Nurse logs are fallen logs that provide safe germination sites for many tree species. In northern temperate rainforests, tree seedling density may be influenced by nurse log age or the moss community supported by the nurse logs. Previous work in northern temperate rainforests on the Olympic peninsula in Washington State has shown that tree seeds cannot germinate beneath the thick moss mats created by Hylocomium splendens and Sphagnum girgensohnii that dominate the forest floor and old nurse logs. Thus, tree regeneration could not only be spatially limited but also temporally limited to younger nurse logs with thin moss mats. Little is known about the timescale on which this cycle occurs and the length of the temporal window of opportunity for successful regeneration. We surveyed the moss depth, visual decay state, seedling number of each tree species and the percent cover of each moss species on 13 Picea sitchensis and Tsuga heterophylla nurse logs in Temperate Rainforest on the Olympic Peninsula in Washington State. Multiple cores were also drilled from each nurse log and a proximal selection of standing trees of the same species. This allowed each nurse log to be aligned within the masterplot from the standing trees so that the year each nurse log fell could be established. Thus, the length of time the log had been on the ground could be compared against the moss community and tree seedlings established on the log’s surface.
Results/Conclusions
We were successful in aligning the nurse log cores within the area masterplot to establish the year each of the 13 nurse logs fell. We found that the abundance of mosses that create thin mats significantly declined over time as the abundance of mosses that create thick mats significantly increased with a crossover period when the tree has been fallen for approximately 120 years. The visual decay state scale was also validated as visual decay levels aligned significantly with actual time since the tree had fallen. Tree seedling density followed a curvilinear relationship with nurse log age with a peak of seedling density at 110-130 years of the log being on the ground. These results suggest that tree regeneration is both spatially and temporally limited in northern temperate rainforests.