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Chapter 3 -- History of the Smith River Watershed (continued)

Fire History and Pattern of Occurrence

Fire has always been a controlling influence on forests in this region, especially forests in inland areas. Because climatic conditions and human use have fluctuated over the last 10,000 years, the fire regime in the Smith River watershed was never static. The intensity of fire varies widely and has a vast influence on forest structure, soil organic matter content, nutrient availability, and erosion hazard. The intensity of the fire influences the proportion of trees that are killed and the amount of wood and soil organic matter that are consumed. Low intensity fires may kill only a few trees and consume smaller fuel materials and a portion of the snags and down logs. A high intensity fire will kill almost all trees and consume virtually all snags and down logs. There have been three main eras in the fire history of the Smith River watershed: pre-settlement, settlement, and post-settlement.

Pre-settlement

As part of the marine coniferous forest region of the Pacific Northwest (Bailey 1996), the Smith River watershed has infrequent crown fires and more frequent light to moderately severe surface fires. Fire-adapted ecosystems must have periodic fire to maintain and regulate productivity, stability, species composition, and forest structure. Fire varies in frequency, intensity, and duration based on numerous factors including climate, topography, vegetation, soil, season, fire causative agents, and previous fire patterns. Fire occurs less frequently on lower slope areas, and more frequently on ridge tops and other high slope positions. Certain vegetation types, such as closed-cone pine forests, have more frequent fires. Aspect is also an important influence on moisture and vegetation, and therefore fire characteristics. Fires tend to burn more intense while moving uphill.

Based on average stand age, stand-replacing fires occur on an average of about 300 years on lower one third slopes, 240 years on upper one third slopes, and 200 years on ridge tops. Low intensity fires occur more frequently. For example, in the mixed evergreen vegetation type, which is dominated by Douglas fir, studies of fire scars indicate that, prior to fire suppression, fires occurred on an average of every 21 years. High intensity fires are more frequent on slopes that are steep, south-facing, and/or capable of funneling wind from dry interior areas. Stands closer to the coast burn less frequently.

The native Tolowa used low intensity fires to encourage production of acorns, bulbs, grass seed, hazels, basket-making materials, and large game. They used annual burning to prevent the conversion of prairies and oak woodlands to dense forests. These fires varied in extent due to weather and moisture conditions (McCain et al. 1995).

Settlement

Fire frequency probably increased during the settlement period due to use of fire in clearing land for grazing and mineral exploration as well as accidental fires. In 1868, the skipper of a ship traveling along the coast reported that they were in sight of a large fire for seven days. This fire began during the summer in southern Oregon, and continued to burn until the fall rains. It reportedly burned inland as much as 30 miles and south to the Klamath River. From 1880 to 1920, large areas of the North Fork drainage and other mining areas were burned to facilitate the location of minerals (California Department of Fish and Game 1980).

Post-settlement

Since about 1920, fire has been dramatically reduced by suppression activities. This has resulted in increased vegetation density and fuel loading. Panoramic views taken from fire lookouts in 1934 compared with today’s forests indicate that at present there are more forests and less shrubs and meadows. Because of the absence of fire for many decades, there have been changes in composition and structure of ecosystems that are adapted to fire. Forest types that depend on fire for regeneration are decreasing. Fire decreases the number of trees in the forest, resulting in forests that are not fully stocked with trees. Because there are fewer trees, there is more moisture available for each individual tree. This becomes important during periods of drought when the fire-thinned forest has more reserve moisture and retains a greater capacity for resisting insect attack. In dense forests where fire has been suppressed, the trees have less reserve moisture during droughts and are therefore more vulnerable to insect attack. This effect is not expected in coastal forests which are not limited by moisture.

Due to fire suppression, many forests now contain unnaturally high fuel loads on the ground and in the understory. These fuels create horizontal and vertical fuel continuity. This allows ground fires to reach the overstory canopy more readily and increases the tendency for intense and destructive crown fires. The resulting stand is more susceptible to damage and mortality from insect epidemics, disease outbreaks, and intense stand-replacing fires (McCain et al. 1995). In addition, young timber plantations are also susceptible to loss by fire. In some areas of the national forest, prescribed burning strategies are being used to remove excess fuel loads and return forests to a less hazardous condition.

Until fire suppression became effective, vegetation composition and patterns in inland areas were dramatically influenced by the patterns of fire occurrence and intensity. The interaction of fire and vegetation is influenced by topography. Vegetation was burned more frequently on upper one-third slopes and ridge tops. This resulted in concentration of the densest forests at lower elevations. Today logging is a major influence on vegetation patterns. Logging occurs more frequently in the productive forests found in valleys and on lower slopes. Forests on ridge tops and upslope areas are less frequently logged because they are less productive and more erosion prone. Also, forests at high elevations are more often preserved as wilderness areas. The result is a decreased proportion of mature and late seral forests at low elevations and an increased proportion at high elevations. Unlike fire which tends to remove vegetation on ridge tops and upper slopes, current forestry activities tend to remove vegetation in downslope areas. The susceptibility of upslope forests to periodic removal by fire has been and remains relatively high except where fog is prevalent in the summer.

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