Background

Forests in many fire-dependent ecosystems of the United States are denser and more spatially uniform, have many more small trees and fewer large trees, and have greater quantities of forest fuels than did their presettlement counterparts. Fire suppression policy, past livestock grazing, timber harvesting, and changes in climate have contributed to current conditions. These practices have resulted in a general deterioration in forest ecosystem integrity and an increased probability of large, high-severity wildfires. Nationally, such conditions are especially prevalent in forests with historically short-interval, low- to moderate-severity fire regimes. The report of the Sierra Nevada Ecosystem Project highlighted these problems and explained the need for large-scale and strategically-located thinning (especially of small trees), fuel treatment, and use of prescribed fire. Modeling studies have indicated that some fuel and silvicultural treatments can be effective in limiting the size and intensity of wildfires, but these predictions need to be validated with field experiments. Furthermore, the extent to which alternative fuel and stand treatments restore ecosystem structure and function is largely unknown.		Prescribed fire in Unit 9 (Early season treatment) – June 20, 2002 (Photo by Eric Knapp)

A team of scientists and land managers has designed an integrated national network of 13 long-term research sites to address this need, with support from the USDA/USDI Joint Fire Science Program. The steering committee and other participants in this national Fire/Fire Surrogate (FFS) study represent a number of federal and state agencies, universities, and private entities, as well as a wide range of disciplines and geographic regions. The study uses a common experimental design to facilitate broad applicability of results. The primary focus is to understand the ecosystem impacts of different fire hazard reduction treatments. As part of the FFS study, USGS researchers at the Western Ecological Research Center are assessing the ecological impacts of early season and late season prescribed fire treatments to reduce fire hazard at one of the sites in the study network - Sequoia National Park. The other twelve sites, including Blodgett Forest (also in mixed conifer forests of the Sierra Nevada) are comparing mechanical thinning, prescribed fire, and mechanical thinning followed by prescribed fire.

Fire in the Sierra Nevada
Fire is an important ecological process in the Sierran mixed conifer forest. Many species of trees and shrubs are dependent on fire to expose mineral soil and create gaps for establishment. In the absence of fire, the species composition has changed. Shade tolerant species such as white fir and incense cedar have increased in abundance while species requiring mineral soil and high light environments, such as ponderosa pine, Jeffrey pine, and black oak have decreased in abundance. Total tree density and amount of woody fuel on the forest floor have also increased substantially. With more and more people choosing to live in and adjacent to forested areas of the Sierra Nevada, the need to address the problem of forests with a high fire hazard is clear.

In the Sierra Nevada, the use of prescribed fire to reduce fuels and restore natural forest processes began in Sequoia and Kings Canyon National Parks in the late 1960’s. While there is general agreement about the potential ecological benefits of re introducing fire, questions remain about the best means of doing so. Due to fire suppression over the past century, fuels have accumulated to unnatural levels and have become more spatially continuous, factors that might cause fire today to have unnatural ecosystem impacts. Concern exists that the resulting fires would lead to high mortality of large trees due to cambium and fine root damage during combustion, and be of uniformly high intensity, erasing the character of the original forest mosaic.

Early fall is currently the season when the majority of prescribed burning in the Sierra Nevada is done. This coincides with or occurs after the period of maximum historical fire activity, as determined from tree fire scar records, and coincides with the period of poorest air quality in adjacent populated areas of the Central Valley. Air quality concerns often severely limit the number of burn days during this time of year, particularly in the southern Sierra. Air quality is typically better during the spring or early summer, due to greater atmospheric instability. However, early season prescribed burning has the potential to affect trees, shrubs, and other forest species in different ways than late season prescribed burning. In addition, concern exists about the potential impact of early-season fire on animal species that may be more active during this time of year.

The Sequoia National Park Site
Forests in the study area are old growth. Tree species, in order of abundance, are, white fir (Abies concolor), sugar pine (Pinus lambertiana), incense cedar (Calocedrus decurrens), red fir (A. magnifica var. shastensis), Jeffrey pine (P. jeffreyi), ponderosa pine (P. ponderosa), mountain dogwood (Cornus nuttallii), and California black oak (Quercus kelloggii). The pre-settlement fire return interval for forests and aspects of the type found at the study site has been estimated to range from 20 to 40 years. However, due to fire suppression, plots had not experienced fire for at least the past 110 years.

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Photos -- National Fire and Fire Surrogate Study -- Sequoia National Park prescribed fire program

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Last update: 15 January 2004