Climate effects on western US forests

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This article is part of the Fall 2014 issue of the Earth Science Matters Newsletter. 

 

WERC Sequoia Field Station

In order to forecast how forests will change in the future, we need to understand how climate affects tree mortality. Indeed, we will likely get substantially different forest landscapes depending on which mechanism or combination of mechanisms are driving tree death.

Forests are a critical component of the terrestrial biosphere, and changes in forests can have substantial effects on the cycling of carbon, energy, and water. To forecast how forests may change in the future, it is important to understand how climate affects tree mortality. Indeed, we may see substantially different forest landscapes depending on which mechanism or combination of mechanisms play a role in tree death. 

The rate of tree mortality in western forests has more than doubled in the past few decades, and that increase appears to be related to a warming climate. It is unclear, however, whether those warming-related increases are driven by increasing drought stress in trees, more favorable conditions for tree-killing organisms, or some combination of the two. 

USGS CLU R&D scientists and colleagues attempted to identify which mechanism dominates climate-related changes in tree mortality across the landscape. Using data from a long-term study of tree demography in the Sierra Nevada of California, they found strong evidence that the drivers of changing mortality rates differ between forests where growth is limited by energy (colder, wetter areas with shorter growing seasons) and forests where growth is limited by water (hotter, drier areas where water is more scarce). In water-limited forests, drought stress on trees seems to dominate changes in mortality rate, while in energy-limited forests climatic effects on tree-killing organisms also appear to play a key role. However, they also discovered that currently available data are inadequate to clearly forecast how these mechanisms will affect tree mortality in the future, with different (and equally supported) models giving radically different outcomes. 

As the climate continues to warm, tree mortality will likely continue to increase. How severe those increases will be and how they will be distributed across the landscape remain key questions. 

The paper was published in the journal PLOS One and is available at: https://pubs.er.usgs.gov/publication/70124440

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