Data from: Forest disturbance accelerates thermophilization of understory plant communities

1. Climate change is likely to shift plant communities towards species from warmer regions, a process termed “thermophilization.” In forests, canopy disturbances such as fire may hasten this process by increasing temperature and moisture stress in the understory, yet little is known about the mechanisms that might drive such shifts, or the consequences of these processes for plant diversity. 2. We sampled understory vegetation across a gradient of disturbance severity from a large-scale natural experiment created by the factorial combination of forest thinning and wildfire in California. Using information on evolutionary history and functional traits, we tested the hypothesis that disturbance severity should increase community dominance by species with southern-xeric biogeographic affinities. We also analyzed how climatic productivity mediates the effect of disturbance severity, and quantified the functional trait response to disturbance, to investigate potential mechanisms behind thermophilization. 3. The proportion of north-temperate flora decreased, while the proportion of southern-xeric flora increased, with greater disturbance severity and less canopy closure. Disturbance caused a greater reduction of north-temperate flora in productive (wetter) forests, while functional trait analyses suggested that species colonizing after severe disturbance may be adapted to increased water stress. Forests with intermediate disturbance severity, where abundances of northern and southern species were most equitable, had the highest stand-scale understory diversity. 4. Synthesis: Canopy disturbance is likely to accelerate plant community shifts towards species from warmer regions, via its effects on understory microclimate at small scales. Understory diversity can be enhanced by intermediate disturbance regimes that promote the coexistence of species with different biogeographic affinities.

1. 气候变化或推动植物群落向暖区物种迁移，这一过程被称为嗜热化（thermophilization）。森林生态系统中，野火等冠层干扰（canopy disturbance）可通过加剧林下的温度与水分胁迫，加速该进程，但目前学界对驱动此类群落转变的潜在机制，以及该过程对植物多样性的影响仍知之甚少。2. 本研究依托美国加利福尼亚州由森林疏伐（forest thinning）与野火因子组合构建的大型自然实验，沿干扰强度梯度对林下植被（understory vegetation）开展取样调查。研究基于物种演化历史与功能性状（functional traits）信息，验证了“干扰强度越高，具有南旱生生物地理亲和性（southern-xeric biogeographic affinities）的物种在群落中的优势度越高”这一假说；同时分析了气候生产力（climatic productivity）对干扰强度效应的调控作用，并量化了物种功能性状对干扰的响应，以探究嗜热化背后的潜在机制。3. 随着干扰强度升高、冠层郁闭度降低，北温带植物区系（north-temperate flora）的占比逐渐下降，而南旱生植物区系（southern-xeric flora）的占比则逐步上升。在气候生产力更高（更湿润）的森林中，干扰对北温带植物区系的抑制作用更强；功能性状分析显示，重度干扰后定植的物种可能适应了增强的水分胁迫。在干扰强度中等的林分（stand）中，南北物种的丰度分布最为均衡，该生境的林分尺度林下多样性最高。4. 综合结论：冠层干扰可通过改变小尺度林下微气候（understory microclimate），加速植物群落向暖区物种迁移的进程；中等强度的干扰制度（intermediate disturbance regimes）可促进具有不同生物地理亲和性的物种共存，从而提升林下多样性。