Bayesian Predictions of Bark Beetle Attack and Mortality of Three Conifer Species During Epidemic and Endemic Population Stages
Bayesian Predictions of Bark Beetle Attack and Mortality of Three Conifer Species During Epidemic and Endemic Population Stages
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DOI：
10.3389/ffgc.2021.679104
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发表时间：
2021-05
期刊：
影响因子：
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通讯作者：
Mario Bretfeld;H. Speckman;D. Beverly;B. Ewers
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作者：
Mario Bretfeld;H. Speckman;D. Beverly;B. Ewers
小蠹虫自然栖息于森林中，当它们的数量达到流行程度时，会导致大量树木死亡。最近一次（20世纪90年代 - 21世纪10年代）主要由山松小蠹（Dendroctonus ponderosae）引发的流行，是美国西部森林中树木死亡的主要原因。小蠹虫种群及其对森林影响的预测模型在很大程度上取决于与寄主相关的参数，例如林分年龄、断面积和密度。我们假设小蠹虫的攻击模式也取决于推断的小蠹虫种群密度：大量流行的小蠹虫种群会优先攻击大直径树木，并凭借数量优势成功杀死它们。相反，小规模的本地小蠹虫种群会伺机攻击受压和小树。我们通过对怀俄明州东南部亚高山森林中三种主要森林树种（扭叶松Pinus contorta、恩格尔曼云杉Picea engelmannii和亚高山冷杉Abies lasiocarpa）进行12年的反复实地观测，并结合贝叶斯建模方法来验证这一假设。这些模型提供了小蠹虫攻击模式的概率预测，无需像频率论模型那样需要一些在这些数据集中经常不成立的假设。此外，我们评估了幼苗/幼树的再生情况以应对上层林冠死亡，并假设在林冠死亡率最高的区域幼苗/幼树的定植率更高，因为资源从竞争的树木中释放出来。我们的研究结果表明，在流行年份，所有树种中，大直径树木比小直径树木更容易被小蠹虫攻击和杀死，但在流行后年份并没有转向优先攻击小直径树木的趋势。然而，与流行年份相比，流行后年份中小直径扭叶松和恩格尔曼云杉被小蠹虫攻击和死亡的概率有所增加。我们还发现，由于小蠹虫导致的上层林冠死亡，林下植被（禾草类、非禾本草本植物和灌木）的整体生长以及幼苗/幼树的定植有所增加，尤其是在以扭叶松为主的林分中。我们的观察结果为三种常见树种在小蠹虫种群从流行到流行后阶段的转变过程中遭受攻击和死亡的轨迹以及森林早期再生提供了证据。
Bark beetles naturally inhabit forests and can cause large-scale tree mortality when they reach epidemic population numbers. A recent epidemic (1990s–2010s), primarily driven by mountain pine beetles (Dendroctonus ponderosae), was a leading mortality agent in western United States forests. Predictive models of beetle populations and their impact on forests largely depend on host related parameters, such as stand age, basal area, and density. We hypothesized that bark beetle attack patterns are also dependent on inferred beetle population densities: large epidemic populations of beetles will preferentially attack large-diameter trees, and successfully kill them with overwhelming numbers. Conversely, small endemic beetle populations will opportunistically attack stressed and small trees. We tested this hypothesis using 12 years of repeated field observations of three dominant forest species (lodgepole pine Pinus contorta, Engelmann spruce Picea engelmannii, and subalpine fir Abies lasiocarpa) in subalpine forests of southeastern Wyoming paired with a Bayesian modeling approach. The models provide probabilistic predictions of beetle attack patterns that are free of assumptions required by frequentist models that are often violated in these data sets. Furthermore, we assessed seedling/sapling regeneration in response to overstory mortality and hypothesized that higher seedling/sapling establishment occurs in areas with highest overstory mortality because resources are freed from competing trees. Our results indicate that large-diameter trees were more likely to be attacked and killed by bark beetles than small-diameter trees during epidemic years for all species, but there was no shift toward preferentially attacking small-diameter trees in post-epidemic years. However, probabilities of bark beetle attack and mortality increased for small diameter lodgepole pine and Engelmann spruce trees in post-epidemic years compared to epidemic years. We also show an increase in overall understory growth (graminoids, forbs, and shrubs) and seedling/sapling establishment in response to beetle-caused overstory mortality, especially in lodgepole pine dominated stands. Our observations provide evidence of the trajectories of attack and mortality as well as early forest regrowth of three common tree species during the transition from epidemic to post-epidemic stages of bark beetle populations in the field.