Data from: Wildfire provides refuge from local extinction but is an unlikely driver of outbreaks by mountain pine beetle

Bark beetle outbreaks and wildfire are important disturbances in conifer ecosystems, yet their interactions are not well understood. We evaluated whether fire injury increased susceptibility of lodgepole pines (Pinus contorta) to mountain pine beetle (Dendroctonus ponderosae Hopkins), how it influenced beetle reproductive success, and whether beetle population phase altered this interaction. Eight sites that experienced wildfire and eight unburned sites were examined in the Greater Yellowstone Ecosystem (U.S.A). Half were in areas where D. ponderosae was undergoing outbreaks and half were in areas with low populations. We examined 2056 trees one year after fire for burn injury and beetle attack. We quantified beetle reproductive success in a random sample of 106 trees, and measured gallery areas of D. ponderosae and competing subcortical herbivores in 79 additional trees. Baited flight traps sampled stand-level populations of subcortical herbivores and predators. Wildfire predisposed trees to D. ponderosae attack, but non-linearly, with moderately-injured trees being most preferred. This tree-level interaction was influenced by stand-level beetle population size, in that both healthy and fire-injured trees of all classes were attacked where populations were high, but no healthy trees, and only low- and moderately-injured trees were killed where populations were low. The number of adult brood produced per female was likewise curvilinear, being highest in moderately-injured trees. This reflected an apparent trade-off, with high intraspecific competition arising from the large number of beetles needed to overcome defenses in healthy trees, versus high interspecific competition and low substrate quality in more injured trees. These results suggest fire-injured trees can provide a reservoir for D. ponderosae during periods when populations are too low to overcome defenses of healthy trees, and might otherwise face localized extinction. However, the likelihood of populations increasing from endemic to outbreak levels in response to increased susceptibility is offset by the opposing constraints of lower substrate quality and higher competitor load in severely-injured hosts, and the relative scarcity of moderately-injured trees. Wildfire may confer some reproductive increases to populations already out-breaking. We present a conceptual model of how these disturbances and inherent feedbacks interact to affect beetle population dynamics.

树皮甲虫暴发与野火是针叶林生态系统中两类重要的干扰因子，然而二者的相互作用机制尚未得到充分阐明。本研究旨在评估火烧损伤是否会提升扭叶松（Pinus contorta）对山松甲虫（Dendroctonus ponderosae Hopkins）的易感性，探究火烧如何影响甲虫的繁殖成功率，以及甲虫种群阶段是否会改变这一相互作用。研究在美国大黄石生态系统内选取了8个火烧样地与8个未火烧对照样地，其中一半样地处于山松甲虫暴发区域，另一半处于甲虫种群低密度区域。在火灾发生一年后，研究人员对2056株林木开展了火烧损伤与甲虫蛀害情况的调查；通过随机抽取的106株林木样本量化甲虫的繁殖成功率，并在额外79株林木中测量了山松甲虫与其他竞争性皮层植食性昆虫（subcortical herbivores）的坑道面积。使用诱饵飞行陷阱对林分尺度下的皮层植食性昆虫与捕食者种群进行了采样调查。野火会使林木更易遭受山松甲虫蛀害，但该效应并非线性，中度火烧损伤的林木最易被甲虫偏好攻击。林木尺度的这一相互作用受林分尺度甲虫种群密度的调控：当甲虫种群密度较高时，所有健康等级的健康林木与火烧损伤林木均会遭受攻击；而当种群密度较低时，无健康林木被蛀杀，仅轻度与中度火烧损伤的林木会被致死。每头雌虫产出的成虫子代数量同样呈曲线变化，在中度火烧损伤的林木中达到峰值。这一现象反映出明显的权衡关系：健康林木中，甲虫需大量聚集以突破寄主防御，导致种内竞争激烈；而在损伤程度更高的林木中，种间竞争加剧且寄主基质质量下降。研究结果表明，当甲虫种群密度过低、无法突破健康林木的防御时，火烧损伤林木可作为山松甲虫的种群储备库，否则甲虫可能面临局地灭绝风险。不过，因寄主易感性提升而使甲虫种群从地方流行水平增长至暴发水平的可能性，会被多重反向约束抵消：重度火烧损伤寄主的基质质量更低、竞争者负载更高，且中度火烧损伤林木相对稀缺。对于已进入暴发阶段的甲虫种群，野火或可提升其繁殖成功率。本研究提出了一个概念模型，用以阐释这两类干扰与内在反馈如何共同作用于甲虫种群动态。