When thousands of acres of oak trees across southern New England died following an outbreak of spongy moth, (Lymantria dispar, formerly known as Gypsy moth), from 2016 to 2018, a team of researchers from throughout the region were curious about why some trees survived while others did not. What they discovered is that a tree’s carbohydrate reserves are crucial to its ability to withstand the damage caused by the caterpillars.
Audrey Barker Plotkin, a senior scientist at the Harvard Forest, and colleagues from the University of Massachusetts, Boston University, and the Massachusetts Institute of Technology, sampled 84 trees in the interior forest of the Quabbin Reservoir watershed in central Massachusetts and roadside trees near Amherst. They assessed how much defoliation each experienced during the outbreak, collected wood samples from the stem and roots, and extracted the sugars and starches – what Barker Plotkin called the “nonstructural carbohydrates” – to measure them in a lab. They did the same thing a year later.
“What we found is that the more defoliated a tree was, the more those carbs were depleted,” Barker Plotkin said. “Second, we identified a tipping point where if a tree’s total sugars and starch stores went below 1.5 percent dry weight – or about 20 to 25 percent of their normal storage capacity – those trees died. It seems like there is a threshold for death, and the actual cause of death is the draining of those reserves.”
Not every tree faced a similar likelihood of death following defoliation, however. The location of the tree mattered as well. The research team found that trees growing along forest edges tended to have more reserves, even those with the same severity of defoliation, making them more resilient than interior forest trees.
“The carbohydrates in roadside trees were drawn down less than trees on the interior,” Barker Plotkin said. “We can’t say why, but our speculation is that perhaps as these roadside trees are in higher light environments, they have more resources and can tolerate defoliation better than interior trees. They may be able to rebound without drawing down their reserves as much.”
This more nuanced understanding of how trees respond to defoliation will help to improve models of forest resilience as impacts from the shifting climate drive other changes in the region.
Barker Plotkin is now working to develop management interventions that can be employed to bolster energy reserves in trees so they can better withstand defoliation events. She is also identifying factors such as tree age, access to light, site conditions, tree density, and other components that might predispose certain trees to carbohydrate depletion.
“What we’ve learned about oaks gives us insights into how trees make energy tradeoffs and when they can’t do that anymore,” she said. “And it raises questions about how other tree species may fare in similar circumstances. While we can’t apply the same specific threshold, the basic idea that there is a threshold is applicable across species.”