2006 photo by J. Lutz
Figure 1. Glaciers and fires have influenced the landscape of the Sierra Nevada. Glacially smoothed Mount Starr King rises behind the area burned by the 1991 Illilouette fire. The matrix of different burn severities can be seen within the fire perimeter (red outline) viewed from Glacier Point.
Fire has physically shaped the species composition and structure of Sierra Nevada forests, just as glaciers have shaped the underlying landscape (fig. 1, above). The long, hot summers that occur in the Mediterranean climate of the Sierra Nevada favor fire, because they dry out vegetation and dead woody debris, creating fuel that readily burns when lightning (also common in the Sierra Nevada) strikes. Fire converts that vegetation—living or dead—into smoke. Smoke from fires contains readily inhalable fine particles that can impair human health, while also obscuring scenic vistas that visitors expect when they visit national parks (Clinton et al. 2006).
Smoke emissions also include greenhouse gases (e.g., carbon dioxide, or CO2) that derive from the carbon in the combusted biomass. While these emissions temporarily contribute to global warming, carbon is returned to the landscape as vegetation takes up CO2 post-fire (Hurteau and Brooks 2011). The resulting net “carbon balance” and the amount of carbon left on the landscape as biomass (i.e., the “carbon stock”) can vary, depending on the period over which that stock is measured and on whether the post-fire vegetation type covering the landscape contains as much carbon as the pre-fire vegetation.
Fire suppression over the last 130 years has changed vegetation types and likely carbon stocks, leaving large portions of Sierra Nevada parks with forest stands that have not burned in almost a century. As a result, small trees and shrubs have grown in under the larger trees, providing “ladder fuels” that could carry fire into the canopy of the larger trees, which are otherwise quite fire-resistant (fig. 2, below). Fire entering such an overly dense understory can burn at higher intensity, grow faster, release more smoke, and kill more (potentially all) trees. Preventing fires in one year can make a future fire even more severe, perhaps even leading to post-fire vegetation characterized by shrubs instead of forest. The increased fire risk that is the legacy of fire suppression in the Sierra Nevada endangers not only carbon stocks but also our ability to manage fires in a way that minimizes air quality impacts and preserves clean and clear air for visitors and local communities. Climate change has the potential to add another dimension of urgency to this issue by creating longer, drier, hotter summers in which these higher-severity, faster-growing fires are more likely (Lutz et al. 2009b).
2011 photos by L. Tarnay (3)
Figure 2. The spatial arrangement of forests affects how much carbon a landscape can contain and also its fire risk. Dense forests (left) contain large amounts of carbon, but the horizontal and vertical fuel continuity increases the risk of high fire mortality and large smoke production. Open forests (center) include large trees that store considerable carbon, and the lower density of smaller trees reduces the risk that fire will rise into the canopy. Patchy forests (right) have areas of both high and low carbon storage, and the lack of continuous fuels both horizontally and vertically increases the chances of a mosaic of burn severities.
This scenario highlights the tension of managing Sierra Nevada forests under a warming climate regime: lightning (and humans) will continue to ignite fires, and each suppressed fire, though minimizing immediate smoke impacts, increases the risk of larger, less manageable fires and smoke impacts in the future. Developing the optimal fire management solution requires that we reconcile what we know about fire, forests, smoke, and projected climate with the objectives of protection of life and property, minimization of smoke impacts, and the need to provide stewardship of these forests.
Forty years ago, the National Park Service realized that fire had been unnaturally excluded from the Sierra Nevada and began allowing fires to burn under prescribed conditions, first in Sequoia and Kings Canyon national parks, and soon after in Yosemite National Park. In Yosemite, NPS fire management and the U.S. Geological Survey have been partnering to develop a more quantitative, science-based approach to managing fire and the ecology of fire-adapted forests. In this article we summarize some of the lessons relevant to fire managers interested in adaptively managing such landscapes.
Tarnay, L. W., and J. A. Lutz. 2011. State of the Science: Sustainable fire: Preserving carbon stocks and protecting air quality. Park Science 28(1):48–55.
Available at https://www.nature.nps.gov/ParkScience/archive/PDF/Article_PDFs/ParkScience28(1)Spring2011_48-55_TarnayLutz_2791.pdf.