Eschtruth Anne K, Battles John J
University of California, Berkeley, Environmental Science, Policy, and Management, 137 Mulford Hall, Berkeley, California 94720-3114, USA.
Ecol Appl. 2008 Mar;18(2):360-76. doi: 10.1890/07-0446.1.
Hemlock woolly adelgid (HWA; Adelges tsugae) infestations have resulted in the continuing decline of eastern hemlock (Tsuga canadensis) throughout much of the eastern United States. While the initial impacts of HWA infestations have been documented, our understanding of forest response to this disturbance remains incomplete. HWA infestation is not occurring in isolation but within a complex ecological context. The role of potentially important interacting factors, such as elevated levels of white-tailed deer herbivory, is poorly understood. Despite the potential for herbivory to alter forest successional trajectories following a canopy disturbance, little is known about herbivory-disturbance interactions, and herbivory is rarely considered in assessing forest response to a co-occurring disturbance. We used repeated censuses of deer exclosures and paired controls (400 paired plots) to quantify the impact of deer herbivory on tree seedling species abundance in 10 eastern hemlock ravines that span a gradient in HWA-induced canopy decline severity. Use of a maximum likelihood estimation framework and information theoretics allowed us to quantify the strength of evidence for alternative models developed to estimate the impacts of herbivory on tree seedling abundance as a function of varying herbivore density and canopy decline severity. The exclusion of deer herbivory had marked impacts on the abundance of the studied seedling species: Acer rubrum, Acer saccharum, Betula lenta, Nyssa sylvatica, Quercus montana, and Tsuga canadensis. For all six species, the relationship between seedling abundance and deer density was either exponential or saturating. Although the functional form of the response varied among seedling species, the inclusion of both deer density and canopy decline severity measures consistently resulted in models with substantially greater support. Canopy decline resulted in higher proportional herbivory impacts and altered the ranking of herbivory impacts by seedling species. Our results suggest that, by changing species' competitive abilities, white-tailed deer herbivory alters the trajectory of forest response to this exotic insect pest and has the potential to shift future overstory composition.
铁杉球蚜（HWA；Adelges tsugae）的侵害导致美国东部大部分地区的东部铁杉（Tsuga canadensis）持续减少。虽然已经记录了铁杉球蚜侵害的初步影响，但我们对森林对这种干扰的反应的理解仍然不完整。铁杉球蚜的侵害并非孤立发生，而是发生在一个复杂的生态环境中。诸如白尾鹿食草量增加等潜在重要相互作用因素的作用，人们了解甚少。尽管食草可能会改变树冠干扰后森林的演替轨迹，但关于食草-干扰相互作用知之甚少，并且在评估森林对同时发生的干扰的反应时很少考虑食草因素。我们对鹿围栏和配对对照（400个配对地块）进行了重复普查，以量化鹿食草对10个东部铁杉峡谷中树木幼苗物种丰富度的影响，这些峡谷跨越了铁杉球蚜导致的树冠衰退严重程度的梯度。使用最大似然估计框架和信息理论，使我们能够量化为估计食草对树木幼苗丰富度的影响而开发的替代模型的证据强度，该影响是食草动物密度和树冠衰退严重程度变化的函数。排除鹿食草对所研究的幼苗物种——红枫、糖枫、桦木、紫树、蒙大拿栎和加拿大铁杉的丰富度产生了显著影响。对于所有六个物种，幼苗丰富度与鹿密度之间的关系要么是指数关系，要么是饱和关系。尽管不同幼苗物种的反应功能形式不同，但同时纳入鹿密度和树冠衰退严重程度指标始终会使模型得到更大的支持。树冠衰退导致更高比例的食草影响，并改变了不同幼苗物种的食草影响排名。我们的结果表明，通过改变物种的竞争能力，白尾鹿食草改变了森林对这种外来害虫的反应轨迹，并有可能改变未来上层林分的组成。
