Allison L. Melcher, et.al, April 2023
RESEARCH BRIEF #48
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MANAGEMENT IMPLICATIONS
Early growing season burns are more effective than dormant season burns for reducing forest canopy closure and increasing structural heterogeneity.
A single fire may not sufficiently restore forest structural heterogeneity; increased number of burns could be required regardless of burn season.
Fire behavior can vary by season; forest managers should apply fire at a time that meets management objectives.
In this study, the authors investigated how fire seasonality affects forest canopy closure, land cover diversity, and length of available edge habitat. Fire seasonality refers to the time of year a fire is set. In the Southern Appalachians, historical fires likely occurred when fuels were dry in late spring and early summer. Prescribed fire is now primarily applied in the dormant season (January-March), in part to reduce the likelihood of wildfire from an escaped fire, but also because fire seasonality and its effects are less understood than other aspects of fire ecology, such as fire intensity or severity.
A century of active fire suppression in the southeastern US has changed historical species composition from shade-intolerant, fire-adapted species to shade-tolerant mesophytic species. The species turnover in turn has produced more cool, damp, and shaded forest microclimates, improving conditions for shade-tolerant species and deteriorating conditions for fire-adapted species. This positive feedback loop is known as forest mesophication, and in the Southern Appalachians, it has degraded or replaced more open oak/pine forests and a rich understory with denser stands of red maple (Acer rubrum), yellow poplar (Liriodendron tulipifera), or black gum (Nyssa sylvatica), and a sparse understory. As a means of restoring degraded oak and pine forests, prescribed fire is often applied with a goal to reduce canopy closure and promote canopy gaps, increasing light levels, structural heterogeneity, and regenerating herbaceous understory species.
The authors conducted their study in national forests in the Southern Blue Ridge Escarpment of the Southern Appalachian Mountains. Canopy cover in study sites was primarily oaks (Quercus alba and Q. rubra), hickories (Carya glabra, and C. tomentosa), and pines (Pinus strobus); with an often dense lower overstory and midstory of mesophytic species including red maple, black gum, mountain laurel (Kalmia latifolia), and rhododendron (Rhododendron maximum), along with a mostly bare or sparse understory. Three locations each contained three treatment units: a dormant season burn, a growing season burn, and an unburned control. Prescribed burns were conducted in either 2018 or 2019. Dormant season burns occurred before tree buds began to break (January 31 – April 5); growing season burns occurred after bud break but before complete leaf-out (April 18 – April 26). All burns were the first prescribed fires on the sites in recent history.
The authors used National Agricultural Imagery Program (NAIP) leaf-on imagery from 2017 (pre-burn) and 2019 (post-burn) to hand-digitize visually distinct areas of “early successional” (< 30% canopy cover) and “open” (30–60% canopy cover) in the study sites; remaining areas were defined as closed-canopy (> 60% canopy cover). They then compared changes in canopy openness, perimeter length of open and early successional polygons (available “edge habitat”) and calculated and compared land cover diversity (amount and evenness of early, open, and closed canopy) among treatments.
Authors found a mean canopy cover decrease of 8.84% following growing season burns, a mean decrease of 5.21% following dormant season burns, and a decrease of 0.01% in unburned units (see figure). Land cover diversity also differed significantly among all treatments, with the lowest diversity in unburned control sites and the highest in growing season burn sites. Similarly, the available edge habitat differed significantly among treatments, averaging 141.3 m (463.6 ft) in growing season treatment units, 104.81 m (343.9 ft) in dormant season treatment units, and 0.544 m (1.8 ft) in unburned units.
The authors suggest that the growing season burns likely had greater intensity and severity than dormant season burns. They conclude that growing season burns may be more effective than dormant season burns at not only reducing closed canopy cover and creating open or early successional conditions, but also at creating a more even distribution of these conditions throughout a unit. Although fire seasonality may be important for structural changes, increasing the number of burns is likely important regardless of season. More work is needed to understand the effect of repeated burns on forest structure in this region. Forest managers may consider expanding their burn programs to include early growing season burns if reduced canopy closure and improved heterogeneity are primary goals.
Allison L. Melcher, Donald Hagan, Kyle Barrett, Beth Ross and Jean Lorber, Fire Ecology, April 2023
The Oak Woodlands and Forests Fire Consortium seeks to provide fire science to resource managers, landowners and the public about the use, application, and effects of fire in the region. www.oakfirescience.com
This research brief was funded by The Joint Fire Science Program. www.firescience.gov
