In western United States, massive mortality events have occurred recently across many tree species, often linked to severe drought and temperature, and are expected to increase with climate change in the next century. Trembling aspen forests recently experienced a severe and rapid drought-driven mortality across several western states, affecting up to a fifth of aspen forests. Despite the prevalence and impact of these events, there is relatively little known about exactly how forests die from drought and temperature stress. The proposed research would help illuminate the physiological mechanisms of how aspen forests succumb to drought stress over multiple years. Understanding the long term effects of severe drought on water uptake and use by trees is critical to informing pathways and thresholds of such mortality events. This research will shed light on the response of forests to climate change, currently a large uncertainty.

Tree death can transform regional landscapes and have severe effects on how forests function and on the services they provide to humans, such as timber, tourism, and wildlife habitat. Furthermore, forest diebacks can lead to dramatic decreases in forest uptake of carbon, resulting in an positive feedback to climate warming. Thus, understanding how and where forests could die in the future would greatly benefit local stakeholders in the western United States. This research will aid in projecting which regions, tree species, and forests could be most vulnerable to drought and climate. Additionally, the research will involve students from local communities in field research and will communicate the results to a broad array of land managers and stakeholders, greatly increasing the impact of the findings.

Project Report

Forest ecosystems provide innumerable benefits to society. They harbor vast amounts of biodiversity, pump water into the atmosphere and help drive the planet’s hydrological cycle, and store more than half of the carbon found in terrestrial ecosystems. They are a large carbon sink and take up more than a quarter of human emissions of carbon dioxide annually. In addition, they provide numerous ecosystems goods and services, such as timber, tourism, recreation, air purification, and water purification, that benefit economies around the globe. Yet forests are increasingly stressed by human-caused climate change. Recent severe droughts in the United States and western North America have triggered widespread forest die-off events in the last decade. These die-offs are concerning because they will lead to losses in the ecosystem goods and services provided by humans, and potentially could drive forests to be less effective carbon sinks and even to become sources, thereby accelerating climate change. Ecologists currently understand relatively little about the physiology of how trees die from drought, which is a major barrier to understanding and predicting forest die-off. This project examined a recent, widespread die-off of trembling aspen across the western United States, known as Sudden Aspen Decline (SAD). The project sought first to determine the type of drought that was lethal to aspens in SAD. It collected an array of aspen physiological data in southwestern Colorado and helped illuminate the unique combination of soil moisture stress and atmospheric water demand that drove aspen die-off. This sort of in-depth physiological work is critical because it can inform what type of drought is lethal, how these droughts are connected to climate change, and whether these droughts are more likely in the future. The project revealed the critical role of temperature in the drought, particularly in driving high atmospheric water demand during the drought, which suggests this type of drought will in fact be more common in coming decades due to climate change. The trends in western Colorado for these temperature increases are very strong. In addition, the project contributed to ecologists’ understanding of how trees die over multiple years, rather than abruptly during the drought itself. It examined how the effects of drought stress accumulate over several years, cannot be repaired, and how aspen trees then die from the effects of drought stress several years after the drought itself. The project revealed a suite of physiological changes, called hydraulic deterioration, that underlie aspen die-off. These extended die-off periods have been observed in other forests around the US and around the world, and thus understanding them is incredibly important for being able to understand and project the future of forest ecosystems with changing climate and drought stress.

National Science Foundation (NSF)
Division of Environmental Biology (DEB)
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Henry L. Gholz
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Carnegie Institution of Washington
United States
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