Widespread increase of tree mortality rates in the western United States.
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.
van Mantgem
Phillip J
PJ
U.S. Geological Survey, Western Ecological Research Center, Three Rivers, CA 93271, USA. pvanmantgem@usgs.gov
Stephenson
Nathan L
NL
Byrne
John C
JC
Daniels
Lori D
LD
Franklin
Jerry F
JF
Fulé
Peter Z
PZ
Harmon
Mark E
ME
Larson
Andrew J
AJ
Smith
Jeremy M
JM
Taylor
Alan H
AH
Veblen
Thomas T
TT
D028202:Abies / Q000033:anatomy & histology / Q000254:growth & development; D002980:Climate; D017753:Ecosystem; D005390:Fires; D015233:Models, Statistical; D017711:Nonlinear Dynamics; D015205:Northwestern United States; D028223:Pinus / Q000033:anatomy & histology / Q000254:growth & development; D013696:Temperature; D064028:Tracheophyta / Q000033:anatomy & histology / Q000254:growth & development; D014197:Trees / Q000254:growth & development; D028225:Tsuga / Q000033:anatomy & histology / Q000254:growth & development; D014481:United States