Sustainable Forest Management Network – SFM
Battling bugs help maintain a healthy Canadian forest
Logging and forest management practices could have a direct impact on
devastating forest tent caterpillar outbreaks in the boreal forest.
Dr. Jens Roland, an insect population ecologist at the University of
Alberta and a researcher with the Sustainable Forest Management Network,
one of Canada's 20 federal Networks of centres of Excellence, has discovered
a correlation between forest tent caterpillar infestations and the amount
of forest left standing after an area has been harvested. The caterpillars
are a principal defoliating insect of trembling aspen in the boreal forest
across the country.
Dr. Roland's work suggests that a forest tent caterpillar outbreak and
the rate of collapse is a critical indicator of the overall health of
Canada's aspen boreal forest. His work also provides various new options
for forest managers who would like to minimize the effects of the caterpillar
outbreaks, which normally
have 10-to-12-year cycles.
"During a major outbreak, forest tent caterpillars can completely
defoliate an aspen forest and virtually stop its growth," says Dr.
Roland. "But while we can't stop a tent caterpillar infestation,
we don't want to do anything to prolong it either, such as rendering its
natural enemies ineffective through the effects of altered forest structure."
The research has focused on four species of parasitic flies that are
the main enemy of these caterpillars, as well as various viral diseases
that also kill forest tent caterpillars. The research also showed that
larger forest stands provide the environment needed for the tent caterpillar
enemies to reduce the duration of outbreaks.
"We were aware that forest cover has an impact on the search behaviour
of these flies as they attempt to find their prey, and on the rate at
which viruses spread," explains Dr. Roland. "We may want to
avoid having lots of small forest stands left behind. We need to log in
a way that leaves areas large and
contiguous so the normal processes can function."
And because the disease and parasites are so tightly linked with the
tent caterpillar (their main source of food), they enter a cyclic pattern
– where the tent caterpillars increase, then a year or so later
the parasites and disease start to increase. "We think that fragmentation
of forest stands in some way
decouples the tent caterpillar from its natural enemies. Their natural
enemies are less able to respond numerically to the forest tent caterpillar
outbreak, and then there's a longer lag and the outbreak lasts longer
in those fragmented stands."
Roland and his team of researchers sampled 127 sites covering an area
of 400 square kilometres near Ministik Hills, Alberta. They studied aspen
stands ranging in size from as small as .28 hectares to 289 hectares.
"What we determined," said Roland, "is that for the parasites
and the viruses to be most
effective, the size of the forest stand should be a minimum of about 100
hectares. Smaller forest stands serve as a caterpillar refuge because
of less effective natural enemies – helping to extend the length
of the infestation by several years."
www.ualberta.ca/sfm
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