Effects of Burning Season on Southeastern Forest Fire Dynamics

Project Description

We conducted an experiment to document and test for potential differences in the rate of fuel regrowth and accumulation following prescribed fires during the dormant and growing seasons. In other words, as a fuel reduction treatment, do growing-season prescribed fires have a different lifecycle than dormant-season prescribed fires?

Purpose and Scope

Land managers in the southeastern United States have actively used prescribed fire, primarily in the winter or dormant season, as a tool to control growth of understory vegetation since the middle of the past century.

The study measured fuel amount and composition annually following dormant-season and growing-season prescribed fires for 3 years in flatwoods ecosystems near Tallahassee, Florida (Apalachicola National Forest and St. Marks National Wildlife Refuge) and Niceville, Florida (Eglin Air Force Base).

Methods

The sampling design employed repeated measures with replicated treatments nested within blocks or locations. Treatments included a dormant-season (December–February) and a growing-season (late March–July) prescribed fires. Selection of study sites and assignment of treatments were dictated by management history and operational burning plans.

At each location, we identified management units with comparable vegetation composition and management history (i.e., season and time since last fire). We selected our study sites from among all management units that were scheduled for dormant-season and growing-season prescribed burning in 2010 with comparable vegetation composition that had been most recently burned during the dormant season in 2008.

Fuel loading and composition were measured before the treatment burns (December 2009–February 2010), immediately after the burns, 1 year after the burns, and 2 years after the burns to evaluate the differences in the fuel treatment lifecycle; that is, the rates of fuel accumulation that result from the different treatments.

We measured fuel amount, fuelbed depth, vegetation height and composition, weather on the day of the burn, and fire behavior and intensity.

Implementation

A 3- or 4-person field crew from the Pacific Wildland Fire Sciences Laboratory conducted preburn, day-of-burn, and postburn fuel inventory measurements at each location.

Key Findings

Postfire fuel regrowth marginally linked to season of burning—The results of our experiment do not show clear differences in postfire live fuel regrowth between growing-season and dormant-season fires. Live shrub and herbaceous fuel loading, coverage, and height were not statistically significantly different for burns in the two seasons.

Fire temperature is not necessarily a function of season of burning—Peak in-fire temperatures spanned a roughly 200 °C range for our 16 prescribed fires. There was no pattern seen in the temperature observations. Neither region nor season of burning affected the distribution of peak fire temperatures.

Regional differences in fuel dynamics—The experimental design of this study called for replicated, repeated sampling of fuel characteristics in a single broad ecosystem type under two treatments (growing-season and dormant-season prescribed fires) in two different locations. A location factor was incorporated into the design of the experiment in an effort to broaden the potential application of the study results.  Although our results do not suggest an effect on fuel dynamics associated with season of burn, we did observe statistically significant differences between the two regions.

Fuel characteristics still recovering after two years—Shrub and herbaceous species loading, coverage, and height were less than prefire levels on average for all regional and seasonal groupings, except for dormant-season herbaceous loading in the western region and shrub coverage in the eastern region, indicating that mesic longleaf pine flatwoods take longer than 2 years to fully revegetate following fires. Potential fire behavior reflects this change in live fuel loading, coverage, and height after prescribed burning. This suggests that repeated burning at intervals of 2 years or less could lead to overall reductions in shrub and herbaceous vegetation loading, potential coverage, stature, and fire potential over time even in longleaf pine flatwoods stands that have been managed with prescribed fire for multiple fire cycles.

Manage fire temperatures—Our results suggest that firing conditions designed to achieve a hotter fire may have the effect of retarding shrub coverage and loading recovery. This includes timing, pattern, and method of ignition; and environmental conditions, including air temperature, wind speed, fuel moisture content, and fuel loading.

Expect regional differences in fire effects—Our results (as well as the results of others) demonstrate that fire effects are best assessed on a case-by-case basis for each fire. It is, therefore, important for the prescribed fire practitioner to keep in mind that the specific characteristics of the place and time in which a fire is implemented will influence the fire’s effects.

Monitoring of growing-season burns necessary—This study highlights the difficulty and danger of uniformly applying principles learned at a single location. Additional data are needed to improve the scientific understanding of fuel succession in ecosystems that are managed with frequent prescribed fire. Basic fuel and fire effects monitoring programs on a selection of prescribed fire management units could contribute greatly to the scientific foundations upon which future fire management will be based.

Growing-season fires have ecological value—Research on seasonal effects of burning on fuel characteristics remains equivocal. There are reasons other than fuel reduction and fuel management, however, for conducting growing-season prescribed fires. Although burning in different seasons may not have marked effects on fuels, corollary ecological benefits, such as the promotion of wiregrass flowering and the protection of sensitive species, warrant continued use of growing-season prescribed fire to achieve multiple resource benefits.

Project Deliverables

Wright, Clinton S.; Vihnanek, Robert E. 2013. Does season of burning affect fuel dynamics in southeastern forests?  Final report. (23 March 2015)

Presentation at the 2012 Association for Fire Ecology Conference, Portland, Oregon.(23 March 2015)

Presentation at the 2013 International Association of Wildland Fire. (23 March 2015) (See Resources tab)

Other

We acknowledge funding from the Joint Fire Science Program under Project #09-1-01-2.