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Nov 2021

Tree mortality response to drought-density interactions suggests opportunities to enhance drought resistance

John. B. Bradford, Robert K. Shriver, Marcos D. Robles, Lisa A. McCauley, Travis J. Woolley, Caitlin A. Andrews, Michael Crimmins, David M. Bell
Arizona
Climate Change
Forest
USGS
Abstract

A future of hotter temperatures and less precipitation under climate change could increase tree mortality under drought conditions in dry forests across the western U.S., causing large-scale tree die offs.  Restoration projects that reduce densities of overgrown forests can result in lower competition for scarce water resources and is one of the few ways in which forest managers can potentially minimize tree mortality and increase forest resilience. This study evaluated the climate and tree density conditions that best explained tree drought mortality in ponderosa pine stands across the western U.S. A key objective of the study was to determine where and how much reducing tree density can help sustain dry forests projected to undergo future drought. We found that basal area, a measure that represents the number and size of trees, was the most important factor that explained ponderosa pine tree mortality from over 3,000 stands monitored by the U.S. Forest Service. Specifically, higher density stands had higher mortality. We also found that mortality was higher in stands that experienced hotter and drier than conditions. However, an unusually wet period could reduce mortality in a stand. Our results suggest that a 50% reduction in forest basal area could reduce tree mortality from 20% – 80% even under adverse climate conditions. This study shows that large-scale forest restoration projects designed to reduce wildfire risk, that are currently being implemented across the West, can also reduce tree mortality from drought.

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(ALL RIGHTS GRANTED TNC) View of Escudilla Mountain. Fall colors heighten the natural beauty of forest and meadow in the White Mountains, one of Arizona’s last wide-open spaces and where TNC works to protect the headwaters of three major Arizona rivers: the Salt, Gila, and Little Colorado and their greenbelts – riparian habitats critical for wildlife and water quality – as well as restore healthy forests within the largest ponderosa pine community in the world, save rare and unique wildlife and plant species, and control non-native, invasive species, such as crayfish, Arizona. © Betsy D. Warner/TNC
Aerial view of the Parks West restoration site overseen by TNC. Taken March 16, 2020.
Conservancy staffers, Dale Turner and Amanda Rebore, helping to map the Sabn Pedro River in Arizona. They use a GPS unit to denote the end of the water flow on the LowerSan Pedro River. They hiked through the Conservancy’s San Pedro Preserve.

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Tree mortality response to drought-density interactions suggests opportunities to enhance drought resistance
A future of hotter temperatures and less precipitation under climate change could increase tree mortality under drought conditions in dry forests across the western U.S., causing large-scale tree die offs.  Restoration projects that reduce densities of overgrown forests can result in lower competit[…]
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