Large wood supports Elwha revegetation by reducing ungulate browsing

Abstract

IntroductionThe increasing number of dams approaching obsolescence drives a need for knowledge about riparian restoration associated with dam removal. Restoring woody vegetation on exposed reservoir beds following dam removal is essential to stabilizing sediment, reconnecting riverine and terrestrial systems, and providing future sources of shade, nutrients, and wood. Revegetation after dam removal on many rivers can be challenging due to rapidly drying sediment, low sediment nutrient content, and heavy ungulate browse pressure. Revegetation in Elwha River restoration, the largest dam removal to date, used large woody debris (LWD) to mitigate moisture and nutrient limitation but ungulate browsing has constrained woody plant growth in many coarse sediment deposits. We evaluated potential for LWD to reduce ungulate browsing following Elwha dam removal.MethodsWe studied LWD mitigation of browsing in the largest former reservoir and a comparable valley upriver with a natural floodplain. We measured browse intensity in randomly located plots stratified by four levels of LWD extent, from no LWD to complete LWD enclosure.ResultsLWD reduced browse intensity four-fold in the former reservoir, but only in plots fully surrounded by LWD. Partial LWD enclosure provided little browse reduction. We obtained similar results in the upriver valley, where browse intensity was somewhat lower except within wood clusters. Wood-mediated browse reduction was slightly greater in the former reservoir than in the upriver valley. Protection from browse was greatest for plant species preferred by ungulates.DiscussionThese results suggest forest restoration after dam removal can be expedited by surrounding young trees with large logs. Planting within LWD clusters or placing LWD clusters in restoration sites can facilitate establishment of forest islands in strategic locations. These forest islands can support dispersal of seeds and marine derived nutrients, reconnect established forest to the river, and potentially advance restoration by decades.

A review of natural and managed revegetation responses in two de-watered reservoirs after large dam removals on the Elwha River, Washington, USA

“…During the first 5 years of light browsing impacts, plantings and naturally occurring vegetation established successfully, reaching average canopy heights above browse levels prior to the regular presence of C. canadnsis roosevelti herds (J. Chenoweth, National Park Service, personal observation). Johnson et al (2023) found that plants on the Elwha that were fully surrounded by large woody debris (LWD) clusters experienced significantly less browse intensity than plants growing in the open or with LWD on one or two sides.…”

Vegetation responses to large dam removal on the Elwha River, Washington, USA

“…secondarily consume large quantities of fruit, and changes in salmon abundance may alter local bear distributions and patterns of seed dispersal (Harrer and Levi, 2018). Taken together with the observed effects of wildlife on revegetation in and around LW accumulations in the former reservoirs (Johnson et al, 2023), these findings illustrate the reciprocal roles that can be played by vegetation and wildlife in ecological restoration: wildlife benefit from restored habitats, and their activities in turn affect patterns of vegetation growth and restoration outcomes (McCaffery et al, 2018).…”

“…In Lake Mills, LW was associated with reductions in wind speed, soil temperature and evaporative stress (Colton, 2018), all of which can inhibit germination and contribute to plant mortality (MaChado and Paulsen, 2001;Wahid et al, 2007). Clusters of LW also likely enhanced seedling growth by impeding browsing by large ungulates (Johnson et al, 2023). At planting sites in Lake Mills, the presence of LW was associated with increased survivorship through the first growing season after planting (Calimpong, 2014).…”

“…Invasive vegetation control was largely effective, as evidenced by the fact that neither of the former reservoirs was dominated by non-native species, and these efforts were bolstered by other forms of active management, particularly seeding (Morgan, 2018;Cendejas-Zarelli, 2021). Similarly, seeding and planting efforts were bolstered by LW placement, as evidenced by the increased rates of survivorship observed in planting sites with LW installations (Calimpong, 2014;Johnson et al, 2023) and the increased species richness observed where seeding was paired with LW (Cendejas-Zarelli, 2021). Together, these findings point to the benefits of multiple forms of active management, undertaken in concert, for revegetating large areas exposed after dam removal.…”