Severe drought and insect outbreaks have caused widespread mortality and dieback of trees in semiarid woodlands. Despite the extent and severity of these dieback events, little is known about how the woodland understory vegetation responds to severe drought and whether that response is mediated by changes to the tree layer. We sampled understory vegetation of 98 plots in 25 pinyon-juniper woodland sites in central Nevada with varying amounts of dieback, during the third yr of a severe regional drought. Twenty-five of these plots had predrought baseline data. We related covers of perennial grasses, perennial forbs, shrubs, and the exotic invasive cheatgrass to gradients of aridity, tree cover, soil water capacity, and tree dieback. Between 2005 and 2015, the covers of perennial grasses and forbs declined, while shrub cover remained mostly unchanged. We found that the greatest understory cover was associated with lower tree cover and greater tree dieback. We did not find evidence for rapid colonization of microhabitats created by tree mortality. The site-level response of vegetation to tree dieback depended on aridity: For both perennial grasses and cheatgrass, tree mortality had a positive effect only on dry sites. Cheatgrass abundance increased the most at dry sites with tree dieback, and almost every site with substantial dieback also had cheatgrass present. Our results show differential effects of drought and tree dieback on understory functional types, which may affect the postdrought successional trajectory of drought-impacted pinyon-juniper stands. Further research is needed to determine the mechanisms by which tree mortality affects the understory, whether through altering the litter layer, light levels, or soil water, and whether these effects persist over longer timescales.

严重的干旱和昆虫暴发已导致半干旱林地中树木的广泛死亡和死亡。尽管这些枯死事件的程度和严重性，人们对林地地下植被对严重干旱的反应以及该反应是否由树木层的变化介导的知之甚少。在严重的区域干旱的第三年，我们在内华达州中部的25个Pinyon-Juniper林地中采集了98个样地的林下植被，并有不同程度的枯萎。这些地块中有25个具有干旱前基线数据。我们将多年生草，多年生forbs，灌木和奇特的侵入性无盖茅草的覆盖率与干旱，树木覆盖率，土壤水容量和树木枯萎的梯度相关联。在2005年至2015年期间，多年生草和Forbs的覆盖率有所下降，而灌木的覆盖率基本上保持不变。我们发现，最大的林下树木覆盖率与较低的树木覆盖率和较大的树木枯萎相关。我们没有找到因树木死亡而迅速定居的微生境的证据。植被对树木枯萎的场所水平响应取决于干旱：对于多年生草和石笋而言，树木死亡率仅对干燥场所具有积极影响。在干旱地区，树木枯萎时，茅草的丰度增加最多，几乎每个严重枯萎的地方也都存在草茅。我们的研究结果表明干旱和树木枯死对林下功能类型的不同影响，这可能会影响干旱影响的Pinyon-Juniper林分的干旱后演替轨迹。需要进一步研究以确定树木死亡率影响林下树木的机制，