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.

在升温背景下发生的干旱事件（即“热浪型干旱（hotter droughts）”）已在全球范围内造成大面积树木死亡；若树木无法完成更新，可能引发生物群系级别的植被转变，取代原有植被类型。本研究针对一处因热浪型干旱与甲虫侵染出现重度上层林木（overstorey trees）死亡的区域，探究上层林木、下层植被（understorey vegetation）以及局地气候与土壤条件（climatic and edaphic conditions）如何相互作用，进而影响树木更新（tree regeneration）——这是生态系统恢复力的核心前提。

我们依托覆盖广泛环境梯度的142个样地的详细野外观测（field observations）数据，评估了气候与近期树木死亡对美国西南部大范围分布的矮松-杜松（piñon (Pinus edulis)-juniper (Juniperus osteosperma, Juniperus monosperma)）林地植被类型中树木更新动态的影响。研究采用结构方程模型（structural equation modelling）框架，解析树木死亡与局地气候、土壤条件如何影响矮松与杜松的更新；同时运用选择性分析（electivity analysis）量化树木幼株与上层林木、下层灌木间的物种特异性关联。

矮松的更新似乎强烈依赖于前期更新（advanced regeneration，即干旱死亡事件前已定植的幼树存活情况）、成年结实树的存活情况（通过存活林木的胸高断面积（basal area）推断），以及上层林木为幼苗定植提供适宜微生境（microsites）的促进效应。模型结果显示，局地土壤气候条件（edaphoclimatic conditions）直接影响矮松与杜松的更新：局地气候更炎热干燥、土壤有效持水量（soil available water capacity）更低的林分（stands），在大规模林木枯死后的树木更新能力极为有限。

结论与综合分析。本研究明确了应对热浪型干旱的四项恢复力指标：（1）树木幼苗存在充足的前期更新种群；（2）拥有足够的冠层覆盖（canopy cover），以保障新生幼苗与现存更新植株的存活；（3）存活林木具备充足的结实能力，可提供足够的种子来源；（4）局地气候更凉爽湿润、土壤有效持水量更高的区域。若不满足上述条件，林地在大规模枯死后更有可能转变为旱生性更强的植被类型（xeric vegetation types）。