Nurse logs, which are fallen trees, play an important role in the regeneration of tree species. In northern temperate rainforests in the Pacific Northwest, tree seedlings are much more abundant on nurse logs than the forest floor. The fidelity of tree seedlings to nurse log microsites for regeneration in these forests has been hypothesized to be due to release from competition with herbs and mosses on the forest floor. However, there is a high degree of bryophytes covering trees and nurse logs in these forests as well as a lot of variation in the distribution of bryophyte species on these substrates, which could have facilitative or competitive interactions with tree seedlings. Building on previous research done in the Picea sitchensis-Tsuga heterophylla forests of the Pacific Northwest, we conducted several studies to examine the importance of nurse logs for tree seedlings focusing on the effects of the nurse log bryophyte community and how that could change with nurse log decay stage. In the Olympic rainforest in the Hoh river basin, we measured tree seedling abundance, bryophyte community composition, bryophyte depth, and canopy coverage on nurse logs and the forest floor in 156 randomly placed 1 m2 plots, and noted decay stage of each nurse log.
Results/Conclusions
Tree seedling density on nurse logs was 467% greater than on the forest floor. On nurse logs, tree seedling density decreased significantly with increasing bryophyte depth, which was driven by a large feather moss species (Hylocomium splendens) that creates thick mats. Bryophyte community composition varied with nurse log decay stage, such that bryophytes that create thin mats (< 4 cm), such as Rhizomnium glabrescens and Scapania bolanderi were more abundant on younger nurse logs and bryophytes that create thick mats (> 4 cm), such as H. splendens were more abundant on older nurse logs. Thus, nurse log decay stage influenced bryophyte community composition, which, in turn, influenced tree seedling density. Our findings corroborate those of previous studies done in these forests that suggested that competition with forest floor herbs and mosses were responsible for the disproportionate number of tree seedlings on nurse logs. Our findings also indicate that tree regeneration may be further limited by nurse logs at later stages of decay as they host bryophytes that create thick mats that hinder seedling growth. Thus, tree seedling regeneration niches in these forests may be more limited than previously thought.