DOI: 10.5061/dryad.5x69p8d34
title: Plant-plant interactions change during succession on nurse logs in a northern temperate rainforest

description: Plant-plant interactions change through succession from facilitative to
 competitive. At early stages of succession, early-colonizing plants can
 increase the survival and reproductive output of other plants by
 ameliorating disturbance and stressful conditions. At later stages of
 succession, plant interactions are more competitive as plants put more
 energy towards growth and reproduction. In northern temperate rainforests,
 gap dynamics result in tree falls that facilitate tree regeneration (nurse
 logs) and bryophyte succession. How bryophyte-tree seedling interactions
 vary through log succession remains unclear. We examined the relationships
 of tree seedlings, bryophyte community composition, bryophyte depth, and
 percent canopy cover in 166 0.5 m x 1.0 m plots on nurse logs and the
 forest floor in the Hoh rainforest in Washington, USA to test the
 hypothesis that bryophyte-tree seedling interactions change from
 facilitative to competitive as the log decays. 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 &gt;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 forest floor plants, such as 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 enable both tree seedlings and smaller bryophyte species to
 avoid competition with forest floor plants, including the dominant
 bryophyte, H. splendens. H. splendens is likely a widespread driver of
 plant community structure given its dominance in northern temperate
 forests. 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. --
description: We counted <i>Tsuga heterophylla </i>and
 <i>Picea sitchensis </i>seedlings in 166 randomly placed 1 m x
 1 m plots on the forest floor (n = 52) and 1 m x nurse log diameter on
 nurse logs (defined as &gt;30 cm diameter and at least 2 m in length;
 nurse log diameter ranged from 30 to 104 cm; n = 114). For a subset of the
 nurse log plots (n = 86) and forest floor plots (n = 12), bryophyte
 composition was surveyed using the point-intercept method whereby each
 bryophyte species found under each of 100 randomly placed dots on a
 transparent sheet was noted. Percent canopy cover was measured as a
 potential confounding factor using a densiometer placed in the center of
 each plot. Bryophyte depth was measured (cm) using a caliper in the center
 of each plot. This study was conducted over the course of three summers
 (2016 – 2018). This data set includes a list of tree seedling
 density, <em>Tsuga heterophylla </em>seedling density, percent
 canopy cover, bryophyte depth, and the percent cover of the most abundant
 bryophyte species in each plot.
description: Not all plots had all measurements. Bryophyte composition was
 only measured in 98 plots and percent canopy cover was only measured in
 132 plots.