Monitoring Amphibians in the Declined Hemlocks at Harvard Forest 2013-2014

Disturbances such as outbreaks of nonnative insects and pathogens can devastate unique habitats and directly reduce biodiversity. The foundation tree species Tsuga canadensis (eastern hemlock) is declining due to infestation by the nonnative insect Adelges tsugae (hemlock woolly adelgid). The decline and expected elimination of hemlock from northeastern US forests is changing forest structure, function, and assemblages of associated species. We assessed changes in occupancy, detection probability, and relative abundance of two species of terrestrial salamanders, Plethodon cinereus (eastern red-back salamander) and Notopthalmus viridescens viridescens (eastern red-spotted newt), in the experimental removal of T. canadensis at Harvard Forest. Four treatments (logging, girdling, hemlock control and hardwood control) have been applied and replicated in eight 0.81-ha plots. Salamanders were sampled under cover boards and using visual encounter surveys in June-July of 2013 and 2014. Removal of the hemlock canopy increased occupancy of P. cinereus but significantly reduced its estimated detection probability and abundance. Estimated abundance of N. v. viridescens also declined dramatically after canopy manipulations. Our results suggest that ten years after hemlock loss due to either the adelgid or pre-emptive salvage logging, and 50-70 years later when these forests have become mid-successional mixed deciduous stands, that the abundance of these salamanders likely will be less than 50% of their abundance in current, intact hemlock stands.诸如外来昆虫与病原体暴发这类干扰事件，可对独特生境造成毁灭性破坏，并直接降低生物多样性。建群树种加拿大铁杉（Tsuga canadensis, eastern hemlock）正因外来昆虫铁杉球蚜（Adelges tsugae, hemlock woolly adelgid）的侵染而种群衰退。美国东北部森林中铁杉的衰退与预期消亡，正在改变森林结构、功能以及伴生物种的群落组成。本研究依托哈佛森林（Harvard Forest）的加拿大铁杉人工移除实验，对两种陆生蝾螈的占据率、检测概率及相对丰度的变化展开评估，这两种蝾螈分别为红背蝾螈（Plethodon cinereus, eastern red-back salamander）与东方红点蝾螈（Notopthalmus viridescens viridescens, eastern red-spotted newt）。实验设置了四种处理组：采伐作业、环剥伐除、铁杉对照组以及硬阔对照组，并在8块面积为0.81公顷的样地中开展重复实验。2013年与2014年的6-7月，研究人员采用掩板式采样法与偶遇式调查法对蝾螈种群进行采样。移除铁杉冠层后，红背蝾螈的占据率有所提升，但其估算检测概率与种群丰度却显著下降；而东方红点蝾螈的估算丰度在冠层干预后同样出现大幅降低。研究结果显示，在铁杉因铁杉球蚜侵染或预防性打捞伐木而消亡十年后，且再过50至70年、该类森林演替至中生演替阶段的混交落叶林时，这两种蝾螈的种群丰度或将不足现有完整铁杉林内种群丰度的50%。