Abstract:
Logs are seedbeds for trees in many Picea sitchensis-Tsuga
heterophylla forests of Washington and Oregon. Factors affecting this
close association, including seed retention and seedling establishment
on logs and successional changes in log characteristics, are explored
In this study.
Field and laboratory experiments indicated that competition with
vegetation on the forest floor favored tree seedling development on
logs. Reciprocal transplants of soil and log blocks revealed differences in substrate quality and not position effects, such as
standing water, produced the "nurse-log" phenomenon. Clearing
vegetation significantly increased survival of planted and natural
conifer seedlings above survival on uncleared plots. Experiments on
the effects of predation and soil pathogens indicated these
interactions were of minor importance. Logs are therefore sites where
competitive effects are sufficiently small to allow abundant seedling
recruitment.
Log surfaces vary widely in their ability to retain seeds and
needles. Moss- and litter-covered surfaces retained 48-98% of seeds
and needles, whereas sound and rotten wood and bare bark retained
0-8%. A model of seedling establishment on log surfaces in
Picea-Tsuga forests, which incorporated the effects of seed retention
and seedling survival rates, indicated 1.3% of the seed crop would
produce one-year-old seedlings on logs but only 0.02-0.18% on
undisturbed forest floor.
Surficial litter accumulations enable Picea and Tsuga seedlings to
establish and grow rapidly on slightly decayed logs. Survival rates
for the first two years of both species increased asymptotically with
litter biomass with a maximum of 62% for Picea and 34% for Tsuga.
Seedling growth was fastest when canopy openness and litter biomass
were high, but slow when either factor was low.
Successional development influences tree seedling recruitment and
survival on logs. Changes in bark, bryophytes, humus and trees were
examined for a chronosequence of Picea, Pseudotsuga and Tsuga logs. Bark fragmentation was a critical process; it removed plants and
humus, reinitiated succession and was responsible for major
differences in successional patterns among log species. Although a
large proportion of tree seedlings establish on logs in Picea-Tsuga
forests, high mortality rates caused by competition, bark
fragmentation and toppling from logs indicated long-term survival on
logs was very low and possibly equivalent to the forest floor.
