Department of Environmental Science and Policy, University of California, Davis, Davis, California, 95615, USA.
Department of Environmental Science and Policy, University of California, Davis, Davis, California, 95616, USA.
Ecol Appl. 2019 Jun;29(4):e01902. doi: 10.1002/eap.1902. Epub 2019 May 15.
Extreme drought stress and associated bark beetle population growth contributed to an extensive tree mortality event in California, USA, resulting in more than 129 million trees dying between 2012 and 2016. Although drought is an important driver of this mortality event, past and ongoing fire suppression and the consequent densification of forests may have contributed. In some areas, land management agencies have worked to reduce stand density through mechanical treatments and prescribed fire to restore forests to less dense, more open conditions that are presumably more resilient to disturbance and changing climate. Here, we evaluate if stand structural conditions associated with treated (e.g., thinned and prescribed burned) forests in the Sierra Nevada of California conferred more resistance to the bark beetle epidemic and drought event of 2012-2016. We found that, compared to untreated units, treated units had lower stand densities, larger average tree diameters, and greater dominance of pines (Pinus), the historically dominant trees. For all tree species studied, mortality was substantially greater in climatically drier areas (i.e., lower elevations and latitudes). Both pine species studied (ponderosa pine [Pinus ponderosa] and sugar pine [Pinus lambertiana]) had greater mortality in areas where their diameters were larger, suggesting a size preference for their insect mortality agents. For ponderosa pine, the tree species experiencing greatest mortality, individual-tree mortality probability (for a given tree diameter) was significantly lower in treated stands. Ponderosa pine mortality was also positively related to density of medium- to large-sized conspecific trees, especially in areas with lower precipitation, suggesting that abundance of nearby host trees for insect mortality agents was an important determinant of pine mortality. Mortality of incense cedar (Calocedrus decurrens) and white fir (Abies concolor) was positively associated with basal area, suggesting sensitivity to competition during drought, but overall mortality was lower, likely because the most prevalent and effective mortality agents (the bark beetles Dendroctonus brevicomis and D. ponderosae) are associated specifically with pine species within our study region. Our findings suggest that forest thinning treatments are effective in reducing drought-related tree mortality in forests, and they underscore the important interaction between water and forest density in mediating bark beetle-caused mortality.
极端干旱胁迫和相关的树皮甲虫种群增长导致了美国加利福尼亚州的一次广泛的树木死亡事件，导致 2012 年至 2016 年间有超过 1.29 亿棵树死亡。尽管干旱是导致这次死亡事件的一个重要因素，但过去和正在进行的火灾抑制以及由此导致的森林密度增加可能也起到了一定的作用。在一些地区，土地管理机构已经通过机械处理和计划火烧来降低林分密度，以使森林恢复到密度较低、较为开阔的状态，这种状态据称更能抵御干扰和气候变化。在这里，我们评估了加利福尼亚内华达山脉中与处理过的（例如，疏伐和计划火烧）森林相关的林分结构条件是否赋予了对 2012-2016 年树皮甲虫流行和干旱事件的更高抗性。我们发现，与未处理的林分相比，处理过的林分的林分密度较低，平均树木直径较大，并且松树（Pinus）的优势更为明显，松树是历史上的优势树种。对于所有研究的树种，在气候较为干燥的地区（即较低的海拔和纬度），死亡率要高得多。两种研究的松树物种（黄松[Pinus ponderosa]和糖松[Pinus lambertiana]）在其直径较大的地区死亡率更高，这表明它们的昆虫致死剂对其直径大小有偏好。对于黄松，这是经历最大死亡率的树种，在给定的树径下，个体树死亡率（probability of individual-tree mortality）在处理过的林分中显著较低。黄松的死亡率还与中到大径同种树木的密度呈正相关，特别是在降水较低的地区，这表明附近的昆虫致死剂宿主树木的丰度是影响松树死亡率的一个重要决定因素。香柏（Calocedrus decurrens）和白冷杉（Abies concolor）的死亡率与基面积呈正相关，这表明它们在干旱期间对竞争敏感，但总体死亡率较低，这可能是因为最普遍和有效的致死剂（树皮甲虫 Dendroctonus brevicomis 和 D. ponderosae）专门与我们研究区域内的松树物种有关。我们的研究结果表明，森林疏伐处理在减少与干旱相关的森林树木死亡方面是有效的，并强调了水和森林密度在调节由树皮甲虫引起的死亡方面的重要相互作用。
