Same streams in a different forest? Investigations of forest harvest legacies and future trajectories across 30 years of stream habitat monitoring on the Tongass National Forest, Alaska.

The effects of timber harvest practices and climate change have altered forest ecosystems in southeast Alaska. However, quantification of patterns and trends in stream habitats associated with these forests is limited owing to a paucity of data available in remote watersheds. Here, we analyzed a 30-year dataset from southeast Alaska's Tongass National Forest to understand how these factors shape stream habitats. First, we examined differences between broad management classes (i.e., harvested and non-harvested) that have been used to guide stream channel restoration goals. Second, we assessed associations between intrinsic landscape characteristics, watershed management, and timber harvest legacies on aquatic habitat metrics. And third, we examined trends in stream habitat metrics over the duration of the dataset to anticipate future management challenges for these systems. Small effect sizes for some harvest-related predictors suggest that some stream habitat metrics, such as pool densities, are less responsive than others, and management practices such as protecting riparian buffers as well as post-harvest restoration may help conserve fish habitats. Large wood densities increased with time since harvest at sites harvested >50 years ago, indicating that multiple decades of post-harvest forest regrowth may contribute large wood to streams (possibly alder), but that it is not enough time for old-growth trees (e.g., spruce, Picea, or hemlock, Tsuga,), classified as key wood, to develop and be delivered to streams. The declining trend in key wood (i.e., the largest size class of wood) regardless of management history may reflect that pre-harvest legacy old-growth trees are declining along streams, with low replacement. The introduction of wood to maintain complex stream habitats may fill this gap until riparian stands again contribute structural key wood to streams. Trend analyses indicate an increasing spatial extent of undercut banks that may also be influenced by shifting hydrologic regimes under climate change.

The influence of weevil herbivory on leaf litter chemistry in dioecious willows.

Leaf litter inputs can influence the structure and function of both terrestrial and adjacent aquatic ecosystems. Dioecy and herbivory are two factors that together have received little attention, yet have the potential to affect the quantity, quality, and timing of riparian litterfall, litter chemistry, and litter decomposition processes. Here, we explore litter chemistry differences for the dioecious Sitka willow (Salix sitchensis Sanson ex. Bong), which is establishing on primary successional habitats at Mount St. Helens (WA, USA) and is heavily infested with a stem-boring weevil (Cryptorhynchus lapathi). Weevil-attacked branches produced summer senesced litter that had significantly higher %N, lower C:N ratios, and lower condensed tannins than litter from branches that were unattacked by the weevil and senesced naturally in the autumn. Weevils more often attack female willows; however, these common litter chemicals did not significantly differ between males and females within the weevil-attacked and -unattacked groups. High-resolution mass spectrometry was used to isolate compounds in litter from 10 Sitka willow individuals with approximately 1500-1600 individual compounds isolated from each sample. There were differences between weevil-attacked litter and green leaf samples, but at this level, there was no clustering of male and female samples. However, further exploration of the isolated compounds determined a suite of compounds present only in either males or females. These findings suggest some variation in more complex litter chemistry between the sexes, and that significant differences in weevil-attacked litter chemistry, coupled with the shift in seasonality of litter inputs to streams, could significantly affect in-stream ecological processes, such as decomposition and detritivore activity.

Aquatic and riparian ecosystem recovery from debris flows in two western Washington streams, USA.

An exceptionally powerful storm struck southwestern Washington in December 2007 causing large debris flows in two adjacent streams. The two affected streams had been studied prior to the storm, providing a rare opportunity to examine ecosystem recovery. We monitored the streams and their riparian zones for six years after the disturbances to determine whether recovery rates of biota, physical habitat, and water temperature differed, and if so, what factors affected resilience. Along both streams, the debris flows removed wide swaths of soil, rock, and coniferous riparian forests, widening the active channel and increasing solar exposure and summer water temperatures. Initially depauperate of vegetation, after four years red alder trees dominated the riparian plant communities. The warmer water, greater solar radiation, and unstable substrates likely contributed to variable benthic insect and tailed frog tadpole densities over time, although benthic insect communities became more similar after three years. The debris flows also decreased channel slopes and removed channel step barriers such that cutthroat trout were able to rapidly occupy habitats far upstream, but sculpins were slower to recolonize and both fish species exhibited some differences in recovery between the two streams. Crayfish were severely impacted by the debris flows; this may be due to attributes of their life history and the timing of the flows. Overall, we found that recolonizing aquatic species exhibited varying levels of resilience and recovery after the disturbances being related to the influence of physical habitat conditions, species dispersal ability, and the presence of nearby source populations.