Research Explains Why Some Oak Trees Are More Resilient After Caterpillars Feast
Very hungry caterpillars have been killing mighty oak trees across southern New England for years.
The Northeast sees a lot of different insect invaders, says Audrey Barker Plotkin. She's a scientist with Harvard Forest.
Some trees will die because of insect defoliation, but Plotkin learned in her recent multiyear study why some oaks have more resilience.
Audrey Barker Plotkin, Harvard Forest: When this insect outbreak happened in 2016, it began and we kind of perked up our heads and thought, "Oh, this hasn't happened for a while." [In] 2017, things got really bad. There were a lot of defoliated trees throughout southern New England. And then in 2018, it continued and that's when we started seeing some trees dying.
We thought, "We've got to find out what's going on." So we went out into the field and did some evaluations of how badly defoliated the trees were, so how much of the leaves were missing. And we did that in the Quabbin Watershed Forest because that's a big intact forest. We also, and this was led by Joe Elkington's lab at UMass, so his lab identified a bunch of trees and did the same thing, where they evaluated how much of the leaves have been munched by the caterpillars. So we joined forces and decided on choosing some of those trees to sample the stored sugars and starches to see, is that associated with which genes are going to die?
Carrie Healy, NEPM: Stored sugars in the trees are kind of like carbohydrates to people, right?
They are carbohydrates. Yeah, so primarily there are sugars. And those are really important for a lot of functions of a tree. So there's starch as well, which is primarily stored fuel. So it's kind of backup energy for, say, when a tree loses a bunch of its leaves, it can create a new set based on drawing on that stored starch. Sugars are even more important for a lot of different functions, like water transport and freeze tolerance and defense against other insects and diseases.
So you were able to measure sugars in these oak tree species from the two locations. What were some of the findings of your research?
Yeah, so one thing that we found was that, you know, the basic question, are the sugars and starches drawn down some more heavily defoliated a tree is? And we found that at both of the sites, that that indeed was the case. We also found that the trees that died, all had stored sugars and starches that were really low. And so that seems like it was clearly associated with their mortality.
The trees on the roadsides in the town tended to have higher carbohydrate stores than the ones in the interior forest. And that might be because the trees on the roadsides get more sun and therefore have a chance to build up their reserves a bit more. And that came through in our results, where we saw less of a draw down in those storage sugars and starches with defoliation than the trees in the interior forest.
So you're saying even though caterpillars were found on trees along roadsides, the damage wasn't as great to the ones along the roadsides as the defoliation deep in a forest?
Yeah, exactly. If you had a tree that had a similar amount of caterpillars eating the leaves, the trees on the roadsides seemed like they had suffered less from them.
What is it that drew you to studying oak trees in this way?
We focused on the oaks because although the Lymantria caterpillar is a generalist — it will see it on a wide variety of trees and shrubs — oaks are its favorite hosts in our region. So it was more that those were the trees that were typically severely defoliated by the caterpillars.
So what we were studying here is [kind of an] iconic example, but it is an example of a really major threat to our forests. There's Lymantria, there's hemlock woolly adelgid, there's emerald ash borer. And based on global trade and the kind of proximity of major trade ports and heavily forested areas in our region, that's going to continue.