Harvey Brian J, Donato Daniel C, Romme William H, Turner Monica G
Ecol Appl. 2014;24(7):1608-25. doi: 10.1890/13-1851.1.
The degree to which recent bark beetle (Dendroctonus ponderosae) outbreaks may influence fire severity and postfire tree regeneration is of heightened interest to resource managers throughout western North America, but empirical data on actual fire effects are lacking. Outcomes may depend on burning conditions (i.e., weather during fire), outbreak severity, or intervals between outbreaks and subsequent fire. We studied recent fires that burned through green-attack/red-stage (outbreaks <3 years before fire) and gray-stage (outbreaks 3–15 years before fire) subalpine forests dominated by lodgepole pine (Pinus contorta var. latifolia) in Greater Yellowstone, Wyoming, USA, to determine if fire severity was linked to prefire beetle outbreak severity and whether these two disturbances produced compound ecological effects on postfire tree regeneration. With field data from 143 postfire plots that burned under different conditions, we assessed canopy and surface fire severity, and postfire tree seedling density against prefire outbreak severity. In the green-attack/red stage, several canopy fire-severity measures increased with prefire outbreak severity under moderate burning conditions. Under extreme conditions, few fire-severity measures were related to prefire outbreak severity, and effect sizes were of marginal biological significance. The percentage of tree stems and basal area killed by fire increased with more green-attack vs. red-stage trees (i.e., the earliest stages of outbreak). In the gray stage, by contrast, most fire-severity measures declined with increasing outbreak severity under moderate conditions, and fire severity was unrelated to outbreak severity under extreme burning conditions. Postfire lodgepole pine seedling regeneration was unrelated to prefire outbreak severity in either post-outbreak stage, but increased with prefire serotiny. Results suggest bark beetle outbreaks can affect fire severity in subalpine forests under moderate burning conditions, but have little effect on fire severity under extreme burning conditions when most large wildfires occur in this system. Thus, beetle outbreak severity was moderately linked to fire severity, but the strength and direction of the linkage depended on both endogenous (outbreak stage) and exogenous (fire weather) factors. Closely timed beetle outbreak and fire did not impart compound effects on tree regeneration, suggesting the presence of a canopy seedbank may enhance resilience to their combined effects.
近期树皮甲虫（西松大小蠹）爆发对火灾严重程度及火灾后树木更新的影响程度，引起了北美西部资源管理者的高度关注，但关于实际火灾影响的实证数据却很匮乏。结果可能取决于燃烧条件（即火灾期间的天气）、爆发严重程度，或爆发与后续火灾之间的间隔。我们研究了美国怀俄明州大提顿地区近期发生的、烧过由扭叶松（Pinus contorta var. latifolia）主导的绿色侵袭/红色阶段（火灾前爆发时间<3年）和灰色阶段（火灾前爆发时间3 - 15年）亚高山森林的火灾，以确定火灾严重程度是否与火灾前甲虫爆发严重程度相关，以及这两种干扰对火灾后树木更新是否产生复合生态影响。利用来自143个在不同条件下燃烧的火灾后样地的实地数据，我们根据火灾前爆发严重程度评估了树冠层和地表火灾严重程度以及火灾后树木幼苗密度。在绿色侵袭/红色阶段，在中等燃烧条件下，几种树冠层火灾严重程度指标随火灾前爆发严重程度增加。在极端条件下，很少有火灾严重程度指标与火灾前爆发严重程度相关，且效应大小具有边际生物学意义。火灾致死的树干百分比和基部面积随绿色侵袭阶段树木（即爆发的最早阶段）相对于红色阶段树木增多而增加。相比之下，在灰色阶段，在中等条件下，大多数火灾严重程度指标随爆发严重程度增加而下降，在极端燃烧条件下火灾严重程度与爆发严重程度无关。火灾后扭叶松幼苗更新在两个爆发后阶段均与火灾前爆发严重程度无关，但随火灾前具球果量增加。结果表明，在中等燃烧条件下，树皮甲虫爆发可影响亚高山森林的火灾严重程度，但在该系统中大多数大型野火发生时的极端燃烧条件下，对火灾严重程度影响很小。因此，甲虫爆发严重程度与火灾严重程度存在适度关联，但关联的强度和方向取决于内源性（爆发阶段）和外源性（火灾天气）因素。时间上紧密相连的甲虫爆发和火灾并未对树木更新产生复合影响，这表明树冠种子库的存在可能增强对其综合影响的恢复力。
