RAPID: Is Carbon Starvation a Proximal Cause of Tree Mortality from Defoliation?

Harvard University, Cambridge MA

Investigators

Abstract

A severe gypsy moth outbreak began in 2015 in southern New England. By 2018, this outbreak had caused dramatic death of trees across thousands of acres of oak forest. It has been more than thirty years since this invasive insect caused such extensive tree damage and death in eastern North America. This event has created a fleeting opportunity to address a fundamental question in ecology: what are the immediate causes of tree death? Hardwood trees draw on stored starches and sugars in the spring to produce new leaves. Gypsy moths eat leaves, limiting a tree's ability to store enough sugars and starches, and if these drop below a critical level, the tree can die. During last year's outbreak, the researchers established a network of plots in oak forests that had experienced differing severity and frequency of gypsy moth activity. If they act fast and sample starches and sugars from oak trees that have experienced defoliation for 0 to 3 years before they leaf-out this spring, then follow these trees through the next growing season, the researchers can determine if starvation directly causes tree death. Better understanding of the timing and causes of tree mortality will inform forest and landscape tree management. This research will also provide student training in answering science questions and how to apply scientific knowledge to solving forest management problems. Temperate deciduous trees draw on stores of non-structural carbohydrates (NSC) in the spring to produce new xylem and foliage. The carbon starvation hypothesis posits that when NSC stores become depleted, secondary growth and flushing is impossible or inadequate, and the tree dies. Defoliation, such as occurs during gypsy moth infestations, is a straightforward mechanism for testing the carbon starvation hypothesis. This study will evaluate NSC levels in relationship to defoliation frequency and severity to determine if there is a critical NSC threshold below which tree mortality occurs. The researchers will sample NSCs by collecting small wood samples from the stem and root of oak trees with known defoliation histories. This study has the potential to offer a mechanistic understanding of tree mortality events and inform the models scientists and policy-makers rely on to forecast ecosystem responses to global change. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.