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.