Stunning fall foliage. Blame it on the bugs?

Credit: Bernard Spragg.NZ

Fundamental research can often challenge long-held assumptions about the way the world works, and when it comes to why and when oaks, maples and other deciduous tree leaves change color and shed, we’ve long thought these changes were due solely to shortening, colder days with less humidity and rain.

Forkner says that one of the major defoliators on red maple is the rosy maple moth ( Dryocampa rubicunda). Credit: Rebecca Forkner

One NSF-funded entomologist believes that leaf-feeding insects may also trigger these changes. Rebecca Forkner at George Mason University is looking at how these insects may both advance and delay processes that determine the level of vibrancy in our fall foliage and also the duration of this color transition before leaves fall from the trees.

Petiole labels help mark and follow the life span of a single leaf. Credit: Rebecca Forkner

Forkner will study red oak, red maple, black gum, and red bud trees, which are common in eastern North America, and contrast the leaf damage from insects in tree species that contain anthocyanins (the pigments responsible for the gorgeous oranges, reds and purples associated with fall foliage) and other trees with other pigments so that she and her research team can identify if there are associated impacts.

Secondarily, she and her research team will follow up on these differences in amount and timing of autumn coloration to see if subsequent impacts arise in the spring in terms of when new leaves appear.

According to Forkner, in the past, we thought of the compounds responsible for fall color as “waste” compounds. In fact, anthocyanins respond to many cues from the environment. Insect feeding is one of those cues. She explained that insect damage alters many aspects of plant nutrient balance, growth, and survival, so it’s not a stretch to predict that insects could have strong impacts on color change and leaf fall in autumn as well.

Studying this phenomenon doesn’t end when autumn ends. Forkner and her team also do measurements in the spring to study correlated impacts then too. This is a spring measure of a red oak bud burst from May 2018. Credit: Rebecca Forkner

“Models of seasonal patterns that don’t incorporate insect damage can underestimate future shifts in spring leaf flush or overestimate the length of the forest growing season,” Forkner said. “Understanding how insects alter fall foliage does more than improve our climate models and carbon budgets, for which information about leaf life spans is critical. The compounds responsible for fall color in leaves affect many ecological processes — from how long it takes leaves to decompose to the number of nutrients a tree stores from year to year to the number of microbe species found in the soil. Leaf-feeding insects are a missing piece in all of those puzzles.”