Background/Question/Methods Plant-plant interactions during succession can vary from facilitation in early stages to competition in later stages. In northern temperate rainforests, gap-phase dynamics alter the microclimate and provide tree boles as sites for forest regeneration (i.e., nurse logs). These nurse logs are often colonized by bryophytes and tree seedlings that vary depending on the decay stage of the log. Thus, interspecific interactions between bryophytes and seedlings could change with nurse log succession. We surveyed tree seedlings and bryophyte communities in 166 plots on nurse logs (n = 112, 1 m x diameter of nurse log) and the forest floor (n = 52, 1m2) in the Hoh rainforest on the Olympic peninsula in Washington to determine how interspecific plant-plant interactions change with nurse log decay. For each plot, we measured percent canopy cover, bryophyte depth, and noted the decay class of each nurse log. We also set up 10 permanent 1.0 x 0.5 m plots on nurse logs and the forest floor to examine the influence of intraspecific interactions (number of conspecific or heterospecific individuals), substrate type (bryophyte species, litter), substrate depth, percent canopy cover, and initial starting height on the mortality of individual seedlings. Results/Conclusions Tree seedling density was highest on young logs with lower canopy cover and early colonizing bryophyte species that create thin mats (< 4 cm), such as Rhizomnium glabrescens, and lowest on old logs and the forest floor with higher canopy cover and bryophytes that create thick mats (>4 cm; Hylocomium splendens). Tree seedling density was positively influenced by substrate type (nurse log decay class and forest floor) and percent canopy cover, and negatively influenced by percent cover of Hylocomium splendens. Thus, interspecific plant-plant interactions shifted from facilitation to competition as the log decayed, and suggest that logs are essential for tree seedlings to escape competition with forest floor bryophytes. We found 17 bryophyte species on nurse logs and only 6 on the forest floor showing that nurse logs are essential for the maintenance of bryophyte diversity. The best predictors of seedling mortality were the number of conspecific neighbors and initial height as positive factors, and percent canopy cover as a negative factor. Tree seedling densities can reach >200 on young nurse logs, which could further increase tree seedling survival. Thus, we found that both interspecific and intraspecific plant-plant interactions are essential drivers of tree seedling dynamics in this northern temperate rainforest.
