Plant-plant interactions change during succession on nurse logs in a northern temperate rainforest

Abstract

Plant-plant interactions can vary depending on the severity of the environment. Positive interactions, such as facilitation, are important in early life stages while negative interactions, such as competition, predominate in later stages. Through succession, plant-plant interactions often change from facilitative to competitive. In northern temperate rainforests, gap dynamics result in tree falls that facilitate tree regeneration (nurse logs) as well as bryophyte succession. While the importance of nurse logs for tree seedlings is known, how the interactions of bryophyte communities and tree seedlings vary through succession of the log remains unclear. We examined the relationships of tree seedlings, bryophyte community composition, bryophyte depth, and percent canopy cover in 166 plots on nurse logs and the forest floor in the Hoh rainforest in Washington, USA. Tree seedling density was highest on young logs with early-colonizing bryophyte species (e.g., Rhizomnium glabrescens), and lowest on decayed logs with Hylocomium splendens, a long-lived moss that reaches depths >20 cm. As a result, bryophyte depth increased with nurse log decay and was negatively associated with tree seedling density. Tree seedling density was 4.6x higher on nurse logs than on the forest floor, which was likely due to competitive exclusion by H. splendens. Nurse logs had 17 species of bryophytes while the forest floor had six, indicating that nurse logs contribute to maintaining bryophyte diversity. Nurse logs are essential for forest dynamics as they enable both tree seedlings and smaller bryophyte species to avoid competition with the dominant forest floor bryophyte, H. splendens. Given that H. splendens has a global distribution and is often dominant in forested systems across the northern hemisphere, it is likely a widespread driver of plant community structure. Our findings indicate that plant-plant interactions shift with succession on nurse logs from facilitative to competitive and, thus, influence forest community structure and dynamics.