School of Environmental and Forest Sciences, University of Washington, Seattle, Washington, 98195, USA.
Quantitative Ecology and Resource Management, University of Washington, Seattle, Washington, 98195, USA.
Ecol Appl. 2022 Jan;32(1):e02474. doi: 10.1002/eap.2474. Epub 2021 Nov 25.
Promoting ecological resilience to increasing disturbance activity is a key management priority under warming climate. Across the Northern Hemisphere, tree mortality from widespread bark beetle outbreaks raises concerns for how forest management can foster resilience to future outbreaks. Density reduction (i.e., thinning) treatments can increase vigor of remaining trees, but the longevity of treatment efficacy for reducing susceptibility to future disturbance remains a key knowledge gap. Using one of the longest-running replicated experiments in old-growth subalpine forests, we measured stand structure following a recent (early 2000s) severe mountain pine beetle (MPB; Dendroctonus ponderosae) outbreak to examine the legacy of historical (1940s) thinning treatments on two components of resilience. We asked: 'How did historical thinning intensity affect (1) tree-scale survival probability and stand-scale survival proportion (collectively "resistance" to outbreak) for susceptible trees (lodgepole pine [Pinus contorta] ≥ 12 cm diameter) and (2) post-outbreak stand successional trajectories?' Overall outbreak severity was high (MPB killed 59% of susceptible individuals and 78% of susceptible basal area), and historical thinning had little effect on tree-scale and stand-scale resistance. Tree-scale survival probability decreased sharply with increasing tree diameter and did not differ from the control (uncut stands) in the historical thinning treatments. Stand-scale proportion of surviving susceptible trees and basal area did not differ from the control in historically thinned stands, except for treatments that removed nearly all susceptible trees, in which survival proportion approximately doubled. Despite limited effects on resistance to MPB outbreak, the legacy of historical treatments shifted dominance from large-diameter to small-diameter lodgepole pine by the time of outbreak, resulting in historically thinned stands with ~2× greater post-outbreak live basal area than control stands. MPB-driven mortality of large-diameter lodgepole pine in control stands and density-dependent mortality of small-diameter trees in historically thinned stands led to convergence in post-outbreak live tree stand structure. One exception was the heaviest historical thinning treatments (59-77% basal area removed), for which sapling dominance of shade-tolerant, unsusceptible conifers was lower than control stands. After six decades, thinning treatments have had minimal effect on resistance to bark beetle outbreaks, but leave persistent legacies in shaping post-outbreak successional trajectories.
在气候变暖的背景下，提高生态系统对日益增加的干扰活动的恢复力是管理的重点。在整个北半球，广泛发生的树皮甲虫爆发导致树木死亡，这引发了人们对森林管理如何促进对未来爆发的恢复力的担忧。密度减少（即疏伐）处理可以增加剩余树木的活力，但减少对未来干扰易感性的处理效果的持久性仍然是一个关键的知识空白。利用北美山区亚高山老林最长的一系列重复实验之一，我们在最近（2000 年代初期）发生严重的山松甲虫（Dendroctonus ponderosae）爆发后测量了林分结构，以检验历史（1940 年代）疏伐处理对两个恢复力组成部分的影响。我们提出了以下问题：“历史疏伐强度如何影响（1）易感树木（白云杉 [Pinus contorta]≥12 厘米直径）的树木尺度生存概率和林分尺度生存比例（统称对爆发的“抵抗力”），以及（2）爆发后的林分演替轨迹？”总体爆发严重程度较高（山松甲虫杀死了 59%的易感个体和 78%的易感基部面积），历史疏伐对树木尺度和林分尺度的抵抗力影响较小。树木尺度的生存概率随树径的增加而急剧下降，并且在历史疏伐处理中与对照（未切割的林分）没有差异。历史疏伐林分中幸存的易感树木的林分尺度比例和基部面积与对照林分没有差异，除了那些几乎移除了所有易感树木的处理，在这些处理中，存活比例增加了近一倍。尽管对山松甲虫爆发的抵抗力影响有限，但历史处理的遗留问题导致了从大直径到小直径白云杉的优势转变，因此，历史疏伐林分在爆发后的活基部面积比对照林分大 2 倍左右。对照林中大直径白云杉因山松甲虫驱动而死亡和历史疏伐林中密度依赖的小直径树木死亡导致了爆发后活树木林分结构的趋同。一个例外是最严重的历史疏伐处理（去除了 59-77%的基部面积），其中耐荫、不易感的针叶树的幼树优势低于对照林分。六十年后，疏伐处理对树皮甲虫爆发的抵抗力影响很小，但在塑造爆发后的演替轨迹方面留下了持久的遗产。
