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.
Highlights
Frequent fire maintains the open canopy structure of longleaf pine (Pinus palustris Mill.) woodlands in part by preventing hardwood trees, which are often abundant in the understory, from growing to fire-resistant size (Provencher et al, 2001; Whelan et al, 2018)
We address the following hypotheses: (1) energy release will increase with proximity to longleaf pine and upland oak species and will decrease with proximity to mesic oak species, (2) intermingling of different tree guilds will have non-additive effects on energy release, and (3) shrub-form hardwoods of upland and xeric habitats will be more resistant to top-kill than hardwoods of mesic habitats at any level of energy release during prescribed fire
Because control of shrub-form hardwoods is a common objective for prescribed fire in longleaf woodlands, we investigated how fire radiative energy density (FRED) affected post-fire fate of four guilds of shrubform hardwood trees common in the understory of longleaf pine woodlands
Summary
Frequent fire maintains the open canopy structure of longleaf pine (Pinus palustris Mill.) woodlands in part by preventing hardwood trees, which are often abundant in the understory, from growing to fire-resistant size (Provencher et al, 2001; Whelan et al, 2018). Reduced fire frequency and intensity may reduce aboveground stem mortality (top-kill) of understory hardwood trees, resulting in Longleaf Pine Neighborhood Fire Energy development of a dense hardwood-dominated midstory (Glitzenstein et al, 1995; Hiers et al, 2007). Dense midstory or overstory tree canopies reduce light availability at the ground, and result in a loss of floristic diversity, which can be exceptionally high in frequently burned longleaf pine woodlands (Kirkman et al, 2001). Overstory trees provide fuel for fire, and the burning characteristics of their litter vary by species (Fonda, 2001; Kane et al, 2008; Kreye et al, 2018). Leaves of hardwood trees common to longleaf pine woodlands burn with varying intensity (Kane et al, 2008). The resulting fire effects may tend to create positive feedbacks whereby trees create conditions that favor their own species (Gagnon et al, 2010)
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