Impacts of fire management on aboveground tree carbon stocks in Yosemite and Sequoia & Kings Canyon National Parks

Abstract

Forest biomass on Sierra Nevada landscapes constitutes one of the largest carbon stocks in California, and its stability is tightly linked to the factors driving fire regimes. Research suggests that fire suppression, logging, climate change, and present management practices in Sierra Nevada forests have altered historic patterns of landscape carbon storage, and over a century of fire suppression and the resulting accumulation in surface fuels have been implicated in contributing to recent increases in high severity, stand-replacing fires. For over 30 years, fire management at Yosemite (YOSE) and Sequoia & Kings Canyon (SEKI) national parks has led the nation in restoring fire to park landscapes; however, the impacts on the stability and magnitude of carbon stocks have not been thoroughly examined. The purpose of this study is to quantify relationships between recent fire patterns and aboveground tree carbon stocks in YOSE and SEKI. Our approach focuses on evaluating fire effects on 1) amounts of aboveground tree carbon on the landscape, and 2) rates of carbon accumulation by individual trees. In 2010, we compiled a database of existing plot data for our analyses. In 2011, our field crews acquired vegetation data and collected tree growth cores from 105 plots. In 2012, we completed an interpretive component and began data analyses. In 2013, processing of tree cores began. In 2014, final processing of tree cores, data analyses, and manuscript preparation was conducted. The work for this project was facilitated through an interagency agreement between the National Park Service and the U.S. Geological Survey, and through a Cooperative Ecosystems Studies Unit (CESU) agreement with the University of Washington. In order to accurately quantify landscape-level carbon stocks, our analyses accounted for major sources of measurement errors, propagating those errors as we scaled plot-based carbon density estimates up to landscape-level totals. Using Monte Carlo simulation methods, we found that vegetation type mapping error was the largest source of uncertainty, while measurement uncertainties contributed by tree diameter measurements and tree diameter–biomass allometry equations were relatively minor. For some forest types, we found differences in aboveground tree carbon densities between burned and unburned areas. For example, mean carbon density in burned red fir forests was estimated to be ~29% lower versus unburned areas. Alternative measures of fire history, such as time since fire and number of times burned, were poorly related to carbon densities. Within YOSE, we evaluated the stability of landscape carbon pools by quantifying carbon stocks in areas of varying degrees of departure from historic fire return intervals. Of the ~25 Tg of total aboveground tree carbon in YOSE, ~10 Tg is contained within relatively stable areas (the next fire is unlikely to be high severity and stand-replacing), ~10 Tg occurs in areas deemed moderately stable, and the remaining ~5 Tg within relatively unstable areas. We compared our landscape carbon estimates in YOSE to remotely-sensed carbon estimates from the NASA–CASA project and found that the two methods roughly agree. Our analysis and comparisons suggest, however, that fire severity should be integrated into future carbon mapping efforts. We illustrate this with an example using the 2013 Rim Fire, which we estimate burned an area containing over 5 Tg of aboveground tree carbon, but likely left a large fraction of that carbon on the landscape if one accounts for fire severity.