Sierra Nevada Network

Stressor: Fire

Altered Fire Regime

NPS staff monitor a lightning-ignited fire in the Mineral King area of Sequoia National Park.

Fire has played a pivotal role in shaping ecosystems and landscapes in the Sierra Nevada for many millennia. It affects numerous aspects of ecosystem dynamics such as soil and nutrient cycling, decomposition, succession, vegetation structure and composition, biodiversity, insect outbreaks, and hydrology.

Historically, fire frequency, size, intensity, and severity varied spatially and temporally across the landscape depending on number of ignitions, climate, elevation, topography, vegetation, fuels, and edaphic conditions. Fires were common, often burning for months and reaching large sizes.

Periodic fires performed many ecological functions within Sierran ecosystems prior to Euramerican settlement. Frequent surface fires in many vegetation types minimized fuel accumulation while their variable nature helped create diverse landscapes and forest conditions. Fires tended to be low to moderately severe, with high-severity portions (intense enough to kill most large trees) generally restricted to localized areas of a fraction of an acre to a few acres. Extensive research in mixed-conifer forests has shown that low intensity surface fires were common and tended to keep the forests open.

Many species and most plant communities show clear evidence of adaptation to recurring fire, indicating that fire occurred regularly and frequently, particularly in the chaparral and mixed-conifer communities, where many plant species have life history attributes tied to fire for reproduction or as a means of competing with other biota. Fire damaged or killed some plants, setting the stage for regeneration and vegetation succession. Many plants evolved fire-adapted traits, such as thick bark, and fire-stimulated flowering, sprouting, seed release, and/or germination. Fire influenced soil and forest floor processes and organisms by consuming organic matter and inducing thermal and chemical changes. It also affected the dynamics of biomass accumulation and nutrient cycling at a variety of spatial scales. These effects in turn influenced habitats and the distribution and occurrence of many species.

Fire regimes are defined according to specific variables including frequency, severity, season, duration, magnitude, spatial distribution, and type of fire. These characteristics may vary through time and across the landscape in response to climatic variation, number of lightning ignitions, topography, vegetation, historic events, and cultural practices. Fire regime types for major Sierra Nevada plant communities vary from short-interval, low-intensity surface fires in ponderosa pine and blue oak woodland to long-interval, variable intensity fires that occur in lodgepole pine forests and include numerous other fire regimes in diverse vegetation types of the Sierra Nevada. Variation in fir frequency exists locally and at large scales, and is affected by site productivity, potential for ignition, and other factors. General patterns of pre- Euramerican fire frequencies are apparent at several scales in the parks. Differences in average fire frequency are also apparent in different vegetation types (Table 1-3 and Figure 1-9). On the west slope of the Sierra, frequencies were reconstructed using firescarred trees. Data show an inverse relationship between number of fires and elevation.

Short-term climatic variation had a significant impact on past burn patterns and fire severity. Historically, on the west slope of the Sierra Nevada, specific fire years have been identified (years in which fires have been recorded at sites throughout the southern Sierra Nevada). These usually occurred during dry years. Analysis of millennial-length fire histories from giant sequoias also document long-term variation (1,000-2,000 years) in the fire regime associated with climatic fluctuations. These data suggest more frequent but smaller fires occurred during the Medieval Warm Period (A.D. 1000 - 1300) and fewer larger fires during cooler periods (A.D. 500 - 1000 and after A.D. 1300). These fluctuations indicate that characteristics of fire regimes are dynamic over long time periods.

Although fire regime characteristics may vary through time and across the landscape, from the late 1890s through the 1960s, Sierra Nevada park and national forest personnel attempted to suppress all fires, and these efforts met with a fair degree of success. Consequently, numerous ecosystems that had evolved with frequent fires have since experienced prolonged periods without fire. This change in fire regime has severely modified ecosystems (See Sierra Nevada Network: Vital Signs Monitoring Plan (2007).

 

 

 

This article is an excerpt from the Sierra Nevada Network: Vital Signs Monitoring Plan (2007).