Monterey pines are considered a fire adapted species meaning they are highly dependent on wildfire for reproduction. Seeds are harbored in serotinous pinecones which open only during warm and dry periods or during exposure to wildfire. Wildfire also creates a favorable seedbed due to the combustion of ground organic material exposing bare mineral soil. Reproduction is most favorable when parent trees survive wildfire. Survival of mature trees is likely if crown scorch remains less than 90% and direct exposure to high intensity wildfire is not sustained. Therefore, Monterey pine forests favor low-moderate intensity wildfire with an average fire return interval of roughly 11-20 years.
Monterey Pine stands vary in surface fuel structures ranging from fine to heavy and may include an understory of grass, brush, pine needles, twigs, branches, and bark litter. Bark and leaf litter can accumulate rapidly beneath Monterey pine trees, resulting in significant fuel loads. Monterey pine litter is a fuel that is generally moderately compacted with a heavy fuel load reaching up to 100 tons per acre. Fuel buildup occurs very rapidly in unmanaged Monterey pine stands in California (USFS 2015). Monterey pine is highly flammable; the pine needles catch fire readily and tend to carry fire into the canopy and to disseminate fire ahead of the main fire front via flying embers (USFS 2015). Broadly, Monterey Pine forests in the Plan Area can be classified by three forest types based on their understory composition.
While historic wildfires, both natural (lightning) and through indigenous burning, benefited the health of Monterey pine forests in the Plan Area, fire suppression associated with European settlement has severely limited wildfire occurrences. This is problematic for a forest type which is so dependent on wildfire for success. While wildfires lead to regeneration, they also mitigate hazardous fuel conditions through the removal of understory vegetation and woody debris. Low-moderate intensity wildfires also improve forest health through reducing competition between individual trees and by removing dead and dying trees. Substantial human development within and adjacent to this fire adapted forest type creates unique challenges for simultaneously managing forest health and community wildfire risk.
Oak stands are composed of fuel structures ranging from fine to heavy. In closed canopy stands, a sparse understory of grass, leaves, twigs, branches, and bark litter may be present. In open stands, understory may include grass, shrubs, leaves, twigs, branches, and bark litter. Fuel buildup occurs very slowly in oak woodland stands in California (USFS 2015), and litter forms a thick, compacted mat resulting in very low surface fuel loads. Oak woodland understory fuel loads are low.
Oak trees are highly flame resistant as the leaves do not readily catch fire. Fires in oak stands tend to smolder in the duff and consume surface fuels without generating enough heat to carry fire into the oak canopy (USFS 2015). Oaks also do not spread fire crown-to-crown readily like many conifers. Oak woodland litter does little to facilitate fire spread as it has a low surface area-to-volume ratio and requires high heat levels to remove fuel moisture and raise fuel to ignition temperature. Oak woodland litter is subject to seasonal drying in the late summer and early fall months, but fog drip, solar shading, and the windbreak provided by oak canopies can sustain high fuel moisture content in the summer when fog is present. Oaks have a low content of VOCs, and the lack of highly combustible oils further reduces the fire hazard associated with oaks and oak woodlands.
Oak woodlands are lacking in features that promote fire spread, but weather and topography have a strong influence on fire behavior. Given extreme fire weather and steep terrain, oak woodlands have the potential for a moderate rate of spread, torching and crown fire, and extreme fire behavior. Fire behavior in oak woodlands and forests is typically much less intense than wildfires burning in chaparral and coastal scrub communities. Low, compacted leaf litter understory, canopy shading of ground fuels, and wind velocity reduction from tree canopies significantly reduces the intensity and spread rates of surface fires in oak woodlands. Transition from ground to canopy fire increases fire intensity, spotting, and tree mortality potential.
Chaparral is considered a moderately fine fuel which is loosely compacted and has a moderate fuel load. Chaparral has a high surface area-to-volume ratio, requiring less heat to remove fuel moisture and raise fuel to ignition temperature. Chaparral is subject to early seasonal drying in the late spring and early summer but does not fully cure in the way that grasses do. The live fuel moisture content reaches its low point in the late summer and early fall months. Chaparral has the potential for a high rate of spread, rapid ignition, and extreme fire behavior given its high content of volatile organic compounds (VOCs).
Grassland/herbaceous fuels in the Plan Area are represented by the annual grassland and perennial grassland vegetation community/land cover types. Grassland types may include scattered and widely spaced trees and/or shrubs, although grasses are the dominant cover type. Grasses are fine fuels that are loosely compacted with a low fuel load. Grasses have a high surface area-to-volume ratio, requiring less heat to remove fuel moisture and raise fuel to ignition temperature. They are also subject to early seasonal drying in late spring and early summer. Live fuel moisture content in grasses typically reaches its low point in early summer, and grasses begin to cure soon after. Due to these characteristics, grasses have potential for a high rate of spread, rapid ignition, and facilitation of extreme fire behavior. Grasses are the vegetation type in the Plan Area with the highest risk for wildfire ignition. Their low overall fuel loads typically result in faster moving fires with lower flame lengths and heat output. Untreated grasses can help spread fire into other adjacent surface fuel types (e.g., shrubs) or facilitate surface to crown fire transition where they exist beneath tree canopies.
High fire risk plant species with the potential to spread fire rapidly may occur within any of the identified vegetation community/land cover types. These plants can increase the frequency of fires by providing more continuous fuels that are more easily ignited (Brooks et al. 2004). Broom and pampas/jubata grass are of primary concern in the Plan Area.
One of the primary high fire risk/rapidly spreading plant types of concern in the Plan Area is broom: French broom (Genista monspessulana), Scotch broom (Cytisus scoparius), and Spanish broom (Spartium junceum). All are identified as Cal-IPC invasive species. Dense broom infestations produce large amounts of dry matter, which can create a serious fire hazard (DiTomaso 1998). Broom spreads by prodigious seed production and may also sprout from the root crown (Bossard 2000) or upper stem (Boyd 1995) when aboveground parts are removed by grazing, cutting, freezing, or fire. A review by Bossard (2000) suggests that broom burns readily and carries fire to the tree canopy layer, increasing both the frequency and intensity of fires in invaded areas.