿美元的损失。在许多地区，野火的频率由于外来的草的入侵而加剧，这些草是高度易燃的，并且在被烧毁的景观重新生长时经常取代本地植物。为了防止入侵物种占领被烧毁的土地，土地管理者越来越多地使用本地植物的种子来促进生态系统的恢复，并阻止入侵植物的入侵。然而，关于决定种子添加成功的因素，在种子物种建立和抑制易燃入侵者的能力方面，还有很多需要了解的。同样不清楚的是，以前人为造成的景观变化，如氮污染或去除吸水灌木（美国西部牧场的一种常见做法），将如何影响本地种子添加和植物从火灾中恢复的成功。此外，还不清楚不同的混合种子是如何抵御外来植物入侵的。本研究项目通过在一个高度入侵的山艾草草原生态系统中调查过去氮污染和灌木清除（最初旨在改善牧场牧草）的影响的长期实验来解决这些问题。实验场地在最近的一场野火中完全被烧毁，这为评估氮污染和灌木移除的历史如何影响野火中植物的恢复提供了一个独特的机会。研究小组将制定标准，创造抑制入侵植物的种子混合物，特别是易燃的一年生草，并将确定它们在研究地点不同的长期实验环境中的有效性。这项研究的结果将帮助土地管理者选择本地种子混合物，这将有助于防止入侵植物的传播，并降低未来野火的风险和成本。该项目研究了营养丰富和优势植物移除的生态记忆如何影响野火后植物的重组，并解决了测试当前模型的适用性的关键需求，以创建能够抵抗变革性入侵物种入侵的火灾后群落。该研究将评估一种基于性状的模型（通过性状选择的群落组合模型），该模型用于建立抵抗入侵禾草和牧草的本土植物组合。该模型将应用于一个新的实验设计中，以辨别种子添加是否通过限制相似性、适应度等级和/或性状互补性而导致生物抗性。虽然很多人都把限制相似性作为一种抵抗入侵的机制给予了关注，但实验支持却参差不齐，因为这种机制很难与适应度等级区分开来，而且生态系统经常同时受到不止一种外来物种的入侵。该项目将通过使用两种不同的种子混合物来克服这些问题，这两种种子混合物旨在传递对入侵的草和forbs的抗性，再加上一种将它们结合在一起的高多样性种子混合物。通过这一实验设置，功能群体内部的生物抗性证据将支持限制相似性，不同群体之间的抗性将支持适应度等级，而高多样性组合中相对较大的抗性将支持性状互补性。混合种子处理将被安置在长期富氮和灌木清除的地块内，这些地块在微生境尺度上以灌木和蚂蚁的生态系统工程引起的环境变化为特征。这种分层设计将使研究人员能够评估景观和微生境水平因素的生态记忆如何影响种子添加的成功，以传递对入侵植物的生物抗性。它还允许评估营养有效性如何影响赋予入侵抗性的机制。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

Wildfires are becoming more common as environmental change progresses, destroying entire landscapes and costing the American public billions of dollars annually. Wildfire frequency in many regions is intensified by the invasion of exotic grasses that are highly flammable, and which often replace native plants as burned landscapes regrow. To prevent invasive species from taking over burned landscapes, land managers are increasingly applying seeds of native plants to encourage ecosystem recovery and keep invasive plants at bay. However, there is still much to learn about the factors that determine the success of seed additions in terms of the ability of seeded species to establish, and to suppress flammable invaders. It is also unclear how previous human-caused landscape changes, such as nitrogen pollution or the removal of water-hogging shrubs (a practice common in western USA rangelands), will affect the success of native seed additions and plant recovery from fire. Moreover, it is unclear how different seed mixes repel exotic plants from invading disturbed areas. This research project addresses these issues by building on a long-term experiment investigating the effects of past nitrogen pollution and shrub removal (originally intended to improve rangeland forage) in a highly invaded sagebrush steppe ecosystem. The experimental site was entirely burned in a recent wildfire, providing a unique opportunity to evaluate how a history of nitrogen pollution and shrub removal will influence plant recovery from wildfire. The research team will develop criteria for creating seed mixes that suppress invasive plants, particularly flammable annual grasses, and will determine their effectiveness within the different long term experimental environments present across the study site. The results from this study will aid land managers in choosing native seed mixes that will help to prevent the spread of invasive plants and decrease the risks and costs of future wildfires.This project investigates how ecological memories of nutrient enrichment and dominant plant removal impact plant re-assembly following wildfire and addresses a critical need to test the applicability of current models for creating post-fire communities that will be resistant to invasion by transformative invasive species. The research will evaluate a promising, but under-studied, trait-based model (the Community Assembly by Trait Selection model) for establishing native plant assemblages that are resistant to invasive grasses and forbs. This model will be applied in a novel experimental design to discern whether seed additions result in biotic resistance via limiting similarity, fitness hierarchies and/or trait complementarity. While much focus has been given to limiting similarity as a mechanism of resistance to invasion, experimental support has been mixed because this mechanism has been difficult to distinguish from fitness hierarchies and because ecosystems are frequently invaded by more than one exotic species simultaneously. This project will overcome these issues by using two separate seed mixes designed to convey resistance to invasive grasses and forbs, plus a high-diversity seed mix that combines these. With this experimental setup, evidence for biotic resistance within functional groups will support limiting similarity, resistance across different groups will support fitness hierarchies, and relatively greater resistance in the high diversity mix will support trait complementarity. The seed mix treatments will be nested within plots of long-term nitrogen enrichment and shrub removal, which at a microhabitat scale are characterized by environmental variation due to ecosystem engineering by shrubs and ants. This hierarchical design will enable the researchers to assess how ecological memories of landscape and microhabitat-level factors affect the success of seed additions for conveying biotic resistance to invasive plants. It also allows evaluation of how nutrient availability influences mechanism conferring resistance to invasion.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.