The natural ‘exclosure effect’ and tree regeneration following post-windstorm salvage logging

Understanding the influence of post-disturbance forest management on tree
regeneration is critical for assessing ecosystem recovery and guiding
future responses. In particular, the influx of elevated coarse woody
material (CWM) following wind disturbance, if left in situ, may impede
herbivore access, thereby protecting saplings from browsing damage through
a natural ‘exclosure effect.’In 2013, a tornado in northcentral Maine, USA
and subsequent salvage logging operations created three clear ‘treatments’
for evaluation of the exclosure effect: blowdown, blowdown plus salvage
logging, and an undamaged control. Nine years post-tornado, we inventoried
tree regeneration within these treatments to evaluate differences in
sapling abundance, species composition, size structure, and browsing
intensity. We also inventoried CWM, including the height above forest
floor.Results revealed significant differences in sapling composition and
browsing intensity among treatments. The salvage treatment had the highest
proportion of browsed saplings (56 ± 28%; mean ± standard error), followed
by the control (9 ± 10%) and blowdown (5 ± 8%). Blowdown had by far the
greatest mean (50 ± 9 cm) and average maximum (169 ± 43 cm) heights for
CWM. Binomial generalized linear models revealed that browsing probability
was a function of mean CWM height and an interaction between sapling
density and proportion of sapling hardwoods. Thus, browsing damage was
less likely in plots with greater CWM heights and more likely in plots
with greater sapling density and more hardwood
saplings. Synthesis and applications: This study furthers our
understanding of ecosystem recovery following blowdown and salvage
logging. Results suggest that salvage logging created important
differences in CWM abundance and height distribution, when compared to
un-salvaged areas, and that these differences in turn altered sapling size
structure and browsing intensity. These findings highlight the potential
long-term effects of successive disturbances, as the differences evident
in these early stages may persist for decades or longer. Importantly, we
provide evidence of the exclosure effect, suggesting that CWM retained in
the un-salvaged area protected saplings from moose browsing. Thus, in
post-disturbance areas where browsing threatens regeneration, we recommend
that managers consider retaining CWM to serve as a natural exclosure.