February 1 2017

11:00 158 Hershey Hall

Michael Goulden
Department of Earth System Science
University of California, Irvine

100 Million Dead Trees Since 2014: Separating the Effects of Drought and Warming on California Forest Mortality


Recent episodes of forest morality have been reported in tropical, semiarid and boreal regions, raising the possibility of accelerating tree death with drought and climate warming. California's Sierra Nevada experienced widespread forest dieback from 2012 to 2015, when below average precipitation and above average temperatures led to the death of millions of trees.The California dieback was especially intense at the Sierra Nevada Critical Zone Observatory (SCZO), where a rich suite of meteorological, plant physiological, ecological and hydrologic observations are allowing us to better understand the roles of drought and warming. Tree death at the SSCZO was greatest at lower elevations and decreased above ~2000 m. California's mountain forests are deeply rooted, with many trees accessing moisture to ~10 m depth and avoiding summer moisture limitation. Precipitation during 2012 to 2015 was inadequate to recharge the full soil column, leading to year-over-year deep drying.This drying was greatest at lower elevations, where warmer temperatures increase transpiration and water withdrawal.The deep soil reservoir at lower elevations was nearly completely dry by 2015, which ultimately led to tree death.Tree mortality at the SCZO reflected both the lack of precipitation to recharge the deep soil and the acceleration of transpiration by warmth.These factors will continue to interact in the future, with projected climate warming accelerating transpiration and leading to more frequent episodes of deep soil moisture depletion and stand dieback.















































































































































































































































































































































































































































































































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