Data from: Fire severity unaffected by spruce beetle outbreak in spruce-fir forests in southwestern Colorado

Recent large and severe outbreaks of native bark beetles have raised concern among the general public and land managers about potential for amplified fire activity in western North America. To date, the majority of studies examining bark beetle outbreaks and subsequent fire severity in the U.S. Rocky Mountains have focused on outbreaks of mountain pine beetle (MPB, Dendroctonus ponderosae) in lodgepole pine (Pinus contorta) forests, but few studies, particularly field studies, have addressed the effects of the severity of spruce beetle (Dendroctonus rufipennis Kirby) infestation on subsequent fire severity in subalpine Engelmann spruce (Picea engelmannii) and subalpine fir (Abies lasiocarpa) forests. In Colorado, the annual area infested by spruce beetle outbreaks is rapidly rising, while MPB outbreaks are subsiding; therefore understanding this relationship is of growing importance. We collected extensive field data in subalpine forests in the eastern San Juan Mountains, southwestern Colorado, to investigate whether a gray-stage (<5 years from outbreak to time of fire) spruce beetle infestation affected fire severity. Contrary to the expectation that bark beetle infestation alters subsequent fire severity, correlation and multivariate generalized linear regression analysis revealed no influence of pre-fire spruce beetle severity on nearly all field or remotely sensed measurements of fire severity. Findings were consistent across moderate and extreme burning conditions. In comparison to severity of the pre-fire beetle outbreak, we found that topography, pre-outbreak basal area, and weather conditions exerted a stronger effect on fire severity. Our finding that beetle infestation did not alter fire severity is consistent with previous retrospective studies examining fire activity following other bark beetle outbreaks and reiterates the overriding influence of climate that creates conditions conducive to large, high-severity fires in the subalpine zone of Colorado. Both bark beetle outbreaks and wildfires have increased autonomously due to recent climate variability, but this study does not support the expectation that post-beetle outbreak forests will alter fire severity, a result that has important implications for management and policy decisions.近年来北美西部本土树皮甲虫大规模严重暴发，引发了公众与土地管理者对该区域火灾活动加剧可能性的担忧。迄今为止，美国落基山脉地区针对树皮甲虫暴发与后续火灾烈度的研究，大多聚焦于扭叶松（Pinus contorta）林内的山地松甲虫（mountain pine beetle, MPB, Dendroctonus ponderosae）暴发情况，但鲜有研究——尤其是野外研究——探讨云杉甲虫（Dendroctonus rufipennis Kirby）侵染烈度对亚高山恩格尔曼云杉（Picea engelmannii）与亚高山冷杉（Abies lasiocarpa）林后续火灾烈度的影响。在科罗拉多州，云杉甲虫暴发侵染的年度面积正快速攀升，而山地松甲虫的暴发态势却在消退，因此厘清二者间的关联愈发重要。我们在科罗拉多州西南部圣胡安山脉东部的亚高山森林中收集了大量野外数据，以探究灰期（即从甲虫暴发至火灾发生间隔不足5年）的云杉甲虫侵染是否会影响火灾烈度。与“树皮甲虫侵染会改变后续火灾烈度”的预期相悖，相关性分析与多变量广义线性回归分析结果显示，火灾前云杉甲虫的侵染烈度几乎未对任何野外实测或遥感反演的火灾烈度指标产生影响。该结论在中度与极端燃烧条件下均保持一致。相较于火灾前甲虫暴发的烈度，我们发现地形、暴发前胸高断面积以及气象条件对火灾烈度的影响更为显著。我们关于甲虫侵染未改变火灾烈度的发现，与此前针对其他树皮甲虫暴发后火灾活动的回顾性研究结果一致，同时再次印证了气候的主导性影响——气候为科罗拉多州亚高山区域大规模高烈度火灾的发生创造了有利条件。受近期气候波动影响，树皮甲虫暴发与野火的发生均呈自主增加趋势，但本研究并不支持“甲虫暴发后的森林会改变火灾烈度”这一预期，该结果对管理与政策制定具有重要意义。