Survey methods
We conducted this study over the course of three summers (2016 – 2018).
We counted Tsuga heterophylla and Picea sitchensisseedlings 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
>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. 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. Because Harmon
& Franklin (1989) found that seedlings that receive less than 0.6% of
light (< 12.5 µmol/m2/sec) would not be able
to survive, and seedling survival was reduced with high moss biomass (an
indicator of moss depth), we also measured light irradiance with a
photometer in 50 pairs under and beside thick bryophyte mats focusing on
the predominant moss in the forest, Hylocomium splendens . Decay
class of each nurse log for each plot was measured using the kick test,
which included three levels of decay classes: 1 = bark still intact upon
contact, 2 = some to various debris falls upon contact, 3 = foot
completely in log, several pieces fragmented off of the log (modified
from Christy & Mack, 1984; Sollins, Cline, Verhoeven, Sachs, &
Spycher, 1987, and Fogel, Ogawa, and Trappe, unpublished report ).
These classes correlate with the age of the nurse log so we used them as
an indicator of nurse log age (Harmon, 1989b).