Western US: Forest Die-off research
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.
