Fire regime characteristics

Grassland fire regimes are characterized by their intensity, frequency, size, and seasonal timing (Reiner, 2007). Fire intensity is the rate of fuel consumed and energy release rate (Neary and Leonard, 2020), and is evaluated by its temperature (low ≅ 250°C, moderate ≅ 400°C, and high ≥ 650°C) (Ngole-Jeme, 2019). Consumption of organic matter can occur even at low intensities, ranging from 200°C to 460°C (Alcañiz et al., 2018). In general, burning grasslands with more aboveground biomass (AGB) will release more heat (Reiner, 2007). Grassland AGB is characterized by the site’s residual dry matter, or the amount of herbaceous plant matter from the previous season immediately prior to the first fall rains (Bartolome and Frost, 2006). Fire frequency is how often a site burns over time, with every three years being optimal for native many native grassland forb species (Carlsen et al., 2017). The seasonal timing of prescribed burns in California are generally in the fall, but late spring/early summer burns can be used to target invasive annual grasses (Bartolome et al., 2019).

Topography and weather conditions

In general, fires on level ground will burn more slowly, but the rate of spread typically increases on ≥25% slopes (Reiner, 2007). Additionally, burns on south-facing aspects wil generally be hotter with lower humidity. However, shaded, northerly slopes can potentially have higher available biomass for burning (Reiner, 2007).

Grass fuels react quickly to temperature and humidity due to their high surface-to-volume ratio, thus sites with drier soil moisture will burn more easily (Reiner, 2007). When the humidity on the day of the burn is between 40-60% with a temperature below 90°F, grasslands typically burn at a moderate intensity (Reiner, 2007). Morning burns tend to have higher moisture and produce less heat. Wind speed is also important, as it delivers oxygen to a fire, increasing the rate of spread of the burn (Reiner, 2007).

Implementing prescribed burns in California

Stackhouse and Quinn-Davidson (2019) summarize four ways for landowners to conduct prescribed burns on their property. The most common method is by requesting a contract through the Vegetation Management Program with the California Department of Forestry and Fire Protection (CAL FIRE), which then covers the costs and liability of the burn. When the prescribed fire is funded by CAL FIRE or another state agency, however, environmental compliance monitoring is required, potentially slowing down the application process. An alternative method is by working with a Prescribed Burn Association (PBA), such as the Central Coast PBA, which brings together community members who have fire safety training and can share pooled fire management equipment. The final two ways include hiring a private contractor or the landowner conducting the burn themselves. The former is effective but is the most expensive option (e.g., sometimes $10,000 per day), whereas the latter is typically only possible for small-scale burns.

Planning for prescribed burns

The first step to plan for a prescribed burn is to develop a Fire Management Plan. This includes the objectives of the burn (e.g. reducing fuel or woody encroachment, increasing native forb cover), and outlines how to control the fire safely, such as listing access roads, water supply, and natural fuelbreaks (Reiner, 2007). From this plan, the Fire Unit Plan or Incident Action Plan will be created, which specifies the required fire staffing, the line of command, required equipment, permits, fire line construction details, ignition pattern, fuel and weather prescription, smoke management, response to accidental escape, and post-fire logistics for the burn prescription day (Reiner, 2007). 

Whether a fire can be ignited and safely controlled is determined by the fuel moisture content, air temperature, humidity, wind speed and direction, and season, however this does not always align with ecological objectives (Reiner, 2007). One tool for predicting fire behavior is using BEHAVE or BEHAVE-Plus, a model developed by the USDA Forest Service which can predict the fire rate of spred, flame length, and fire intensity.

It is important to check with the local Prescribed Fire Incident Reporting System (PFIRS) which provides a direct interface between air quality managers, land management agencies, and individuals conducting prescribed burning in California. For burn decisions along the California Central Coast, this includes:

• San Francisco Bay Area PFIRS
• North Central Coast PFIRS

Another available resource to assess potential smoke impacts includes the California Air Resources Board – Smoke Spotter App.