PS 29-151 - Down woody debris microsite facilitation for black huckleberry (Vaccinium membranaceum) seedlings

Abstract: Black huckleberry (Vaccinium membranaceum) is an economically, culturally, and ecologically important ericaceous shrub of the Pacific Northwest. Climate change, including increasing drought stress, and land use changes, including fire suppression, have driven widespread declines in black huckleberry production, habitat loss, and range shifts. As a result, Indigenous communities, foresters, and landowners in the Pacific Northwest have explored various management and restoration tools to promote continued black huckleberry survival and recruitment. One potential restoration practice is the placement or retention of down woody debris (DWD) to manipulate microsite conditions in open habitats and post-disturbance forest sites. The benefits of DWD microsites, including increased shade and soil moisture, have long been recognized but have rarely been directly studied.

We explored the potential of DWD-generated microsites to promote the growth and survival of black huckleberry seedlings. We hypothesized that seedlings would exhibit higher survival and more growth when planted closer to DWD, and the benefit would be greatest on the north side where microsite features like shade, high soil moisture, and low surface temperatures would be strongest. We planted one-year-old black huckleberry seedlings at 0.0, 0.25, 0.5, and 1.5 meters from DWD on the north and south aspects of east-west oriented DWD segments. Over two growing seasons (April-August), we made monthly measurements of seedling vigor, survival, height, photosystem II fluorescence, soil surface temperature and soil moisture. We used linear mixed effect models and cumulative-link mixed models to examine how seedlings responded to planting location relative to each DWD segment and the associated microsite.

Black huckleberry seedlings exhibited highest survival on the north aspect at the 0.0 m and 0.25 m distances, locations that were associated with significantly higher soil moisture and lower soil surface temperatures compared to the south aspects and 0.5 m and 1.5 m distances. Seedlings at the near distances on the north side also exhibited highest vigor, tallest heights, and highest values of photosystem II fluorescence, all traits which indicate overall healthier seedlings at these locations compared to the farther distances and the south aspect.

Our results illustrate the importance of DWD microsites and their facilitation of black huckleberry survival when habitats might otherwise be unsuitable. Microsite phenomena are applicable to other understory and tree species that face habitat loss from climate change and may serve as a restoration tool both through DWD retention after a disturbance or clear-cut and intentional DWD placement, as demonstrated in this study.