Mountain Pine Beetle Dynamics and Reproductive Success in Post-Fire Lodgepole and Ponderosa Pine Forests in Northeastern Utah

Fire injury can increase tree susceptibility to some bark beetles (Curculionidae, Scolytinae), but whether wildfires can trigger outbreaks of species such as mountain pine beetle (Dendroctonus ponderosae Hopkins) is not well understood. We monitored 1173 lodgepole (Pinus contorta var. latifolia Doug.) and 599 ponderosa (Pinus ponderosa Doug. ex Law) pines for three years post-wildfire in the Uinta Mountains of northeastern Utah in an area with locally endemic mountain pine beetle. We examined how the degree and type of fire injury influenced beetle attacks, brood production, and subsequent tree mortality, and related these to beetle population changes over time. Mountain pine beetle population levels were high the first two post-fire years in lodgepole pine, and then declined. In ponderosa pine, populations declined each year after initial post-fire sampling. Compared to trees with strip or failed attacks, mass attacks occurred on trees with greater fire injury, in both species. Overall, a higher degree of damage to crowns and boles was associated with higher attack rates in ponderosa pines, but additional injury was more likely to decrease attack rates in lodgepole pines. In lodgepole pine, attacks were initially concentrated on fire-injured trees, but during subsequent years beetles attacked substantial numbers of uninjured trees. In ponderosa pine, attacks were primarily on injured trees each year, although these stands were more heavily burned and had few uninjured trees. In total, 46% of all lodgepole and 56% of ponderosa pines underwent some degree of attack. Adult brood emergence within caged bole sections decreased with increasing bole char in lodgepole pine but increased in ponderosa pine, however these relationships did not scale to whole trees. Mountain pine beetle populations in both tree species four years post-fire were substantially lower than the year after fire, and wildfire did not result in population outbreaks.

火灾损伤可提升树木对部分树皮甲虫（Curculionidae, Scolytinae）的易感性，但野火是否会诱发山松甲虫（Dendroctonus ponderosae Hopkins）这类物种的种群爆发，目前仍未得到充分阐明。我们在犹他州东北部尤因塔山脉的一处本地存在山松甲虫种群的区域，对野火后的1173株洛矶山扭叶松（Pinus contorta var. latifolia Doug.）与599株美国黄松（Pinus ponderosa Doug. ex Law）开展了为期三年的监测。本研究探究了火灾损伤的程度与类型如何影响甲虫侵袭、子代种群繁育以及后续树木死亡情况，并将上述指标与甲虫种群随时间的动态变化相关联。在洛矶山扭叶松林中，山松甲虫种群密度在野火后的前两年处于较高水平，随后逐渐下降；而在美国黄松林中，种群密度在野火后首次采样后的每一年均呈下降趋势。相较于出现条带状侵袭或侵袭失败的树木，两个树种中火灾损伤更严重的植株均发生了大规模甲虫侵袭。总体而言，美国黄松的树冠与树干损伤程度越高，其甲虫侵袭率也越高；但洛矶山扭叶松的额外火灾损伤则更可能降低其侵袭率。洛矶山扭叶松的甲虫侵袭最初仅集中于火灾损伤植株，但在后续年份中，甲虫会侵袭大量未受损伤的个体；而美国黄松林分因受火程度更严重，几乎无未受损伤的树木，因此每年的甲虫侵袭均主要发生在受损植株上。整体来看，共有46%的洛矶山扭叶松与56%的美国黄松受到了不同程度的甲虫侵袭。在笼养树干区段的试验中，洛矶山扭叶松的子代成虫羽化率随树干炭化程度升高而降低，而美国黄松则呈相反趋势，但上述相关性均无法推广至整株树木。野火发生四年后，两个树种林分中的山松甲虫种群密度均显著低于火灾后第一年，且野火并未诱发甲虫种群爆发。