Photosynthesis feeds trees and has a significant benefit for people, too, namely the removal of carbon from the atmosphere and into live tree biomass through a process called “sequestration.” However, USDA Forest Service scientists and a colleague found that increased tree mortality from the impacts of nonnative insects and diseases results in the transfer of carbon stored in live trees into dead material, much of which will eventually return to the atmosphere by decomposition. This threatens the estimated 76 percent of carbon sequestration in North America that comes from forests.

Worldwide, forests are increasingly affected by nonnative insects and diseases, some of which cause substantial tree mortality. U.S. forests have been invaded an estimated 450 tree-feeding pest species. Researchers with the Northern and Southern Research Stations and Purdue University used 92,978 forest plots distributed across the conterminous United States, established and monitored by the Forest Service’s Forest Inventory and Analysis (FIA) program, to estimate biomass loss associated with elevated mortality rates caused by the 15 most damaging nonnative forest pests. Insects such as the emerald ash borer, gypsy moth, and hemlock woolly adelgid and diseases including Dutch elm disease, beech bark disease, and laurel wilt disease are among the 15 most damaging nonnative species. The results indicate that these species caused an additional annual loss of 5.53 Tg C of increased tree mortality, an amount comparable to carbon emissions from 4.4 million cars or one-fifth of all wildfires annually. In addition, 41 percent of the total live forest biomass in the conterminous United States is at risk to future loss from these 15 pests. This study demonstrates the value of FIA data to assess broad-scale changes in forests and indicates that forest pest invasions, driven primarily by globalization, are creating a large risk to forests in the United States and have significant impacts on carbon dynamics.