Overstory Longleaf Pines and Hardwoods Create Diverse Patterns of Energy Release and Fire Effects During Prescribed Fire

Abstract

Litter from pine trees in open woodlands is an important fuel for surface fires, but litter from hardwood species may quell fire behavior. Lower intensity fires favor hardwood over longleaf pine regeneration, and while overstory hardwoods are important sources of food and shelter for many wildlife species, too many could result in canopy closure and a loss of ground layer diversity. Although some researchers have found synergies in fire effects when leaves of different species are combined, field tests of effects of tree guild diversity on fire behavior are lacking from the literature. We used neighborhood modeling to understand how diverse overstory trees in longleaf pine forests affect fire radiative energy density (FRED), and to determine the effect on top-kill of shrub-form hardwood trees. We measured the effects of three guilds of overstory trees (longleaf pine, upland oaks, and mesic oaks) on FRED, and related FRED to post-fire damage in four guilds of understory hardwoods (sandhill oaks, upland oaks, mesic oaks, and fleshy-fruited hardwoods). We found that FRED increased 33–56% near overstory longleaf pine but decreased 23–37% near overstory mesic oaks. Additive models of FRED performed well and no synergies or antagonisms were present. Seventy percent of stems of understory hardwoods survived fire with energy release typical of dormant-season fires in canopy gaps and near overstory mesic oaks. We also found that among understory trees >2 m tall, upland and sandhill oaks were more likely than mesic oaks or fleshy-fruited hardwoods to avoid top-kill. We conclude that neighborhood models provide a method to predict longleaf pine forest structure and composition that allows for the ecological benefits of overstory hardwoods while maintaining ground-layer diversity. To maintain hardwood control, fire practitioners may need to select fire weather conditions to increase fire behavior especially during dormant-season burns.

Litter Flammability of 50 Southeastern North American Tree Species: Evidence for Mesophication Gradients Across Multiple Ecosystems

“…Recent studies have highlighted how flammable litter often corresponds with fire tolerance at the species level and may itself be a fire-adapted trait (Varner et al, 2016;Stevens et al, 2020;Varner et al, in review). Field measurements in forests, woodlands, and savannas validate the relative importance of senesced litter as the primary mechanistic driver of surface fire behavior (Wenk et al, 2011;Whelan et al, 2021).…”

“…Litter is a primary fuel in woodlands and forests, but the contribution of other fuels should be better integrated. In studies of fire behavior drivers in eastern woodlands and forests, litter has been repeatedly shown to drive intensity and spread (Dickinson et al, 2016;Whelan et al, 2021). Where herbaceous fuels are proportionally more dominant, speculation exists over the importance of comparatively rare tree litter.…”

Life Stage and Neighborhood-Dependent Survival of Longleaf Pine after Prescribed Fire

“…Microsite conditions can substantially impact local fuel loads and fire intensity [21,46,63]. We observed mixed results regarding the importance, direction, and magnitude of microsite characteristics' relation to fire temperatures.…”

Turning up the heat: fire ecology and climate adaptive silviculture in longleaf pine forests

“…Local variation of fuels can affect fire behavior, with important implications for the establishment and persistence of woody species (Bigelow & Whelan, 2019;Mugnani et al, 2019;O'Brien et al, 2016). The establishment of fire-intolerant species with low litter flammability can create a negative feedback, altering understory composition and creating areas where fire will not permeate even with regular fire return intervals (Whelan et al, 2021;Martin & Kirkman, 2009). Local variability in fire behavior has also been associated with longleaf pine seedling establishment (Grace & Platt, 1995;Robertson et al, 2019).…”

“…and sand post oak (Q. margaretta Ashe; Hiers et al, 2014). By contrast, fire sensitive species, termed mesophytic oaks, include those that decrease flammability or entirely prevent fire penetration where their litter falls (Kreye et al, 2018;Loudermilk et al, 2016;Whelan et al, 2021). Examples of mesophytic oaks in the longleaf ecosystem are coast live oak (Q. virginiana Mill), water oak (Q. nigra L.) and laurel oak (Q. hemisphaerica spp.).…”