About the Southern Cascades
The Southern Cascades are bounded on the west by the Sacramento Valley and Klamath Mountains and on the east by the Modoc Plateau and Great Basin. Elevations in the region range from 196 ft in the southwestern foothills adjacent to the Sacramento Valley to 14,162 ft, which is the summit of Mt. Shasta. The headwaters of the Klamath and Pit Rivers originate east of the Cascade crest and flow west towards the Pacific ocean.
The climate of the Southern Cascades is Mediterranean, with warm, wet winters and hot, dry summers. Most precipitation falls as snow at higher elevations. There is a west-east gradient in precipitation and winter temperatures where wetter and warmer conditions are more prevalent on the west side of the range south of Mt. Shasta. There is also a north-south gradient in precipitation on the western side of the Cascade Range north of Mt. Shasta due to a rain shadow effect from the Klamath Mountains. These gradients in precipitation causes distinct vegetation types to develop in response to the different climatic regimes.
Generally, years that are drier and warmer than normal have larger and more widespread fires.
Fire History of the Southern Cascades
Indigenous burning
Historically, intentional ignitions from Indigenous people contributed to the frequent presence of fires in these landscapes. The Southern Cascades region is the ancestral lands of many Indigenous tribes that still steward these lands today, including the Modoc, Mountain Maidu, Pit River, Shasta, and Wintu tribes to name a few. Tribes burn for many reasons, including for the production of food and basketry materials, to aid in hunting efforts, and for cultural and ceremonial purposes. To learn more about other Indigenous tribes in the Southern Cascades region, visit https://native-land.ca/.
Historic Fire Occurrence
In several of the dry, conifer forest types of the Southern Cascades, the Mediterranean climate, common occurrence of lightning ignitions, and widespread use of fire by Indigenous people historically promoted frequent surface fires. These fires consumed woody debris and killed smaller trees, so that when the subsequent fire occurred it resulted in low- to moderate-severity effects. There was not enough time between fires to accumulate enough fuel to promote high severity across broad areas. This fire regime also encouraged and maintained plant species with fire-adaptive traits across ecosystems in the Southern Cascades region.
After Euro-American settlement in the mid-1800s, there was a substantial decline in fire activity – this is likely due to the removal of Indigenous populations and their use of fire, as well as historic grazing and logging practices. The lack of fire and historical land management practices combined to result in dramatic increases in tree densities and coarse woody debris.
Because of past management and climate change, in areas were low- to moderate-intensity surface fires were more frequent and extensive, fires are now often suppressed when they are very small, enabling the continued build-up of woody fuels and tree densities. When they escape those initial suppression efforts, it is usually because of very hot dry weather, which results in large, mostly high intensity fires.
Fire Ecology of the Southern Cascades by Ecoregion
Fire in the Foothills
Fire season in the southwestern foothills is long, beginning in early summer and lasting through late fall. Foothill conifer stands and woodlands typically experienced fire regimes of high frequency and low- to moderate-intensity and severity, while foothill shrub patches experienced fires of moderate- to high-severity every 30-90 years.
Generally, in the northwestern foothills, sagebrush ecosystems experienced fire every 13-100 years depending on aridity and amount of fuel available. Historically, most fires burned between mid-summer and early fall with the likely ignition sources being from lightning and Indigenous peoples. Over the last 100 years, things like fire suppression, grazing, and introduction of exotic weeds have significantly altered fire frequency, intensity, and severity in these ecosystems.
Fire in Mid-Montane Forests
Historically, fire return intervals were short and intensity and severity were low to moderate. The dominant conifer species in this zone all evolved with these fire regimes; which allowed them to develop various fire-adaptive traits. Mixed-conifer forests have changed since the onset of fire suppression, historic logging practices, and climate change. Forest density has increased along with a shift in species composition toward increasing density overall, particularly of fire-sensitive white fir and incense cedar. Contemporary forest stands with increased stand densities and fuel accumulation are more likely to burn severely. I addition, these dense forests are more susceptible to mortality from drought and insects.
As a result of these increased woody fuels and tree densities, there has been a significant increase in the proportion of area that burns at high severity in this zone. Historically, high severity fire was present in this system, just at smaller scales – likely a few acres to a few hundred acres. In recent years, we have seen high severity patches that are thousands of contiguous acres, which can affect postfire succession and the associated trajectory of the ecosystem. Succession in such fires usually begins with forests initially replaced with a combination of snags, herbaceous plants, and shrubs.
Generally, the larger the high-severity burn patch, the fewer the available seed trees, with increasingly less conifer regeneration except along edges of the patches. Most dominant conifer species in this zone rely on wind-driven seed dispersal for regeneration, with seeds generally travelling only ~200 feet from the parent tree. This means that in very large high severity patches, significant area is likely outside of the dispersal range of surviving mature trees. As a result, forest managers are increasingly working to actively reforest these areas. Where natural regeneration is lacking and no reforestation is undertaken, shrubs tend to come to dominate. In addition, all of the fire-killed trees in these areas will eventually fall and create heavy surface fuel loads. Taken together, these characteristics tend to result in high severity re-burns, which could result in a persistent type conversion to shrubland.
In drier, eastside forests, historically, fire return intervals were short and intensity and severity were low. The effects of fire suppression and changes in fire regimes of eastern forests are similar to those of western montane forests.
Fire in Upper Montane Forests
Fire regimes in red fir-dominated forests are characterized by mixed-severity burns that occur in the late summer or fall. Fire return intervals ranged from 25-110 years. However, following fire suppression, the occurrence of fire in red fir-dominated forests has decreased and the density of younger trees has increased in response. Yellow pine forests had relatively shorter fire return intervals compared to other forest types (roughly every 13-32 years). Yellow pine stands have gone from relatively open stands with larger trees to much denser stands of smaller trees, which are more susceptible to mortality from drought, disease, insects, and other fires.
On average, fires in montane shrub fields occurred every 26 years. Many important Ceanothus and Arctostaphylos shrubs in this region germinate post-fire from soil seedbanks. Historically, frequent, low- to moderate-intensity fires under dry, late summer conditions likely limited seedbanks of montane shrub species.
Fire in Subalpine Forests
Historically, fire regimes in this ecosystem tended to be of mixed-severity, burning in the late summer and early fall. Fire return intervals varied due to topography, vegetation, and fuel availability, but generally, these high elevation sites had longer fire return intervals. Lightning is frequent in this zone, but fire does not play a significant role in these ecosystems. Because of late melting snowpack and fuel scarcity, the few fires that do ignite do not spread beyond individual or small clusters of trees. Trees in this zone are sensitive to even low-intensity fires.
Additional Resources
Check out these resources below to learn more about fire ecology and history in the Southern Cascades!
- Skinner, C. N. and A. H. Taylor. 2018. Southern Cascades Bioregion. In van Wagtendonk, J. W., N. G. Sugihara, S. L. Stephens, A. E. Thode, K. E. Shaffer, J. A. Fites-Kaufman (Eds.), Fire in California’s Ecosystems. 363-397.
- van Wagtendonk, J. W., N. G. Sugihara, S. L. Stephens, A. E. Thode, K. E. Shaffer, J. A. Fites-Kaufman. 2018. Fire in California’s Ecosystems (2nd ed.). University of California Press. https://www.ucpress.edu/books/fire-in-californias-ecosystems/hardcover.