Effects of fuel and weather on fire behavior in longleaf pine-wiregrass communities
By Kevin Robertson, PhD, Fire Ecology Program Director
Upon wrapping up the ninth year of work at the Pebble Hill Fire Plots long-term research project on fire in longleaf pine-wiregrass communities, we have completed the first set of analyses examining the complex relationships among fire regime (1-4 year fire intervals in May-September for growing season burns and January-February for dormant season burns), fuel characteristics, and fire behavior. Our primary interest for this part of the project was to determine how time since fire changes the fuel bed, which leads to changes in fire behavior. This kind of information helps us predict what effects prescribed fire and wildfire have on fuel consumption, smoke production, and effects on plants and wildlife habitat.
We analyzed our data using Structural Equation Modeling, which is a method for analyzing multiple complex relationships among variables at the same time to better interpret how the system works as a whole. One of our preliminary findings was that the moisture of dead fine fuels, such as dead grasses and pine needle litter, does not have a strong influence on how much or how fast the fuel is consumed, although it appears to affect how fast fire spreads. Apparently the moisture in dead fine fuel is driven off by pre-heating prior to combustion as the fire approaches, such that the combustion processes themselves are similar regardless of initial moisture. Considering that this community type has an abundance of fine fuels in the form of grass and pine needles, this result helps explain why it is so flammable even under conditions of high relatively humidity and following very recent rain.
We also found that time since fire, which corresponds to fine fuel accumulation over time, surprisingly did not have a strong overall effect on reaction intensity, which is how fast heat is released per area of ground within the flaming front. This observation is in contrast to the typical prediction by fire behavior models such as BEHAVE, which is that reaction intensity increases with higher fuel loads. The lack of overall effect appears to be because of the canceling influences of higher fuel loads and total heat released tending to increase reaction intensity, while greater density of the fuel bed with time since fire and fuel accumulation tend to decrease reaction intensity. These results suggest that even relatively high fine fuel loads in pine savannahs following several years of fire exclusion, which is typical of restoration burns, can be managed with appropriate ignition techniques to minimize reaction intensity and associated tree crown scorch and rate of smoke production.
Our results will be used to evaluate and suggest improvements to fire behavior models where needed in order to provide more accurate fire behavior and effects predictions for fire practitioners. Our ultimate goal is to more effectively use fire to restore and maintain healthy pine communities and reduce wildfire risk in a manner protects public health and safety.
Photos: top, Michelle Smith lights a head fire at a 3-year plot; bottom, Eden Robertson and Jessie Chi take post burn residue samples.