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Sierra Nevada Forest Dynamics: Studying Forest Health

Giant Sequoia

Forests provide humans with economically important and often irreplaceable ecosystem products and services, such as clean water, wood, biodiversity, and recreational opportunities.  Additionally, forests store the majority of the world's land-based carbon, making them key components of the global carbon cycle.  However, ongoing changes in land use and disturbance regimes have the potential to rapidly alter forests in undesirable ways, such as by converting forest to shrubland as a result of catastrophic wildfire.  For society to anticipate and mitigate these increasing threats to forests, we must greatly improve our ability to understand and predict forest responses to severe drought and other disturbances.  Yet such efforts currently are severely limited by our poor understanding of many of the basic processes that drive forest dynamics.

Project Details

Together with partners from the National Park Service (NPS), Stanford University and the University of California, Berkeley, Dr. Nate Stephenson is investigating the ways that environmental changes can affect Sierra Nevada forests. These forests' health is closely linked to water supply and quality, air quality and threats to property from wildfires, and tourism. Perfectly situated on-the-ground in Sequoia and Kings Canyon National Parks, Dr. Stephenson studies the delicate balances that influence tree growth and mortality, including links between drought and disease, drought and wildfire, and resource (water, sunlight, nutrients) competition between trees and tree mortality levels. Major areas of focus are:

(1) Improving scientists' and resource managers' understanding of forest and carbon dynamics: This long-term project delves into the basic processes that determine how forests work. What are the interactions between internal (e.g. competition between trees) and external (e.g. air temperature) drivers of forest change? How do short- and long-term fluctuations in water balance affect tree recruitment, growth, and mortality? What is the relative importance of organic (e.g. insects) and inorganic (e.g. wildfire) causes of tree mortality?

(2) Detection and attribution of drought effects on forests:  Current efforts focus on how "hotter droughts," unprecedented severe droughts compounded by high air temperatures and water stress on trees, affect forests of the Sierra Nevada. Dr. Stephenson addresses several fundamental questions: Are patterns of forest mortality predictable across geographical space? What are the major causes of mortality during "hotter droughts"? What are the consequences for carbon storage? What is the nature and timing of forest recovery?

(3) Adaptation options: USGS science can help inform land managers and policy makers from multiple entities, including the NPS and U.S. Forest Service. Findings  help managers and policy makers reassess agency missions and policies in the long-term, develop broad concepts relevant to adaptation planning in response to trends in forest health, and support USGS efforts to provide assistance during adaptation planning.

USGS Contact For This Project
Nathan Stephenson
(559) 565-3176
Sequoia and Kings Canyon Field Station
47050 Generals Highway #4
Three Rivers, CA 93271-9651
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