Widespread Increase of Tree Mortality Rates in the Western United States

TLDR

Tree mortality changes can alter forest structure, composition, and ecosystem services such as carbon sequestration. The study examines whether regional warming and increased water deficits contribute to rising tree mortality rates. Longitudinal data from unmanaged old forests in the western United States reveal that background tree mortality rates have rapidly increased—doubling every 17–29 years—with widespread effects across elevations, sizes, genera, and fire histories, slight declines in density and basal area indicating competition is not the driver, and the rise in small‑tree mortality showing aging of large trees alone cannot explain the trend.

Abstract

Persistent changes in tree mortality rates can alter forest structure, composition, and ecosystem services such as carbon sequestration. Our analyses of longitudinal data from unmanaged old forests in the western United States showed that background (noncatastrophic) mortality rates have increased rapidly in recent decades, with doubling periods ranging from 17 to 29 years among regions. Increases were also pervasive across elevations, tree sizes, dominant genera, and past fire histories. Forest density and basal area declined slightly, which suggests that increasing mortality was not caused by endogenous increases in competition. Because mortality increased in small trees, the overall increase in mortality rates cannot be attributed solely to aging of large trees. Regional warming and consequent increases in water deficits are likely contributors to the increases in tree mortality rates.