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Low severity fire regime continued
encourages the development of large, open-grown pine and oak, open habitats, and diverse, patchy structural conditions. Prior to fire suppression, fire return intervals in these low elevation ecosystems varied from one to thirty years. These intervals varied depending on the resources available, the proximity to village or harvesting sites, the abundance of game, weather conditions, as well as geographic and topographical conditions. Evidence suggests annual burning took place in the valley bottoms and adjacent to certain ceremonial sites. The frequency of this anthropogenic fire kept fire intensities fairly low and the risk of crown-fires were minimal. Shade tolerant and fire intolerant species were kept at bay, while fire dependent, sun-loving species thrived. Although open habitat conditions were historically maintained across vast acreages at low elevations, considerable acreages have also been documented as having been covered in dense chaparral or closed forest types.

Mixed severity fire regime
The mixed severity fire regime is perhaps the most widely distributed fire regime in the Klamath-Siskiyou Mountains. Much of the region’s mixed conifer forest at low, mid, and sometimes even high elevations has experienced mixed severity fire, creating intensely diverse and patchy landscapes representing a mosaic of soil conditions, exposure and fire history.

Although mixed conifer forests are recognized as variable and complex, such forest communities were once more

open and diverse then they generally are today. Such open conditions allowed for the development of a patchy forest consisting of groupings of large trees, filtered forest canopies, hardwood stands, brush fields, and in some places dense, closed-canopy forests. It is believed that over long periods of time and through the sustained influence of fire, mixed conifer forests developed the complex, layered canopies found in old-growth forests today. The historic role of fire has enabled hardwoods and shade intolerant conifer species, such as ponderosa and sugar pine, to thrive.

The combination of anthropogenic fire and some of the Northwest’s highest lightning occurrences created a fire regime in the Klamath-Siskiyou of mixed intensity and return. Fire return intervals in mixed severity regimes have been estimated at ten to thirty years, punctuated by occasionally longer fire-free periods. Fires in this regime are extremely variable, diverse, patchy, and irregular in frequency and severity, leaving a chaotic and complex pattern on the stand and landscape scale. Studies on Thompson Ridge above the community of Happy Camp on the Klamath River have shown a strong correlation between fire severity and exposure. Fires have historically burned at low severity on the lower slopes of the ridge and in canyon bottoms, as well as on north and east facing slopes, creating complex, old-growth forests. Fires associated with the upper third of a slope and on south or west facing slopes have a tendency toward more moderate and high fire severity, creating hardwood stands, brushfields, and open canopied conifer groves. The entire spectrum of fire severity, from...

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