Sustain Our Nation's Forests and Grasslands

New study quantifies the effects of invasive species in relation to tree mortality

Emerald ash borer
Of the estimated 450 non-native insects that have landed in U.S. forests, the emerald ash borer is one of the most damaging. USDA Forest Service photo by Leah Bauer.

MINNESOTA – More than 450 non-native insects and diseases have found their way to the United States and successfully established populations in the Nation’s forests. Most of these species have little known effects on forests, however 83 have caused noticeable damage and of these, 15 have caused extensive tree mortality.

A new study by USDA Forest Service scientists and Songlin Fei of Purdue University, “Biomass losses resulting from insect and disease invasions in USA forests,” is the first to comprehensively quantify the cumulative losses of trees following invasion by all species of non-native insects and diseases at a national scale. The study was published this week in the journal Proceedings of the National Academy of Sciences.

With lead author Fei, scientists Randall Morin and Andrew Liebhold of the Northern Research Station and Christopher Oswalt of the Southern Research Station found that the combined effect of invasive insects and disease cause an additional tree mortality rate of about 6.1 million tons of per year beyond the normal baseline mortality rate that occurs in U.S. forests.

In North America, forests account for an estimated 76 percent of carbon sequestration, or removal from the atmosphere and storage globally. Scientists emphasized that the study does not suggest that insect-killed trees become instant sources of carbon emissions. Carbon transfers from living trees and plants to dead organic matter, and the release of carbon occurs gradually with decomposition of organic matter. The total amount of carbon in these dead materials is comparable to carbon emissions from 4.4 million cars or nearly one-fifth of all wildfires in the United States annually.