Data from: Woodland resilience to regional drought: Dominant controls on tree regeneration following overstorey mortality

Drought events occurring under warmer temperatures (i.e. “hotter droughts”) have resulted in widespread tree mortality across the globe, and may result in biome-level vegetation shifts to alternate vegetation types if there is a failure of trees to regenerate. We investigated how overstorey trees, understorey vegetation, and local climatic and edaphic conditions interact to influence tree regeneration, a key prerequisite for resilience, in a region that has experienced severe overstorey tree mortality due to hotter droughts and beetle infestations. We used detailed field observations from 142 sites that spanned a broad range of environmental conditions to evaluate the effects of climate and recent tree mortality on tree regeneration dynamics in the spatially extensive piñon (Pinus edulis)-juniper (Juniperus osteosperma, Juniperus monosperma) woodland vegetation type of the southwestern USA. We used a structural equation modelling framework to identify how tree mortality and local climatic and edaphic conditions affect piñon and juniper regeneration and electivity analyses to quantify the species-specific associations of tree juveniles with overstorey trees and understorey shrubs. Piñon regeneration appears to be strongly dependent upon advanced regeneration, (i.e. the survival of juvenile trees that established prior to the mortality event), the survival of adult seed-bearing trees (inferred from basal area of surviving trees) and the facilitative effects of overstorey trees for providing favourable microsites for seedling establishment. Model results suggest that local edaphoclimatic conditions directly affected piñon and juniper regeneration, such that stands with hotter, drier local climatic conditions and lower soil available water capacity had limited tree regeneration following large-scale dieback. Synthesis. We identify four indicators of resilience to hotter drought conditions: (1) abundant advance regeneration of tree seedlings; (2) sufficient canopy cover for survival of emergent seedlings and existing regeneration; (3) sufficient seed source from surviving trees with high reproductive output; (4) areas with cooler and wetter local climates and greater soil available water capacity. In the absence of these conditions, there is greater likelihood of woodlands transitioning to more xeric vegetation types following dieback.

在全球变暖背景下发生的高温干旱事件（即“极端高温干旱”）已在全球范围内造成大面积树木死亡；若树木无法完成自然更新，则可能引发生物群系级别的植被转变，更替为其他植被类型。本研究针对因极端高温干旱与甲虫侵染经历了上层乔木大规模死亡的区域，探讨了上层乔木、林下植被、局地气候与土壤条件如何相互作用影响树木更新——而树木更新是生态系统恢复力的核心前提。本研究依托覆盖广泛环境梯度的142个样地的详细野外观测数据，评估了气候与近期树木死亡对美国西南部大面积分布的矮松（Pinus edulis）-杜松（Juniperus osteosperma、Juniperus monosperma）林地植被类型中树木更新动态的影响。本研究采用结构方程模型（structural equation modelling）框架，解析了树木死亡、局地气候与土壤条件对矮松与杜松更新的影响；同时通过物种选择性分析，量化了幼树与上层乔木及林下灌丛的物种特异性关联。研究发现，矮松更新强烈依赖于前期更新苗（即干旱死亡事件发生前已定植的幼树存活率）、成年结籽树木的存活率（由存活树木的胸高断面积推断），以及上层乔木为幼苗定植提供有利微生境的促进作用。模型结果显示，局地土壤气候条件直接影响矮松与杜松的更新：局地气候更炎热干旱、土壤有效持水量更低的林分，在大规模枯死后的树木更新能力极为有限。结论：本研究确定了应对极端高温干旱的四项恢复力指标：（1）树木幼苗前期更新储备充足；（2）冠层盖度足以支持萌苗及现有更新植株存活；（3）存活树木提供充足的种子来源且繁殖能力较强；（4）局地气候更凉爽湿润、土壤有效持水量更高。若不满足上述条件，林地在枯死后更有可能转变为更耐旱的植被类型。