By ecoRI News staff
Pitch pine forests are at
greater risk of attack from the southern pine beetle than forests with a mix of
tree species, according to recent research by Dartmouth College. The study shows that the composition of forests is more important than other factors when predicting where the destructive pest will strike next.
The research, published in Forest
Ecology and Management, adds to understanding of the southern pine
beetle and confirms previous research from the beetle’s southern habitat on the
importance of characteristics that increase forest susceptibility to the pest.
The research finding has
important implications for forest managers who need to predict and prevent
infestation by a pest that is already responsible for significant forest damage
and that is continuing its climate-induced move northward, according to
researchers.
“Knowing which tree stands may be most susceptible to this beetle is extremely important information for managers working to protect our forests,” said Carissa Aoki, a post-doctoral research associate at Dartmouth and lead author of the study. “This research not only tells us that preventative treatment such as thinning can be effective, but also helps prioritize tree stands for treatment based on structural characteristics."
Southern
pine beetles have a hard reddish brown to black exoskeletons
and measure about 0.12 inches, about the size of a grain of rice.
For the study, researchers
focused on southern pine beetle infestations in the New Jersey Pinelands, a
forested area that spans the southern and central portions of the state. The
coldest night of the winter in this region has warmed by about 7 degrees
Fahrenheit in the past 50 years, creating favorable conditions for the southern
pine beetle to increase its range.
The last documented outbreak
of southern pine beetle in New Jersey occurred more than 80 years ago, but the
mid-Atlantic states may see more regular outbreaks in the future. The northern
New Jersey Pinelands, and pitch pine stands in the New England states with
similar structural characteristics, are particularly at risk of infestation as
the southern pine beetle continues to move northward, according to researchers.
As of 2014, a new outbreak
was detected on New York’s Long Island, and scattered throughout Connecticut
the following year. Beetles have additionally been trapped as far north as
Rhode Island and Massachusetts, though large-scale tree mortality hasn’t yet occurred
in these southern New England states.
“The northward movement of
the southern pine beetle is just one example of how climate warming is
permitting rapid range expansions,” said Matthew Ayres, a professor of
biological sciences at Dartmouth. “We can expect many more cases because the
warming continues. This might mean you can grow a cherry tree where you
couldn’t before, but you and your plants can also expect a growing battery of
pests that weren’t there before.”
The researchers found that
southern pine beetle infestations in both wetland and upland areas were far
more likely to occur in pure conifer stands than mixed stands of oak and pine.
While wetland conifer areas were especially affected, wetland mixed sites had
fewer spots than expected.
While the study confirms
some of what was previously known about the southern pine species, the finding
in the Pinelands — that a high percentage of pine trees in a stand is a more
important factor than moisture levels — is in contrast to previous research that
indicated strong evidence for the connection between high moisture and stand
susceptibility.
Researchers also found that
stands of intermediate age — about 25 to 75 years for pitch pine — were
disproportionately infested. Forest stands comprised of older, larger trees
tended not to be very susceptible, while young trees are known not to be
susceptible and weren’t sampled. The volume of trees in a stand and the
percentage of each tree that is green were also found to contribute to stand
susceptibility.
The results indicate that
the same tactics that have been effective at limiting beetle impacts in the
South could also be effective in newly occupied northern ranges. Those tactics
include monitoring to detect population increases, rapid suppression of spots when
they are still rare, and thinning of trees for prevention.
Southern pine beetle
activity occurs in extremes — either very rare or through infestations that
involve millions of pests. During episodic outbreaks, the beetles readily kill
even the healthiest pines through synchronized attacks that overwhelm tree
defenses.
“The southern pine beetle is
one of the most aggressive tree-killing insects in the world. Outbreaks tend to
be self-sustaining because the more beetles there are, the better they succeed,”
said Ayres, a co-author of the report with 25 years of experience studying the
beetle species.