In this post, Audrey Barker Plotkin of Harvard University talks to us about her latest research where she investigates how invasive insects can starve trees and the importance of protecting temperate forest land.
I’m a forest ecologist based at the Harvard Forest in the northeastern United States. Because of high levels of global trade and forest cover, our region is especially vulnerable to forest insect introductions. Therefore, one of my major research themes is exploring the consequences of insect outbreaks on forest structure and function. We recently witnessed a severe outbreak of a defoliating moth that ended up killing many mature oak trees. I’m interested in why some trees died and others survived. One seemingly obvious cause of death is that when insects eat a tree’s leaves – their sugar-making organs – the tree draws down their energy stores and that tree can starve to death if those reserves get too low. This ‘carbon starvation’ hypothesis makes a lot of sense, but past work doesn’t give a clear answer, as some studies found that trees died well before their reserves were spent, in part due to additional stressors like drought. Also, most prior studies included sampled surviving trees.
To answer this question, I worked with team of collaborators, including an entomologist, a physiological ecologist, and a remote sensing expert. Based on satellite imagery, we identified sites with varying intensity of defoliation during the outbreak. We included trees growing in a large, forested watershed, along with trees growing on forest edges such as along roads. We visited each tree to assess how much of its leaves were munched by the caterpillars, and then went back in the winter to find out how that related to the trees’ stored starches and sugars.
To measure the stored sugars and starches (these are collectively known as nonstructural carbohydrates), we collected small wood samples from oak stems and roots. The midwinter sampling campaign included chipping through ice and snow – and in some cases poison ivy vines – to sample oak tree roots. Sampling in the summer would have been simpler, but since carbohydrate reserves fluctuate seasonally, sampling in mid-winter was the best time to understand what the trees had stored up to fuel (or not) spring leaf-out. This is how I ended up with a severe poison ivy rash in the middle of a New England winter!
My amazing co-author Meghan Blumstein, a physiological ecologist with experience leading studies of nonstructural carbohydrates, then took the samples and extracted the sugars and starches. The lab protocol included a series of chemical extractions and a new spectrophotometer, along with more familiar items including a retrofitted deep-fat fryer. Blumstein processed all the samples in a marathon eight 15-hour days during Summer 2020, since lab access was extremely limited during the height of the pandemic.
All that dedication to the project paid off; in our new study published in Functional Ecology, we demonstrated that increasingly severe defoliation indeed draws down energy reserves – sometimes to zero – providing direct evidence that stressed trees can starve to death.
Forest edge trees had higher sugar and starch levels that were less sensitive to defoliation than interior forest trees. The forest edge trees may be more resilient to the effects of defoliation because they receive more light, but it may be instead a result of somewhat differing defoliation history between the forested and edge sites. I’d be interested in future studies that explore this more deeply.
All the trees that died had extremely low energy reserves (<1.5% dry weight of sugars and starches in their roots and stems), providing empirical evidence that these trees starved to death. We were surprised that trees could survive with such low reserves, since they are used not only as back-up fuel to produce a new set of leaves, but are involved in water transport, defense, freeze-tolerance, and many other basic physiological functions.
Our findings will improve global change forecasts and help target management interventions. Now that we have a better understanding of what causes an individual tree to live or die after defoliation, I’m next working to better understand site-level mortality risk factors, and to quantify how much this oak loss will diminish forest productivity and alter the future forest.
Oak is a major tree species worldwide and dominates much of the forests in southern New England. Its acorns are important for wildlife, it is a major driver of the temperate forest carbon sink, and it is a valuable timber tree. I hope this work helps us better understand the risks to this tree, and to inform work to increase oak forest resilience.