Logging effects on liana diversity and abundance in Central Guyana

Executive Summary
<strong>Logging</strong> had a large impact on the composition of liana communities, but how large depended
on the logging intensity and the species concerned. Nineteen species, of 102 present before
logging, disappeared from the plots after logging. All species lost were rare, and it is likely
that chance <strong>effects</strong> played a large role in determining which species disappeared rather than
their sensitivity to logging. Thirty-five species appeared after logging, so the net effect of
logging was an increase in the number of species present.
The most obvious effect of logging on liana communities was an increase in overall liana
abundance. However, this increase was only found in the most heavily logged plots, while in
plots with light or moderate logging intensity no increase in abundance was noted.
Species diversity, whether expressed as the number of species present or Fisher’s α, increased
with logging intensity. Two factors contributed to this increase. In the first place, increased
stem density in the most heavily logged plots increases the probability that a species is present
(the density effect). In the second place, logging creates habitats that are rare or not present in
undisturbed forest: gaps, and more in particular, skidded gaps. Several species are specialists
of high light environments or are stimulated by the soil disturbance caused by skidders.
Hence, many species that are rare of absent in undisturbed forest find suitable growth
conditions in gaps or skidded gaps.
These trends are also found on the smaller scale of habitats. Forest patches that are untouched
by logging activities have the lowest diversity, while gaps have the highest. Skidded gaps
have intermediate diversity, but because of the complete removal of pre-existing vegetation
and the different growth conditions on bare and compacted soil of skidtrails, their species
composition is least similar to the undisturbed forest.
Comparing the responses of individual species, it was evident that certain species responded
much stronger to the opportunities offered by increased gap area and soil disturbance than
others. Of 59 common species, sixteen showed relatively strong and consistent positive
responses to logging and logging-related habitats, while only three showed negative
responses. Twenty-two species can be considered indifferent to logging and logging intensity,
while the remaining eighteen species showed variable or inconsistent responses. Other species
were not abundant enough to draw conclusions. A small number of species, exemplified by
the common species passion flower (Passiflora glandulosa) and fire rope (Pinzona coriacea),
were strongly associated with skidded gaps. Another set was more strongly associated with
gaps. Together these species demonstrate pioneer-like ecological behaviour.
In spite of the changes caused by logging, pre-existing spatial patterns of species composition
remain relatively strong, even in the small geographic area of Pibiri. Logged plots were still
very similar in species composition to nearby unlogged plots, while similarity with plots
logged at the same intensity decreased with increased distance.
SUCCESSIONAL DEVELOPMENT OF LIANA COMMUNITIES AFTER LOGGING
After logging, a succession takes place during which sites opened by logging regain
vegetation cover and biomass. Gradually, sun-loving species that colonise the open patches
give away to species that can grow in the forest understorey. In principle, it is expected that
liana communities will gradually grow back into an “undisturbed” state closely resembling
pre-harvest species composition and abundance. To study this process, liana communities
were compared among 18 plots that differed in age since they were logged. Plot age varied
from 0-16 years. All plots were heavily logged, comparable to the heaviest treatment of the
logging intensity study. As is usual in these so-called “chronosequence studies”, species
composition of liana communities of the plots varied with other environmental and site
parameters and with logging method along with plot age. This problem was partly overcome
by comparing logged plots at each site with nearby unlogged controls.
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