Increasing rates of subalpine tree mortality linked to warmer and drier summers

Authors: Robert Andrus*, University of Colorado, Boulder, Rachel Chai, University of Colorado, Kyle Rodman, University of Wisconsin - Madison, Brian Harvey, University of Washington, Thomas Veblen, University of Colorado
Topics: Biogeography
Keywords: Climate change, forest demography, tree mortality, permanent plots, long-term study
Session Type: Virtual Paper
Day: 4/7/2021
Start / End Time: 11:10 AM / 12:25 PM
Room: Virtual 9
Presentation File: No File Uploaded

Rates of tree mortality are increasing across the earth. In addition to widespread increases in disturbance-driven mortality (e.g., bark beetles, fire, drought), rates of background mortality (e.g., tree mortality in the absence of a disturbance) may also be increasing in association with warming temperatures and rising moisture deficits. Tree mortality is an important forest ecosystem process and increases in tree mortality may signal important shifts in forest dynamics. We examine how temporal variability in rates of background tree mortality (~0.5-2% per year) for four subalpine conifers reflects variability in climate and climate teleconnections using observations of tree mortality from 1982-2019 at Niwot Ridge, Colorado, USA. Individually marked trees (initial population 5,043) in 13 permanent plots—located across a range of site conditions, stand ages, and species compositions—were censused for new mortality nine times. Higher rates of tree mortality were related to warmer maximum summer temperatures, greater summer moisture deficits, and negative anomalies in ENSO (La Niña), with greater effects of drought in some subpopulations (tree size, age, and species). For example, in older stands (> 250 years) larger and older trees were more likely to die than smaller and younger trees. Differences in tree mortality rates and sensitivity to climate among subpopulations that varied by stand type may lead to unexpected shifts in stand composition and structure. A strong relationship between higher rates of tree mortality and warmer, drier summer climate conditions implies that background mortality rates are likely to continue increasing in subalpine forests under ongoing climate warming.

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