Emergence of Unexpected Tree Die-Offs in Global Forests under Changing Climate

Globally distributed, unexpected events of regional-scale mortality in forests.

Widespread spruce mortality in the Colorado Rocky Mountains in 2019, resulting from combined disturbances of drought, warmer temperatures, and insect outbreak. Surviving aspen trees suggest a large change in plant functional types from conifer to broadleaf trees is occurring as a result of forest mortality.

[Courtesy Pacific Northwest National Laboratory.]

The Science

Researchers summarized the known die-off events through literature analyses as well as personal anecdotes to identify the level of expectedness of regional die-off events. They subsequently synthesized the literature not only on remote sensing of die-offs at the global scale but also on the path forward for prediction of die-offs.

The Impact

Most recent tree die-offs (regional-scale mortality events) greatly exceeded expectations regarding their occurrence, speed of onset, and magnitude, indicating a need for improved detection and prediction capabilities. This study provides methods for improved die-off monitoring and model simulations, as well as a road map for future research on the patterns, causes, and future trends in tree mortality.


Tree mortality in global forests, particularly in tropical forests, reduces the carbon storage potential of terrestrial ecosystems. Tropical forests are an important terrestrial carbon sink globally but are experiencing increasing rates of tree die-off at regional scales. This study’s discovery of the unexpected nature of mortality events was particularly alarming in the tropics, which were long assumed to be resilient to drought and a changing climate, highlighting the importance of better understanding these events. The study’s identification of paths forward for improved monitoring and prediction of die-offs provides a road map for future research.

Principal Investigator

Nate McDowell
Pacific Northwest National Laboratory
[email protected]

Program Manager

Brian Benscoter
U.S. Department of Energy, Biological and Environmental Research (SC-33)
Environmental System Science
[email protected]


This work was funded by the Next-Generation Ecosystem Experiments–Tropics (NGEE–Tropics); the Department of Energy’s (DOE) Coastal Observations, Mechanisms, and Predictions Across Systems and Scales program; and a European Research Council grant.


Hartmann, H., et al. "Climate Change Risks to Global Forest Health: Emergence of Unexpected Events of Elevated Tree Mortality Worldwide." Annual Review of Plant Biology 73 673–702  (2022). https://doi.org/10.1146/annurev-arplant-102820-012804.