Cascading effects of mountain pine beetle outbreak: Pine seedling survival is reduced in beetle-killed stands

Background/Question/Methods

Following the mountain pine beetle (MPB; Dendroctonus ponderosae) outbreak, entire landscapes of mature Pinus contorta forests will be replaced by a new cohort of seedlings. Identifying potential limitations on seedling survival will be critical for predicting trajectories of beetle-killed stands. Toward this goal, we tested the importance of three resources necessary for the establishment of P. contorta, and likely affected by tree mortality: access to ectomycorrhizal fungal networks, light and moisture availability. We predicted that tree mortality would decrease the presence of fungal networks and thus, seedling survival should be less in the absence of hyphal connections in undisturbed than that in stands experiencing high tree mortality. We tracked survival of 660 seedlings sowed across sites representing a gradient of beetle-induced tree mortality (0-80% basal area killed), and manipulated hyphal and root access to seedlings via mesh. Light and soil moisture were measured for the two years seedling survival was tracked. 

Results/Conclusions

Seedling survival decreased with increasing tree mortality, though along this gradient, both availability of light and soil moisture increased. Contrary to our prediction, in undisturbed stands, access to ectomycorrhizal fungal networks did not increase seedling survival, nor did it affect seedling survival in stands with high pine mortality. Seedling survival was decreased by the presence of roots to the same extent along the tree mortality gradient indicating that the strength of root competition remained constant. Compensatory growth of understory plants likely offset the loss of roots due to pine mortality. Seedlings establishing in beetle-killed stands do not appear limited by essential resources, i.e., light, water and access to ectomycorrhizal networks, but their survival is impaired nonetheless.