Terrestrial amphibian distribution, habitat associations and downed wood temperature profiles in managed headwater forests with riparian buffers in the Oregon Coast Range

Graduation date: 2008Western forests have become increasingly fragmented landscapes dominated\ud by young stands. Given that western Oregon forests largely consist of headwater\ud systems, there is a need to better understand how headwater forest taxa and their\ud habitats are impacted by forest management practices. Several amphibian species\ud associated with forested headwater systems have emerged as management concerns.\ud Forest management strategies, such as harvests that remove only part of the canopy\ud and retention of riparian buffer strips, may help ameliorate some of the negative\ud effects on amphibians in managed forests. Pre-existing site conditions, such as legacy\ud downed wood, also may play a role in buffering the impacts of silvicultural practices\ud on terrestrial amphibians. Downed wood is an important habitat component for many\ud amphibians, because the cool, moist microclimates of downed wood can provide\ud refugia for terrestrial amphibians during warmer summer months. However, downed\ud wood habitat suitability is another emerging concern as the rate of input and size of\ud downed wood declines in managed forests.\ud As part of the USDI Bureau of Land Managements Density Management\ud Study, we investigated how untreated streamside buffers modify impacts of upland\ud thinning on headwater forest terrestrial amphibians and their habitat at three sites in\ud the Oregon Coast Range. To further assess habitat associations of these animals, we\ud conducted a field experiment to address amphibian cover use, including downed\ud wood, moss and coarse and fine substrates. In addition, we examined how temperature\ud profiles inside small- and large-diameter downed wood and soil temperatures differed\ud from ambient air temperatures. Temperatures of wood and soil were monitored at\ud different slope positions (near streams and upslope) and overstory regimes (thinned\ud and unthinned stands) to assess potential habitat suitability and buffering capabilities\ud against seasonal temperature extremes for plethodontid salamanders.\ud Our results suggest that pre-existing site conditions (e.g., amount of rocky or\ud fine substrate) play an important role in determining the response of terrestrial\ud amphibians to upland forest thinning. However, retention of stream buffers is\ud important in maintaining unaltered stream and riparian conditions. Moderate thinning\ud and preservation of vital habitat in riparian and nearby upland areas by way of\ud variable-width buffers (15 m minimum width) may be sufficient in maintaining\ud suitable habitat and microclimatic conditions vital to amphibian assemblages in\ud managed headwater forests.\ud Additionally, logs of a wide size range and soils may provide sufficient\ud protection against thermal extremes harmful to plethodontid salamanders in thinned\ud stands with limited overstory. However, this alone cannot support plethodontid\ud salamanders. These salamanders require exposed areas (e.g., leaf litter, soil surface,\ud rock faces) where much of their foraging and well as courtship occurs. Partial\ud retention of the canopy through moderate thinning coupled with variable-width\ud riparain buffers that may increase in width when suitable terrestrial habitat is\ud encountered, may provide sufficient microhabitat, microclimate, and protection in\ud maintaining terrestrial amphibian assemblages in managed headwater forests