Exploring seedling-based aspen (Populus tremuloides) restoration near range limits in the Intermountain West, USA

With the effects of climate change expected to intensify over the coming century, land managers will require more proactive and novel approaches to conserve and restore threatened ecosystems. In the US Intermountain West, quaking aspen (Populus tremuloides Michx.) is a foundation species of considerable conservation interest. However, traditional silvicultural practices in the region focus exclusively on its vegetative regeneration potential, limiting restoration efforts to locations where aspen is already established, and precluding approaches such as afforestation and assisted migration. Planting nursery-grown aspen seedlings could overcome these limitations. This approach has become common for forest land reclamation and afforestation projects in the boreal forests of western North America, but has received little attention in the US Intermountain West to date. In this study we explored the potential for seedling-based aspen restoration in an Intermountain West landscape. We planted 7,200 nursery-grown aspen seedlings across 15 unirrigated plots at three high elevation (~2,800 m) sites in southwestern Utah, USA and monitored their growth and survival rates for two growing seasons. Overall seedling survival was only 10% by the end of the study, with 49% of mortality due to apparent drought stress, 38% due to small mammal herbivory, and 3% due to unknown causes. 79% of the surviving seedlings were located in just two plots, where higher levels of soil moisture were present during the driest summer months. A subsequent test of wood mulch to retain moisture in one of the plots increased seedling survival to 62%, compared to 0% in a non-mulched treatment. Together, these findings indicate that seedling-based aspen restoration in these habitats was limited primarily by site conditions. For this approach to become operational, additional study on seedling quality, site selection and site preparation will be necessary.