Tree encroachment threatens the biodiversity and ecological functioning of grasslands worldwide. Reversing effects of encroachment requires not only removing trees, but limiting subsequent invasions, which can stall grassland reassembly. We quantified rates and patterns of conifer reinvasion of mountain grasslands (meadows) in western Oregon, 7 and 15 years after experimental tree removal with or without burning. We assessed frequency (percentage of 100 m² subplots) and density of reinvading trees in six 1-ha plots as a function of burn treatment, abundance in the adjacent forest (a proxy for seed availability), and distance to or orientation of forest edges. After 15 years, frequency and density of colonists varied widely among plots (24–57% of subplots and 60–250 trees/ha, respectively), but burning had no effect on either measure. Abies grandis, which dominated adjacent forests, was the principal invader. Colonization by less-common Pinus contorta and A. procera was correlated with species' density and basal area in adjacent forests, suggesting that establishment was seed limited. Annualized rates of invasion were similar between early and late sampling intervals (9–10/trees/ha). Establishment was generally greater along the forested edges of plots (0–10 m) than in the core (10–50 m). Prominence of shade-tolerant A. grandis along northerly-facing edges suggests strong microclimatic controls on establishment. In sum, tree removal, with or without prescribed fire, is successful in reversing effects of conifer encroachment. Despite abundant seed rain, reinvasion is slow and spatially restricted. Periodic hand removal of colonizing trees, while they are small, can be done efficiently and economically.

中文翻译：

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大规模草地恢复处理 15 年后树木再入侵的速率、模式和驱动因素

树木侵占威胁着全球草原的生物多样性和生态功能。逆转侵占的影响不仅需要移除树木，还需要限制随后的入侵，这可能会阻碍草地重新组装。我们量化了俄勒冈州西部山区草原（草地）针叶树再入侵的速率和模式，在有或没有燃烧的实验性树木移除后 7 年和 15 年。我们评估了频率（100 m ^{2 的}百分比子地块）和 6 个 1 公顷地块中重新入侵树木的密度作为燃烧处理、相邻森林中的丰度（种子可用性的代表）以及到森林边缘的距离或方向的函数。15 年后，殖民者的频率和密度在地块之间变化很大（分别为 24-57% 的子地块和 60-250 棵树/公顷），但燃烧对这两种测量都没有影响。控制邻近森林的冷杉是主要的入侵者。不太常见的松树和A. procera 的殖民化与邻近森林的物种密度和基面积相关，表明建立是种子有限的。早期和晚期采样间隔（9-10/树木/公顷）之间的年化入侵率相似。沿着地块的森林边缘（0-10 m）的建立通常大于核心（10-50 m）。沿朝北边缘突出的耐阴A. grandis表明对建立的强烈微气候控制。总而言之，无论有没有规定的火灾，树木移除都可以成功逆转针叶树侵占的影响。尽管有大量的种子雨，但重新入侵是缓慢的并且在空间上受到限制。定期手动移除定植树木，虽然它们很小，但可以有效且经济地进行。