OOS 61-4 - Exploring the effects of fire return interval on resource-use strategies of pyrophytic and mesophytic species

Fire exclusion in fire-structured forests in the southeastern United States has led to changes in forest species composition. The hypothesized process explaining these changes is mesophication, a phenomenon characterized by fire-sensitive, shade-tolerant species (i.e., mesophytes) replacing fire-tolerant, shade-intolerant species (i.e., pyrophytes), altering understory microclimate and flammability. To address mesophyte encroachment and restore fire-tolerant vegetation, managers reintroduce low-intensity, surface fires in these systems. However, fire often fails to alter species demographics, and even in frequently burned forests mesophyte recruitment often exceeds that of pyrophytes. We hypothesize that trait plasticity and a shift to a more conservative strategy can help explain the persistence of mesophytes in fire-disturbed areas, while a prolonged period of fire exclusion may result in pyrophytes adopting an acquisitive strategy. To test this hypothesis, we investigated the effects of fire return intervals and fire exclusion on functional traits of three species: one pyrophyte, southern red oak (Quercus falcata), and two mesophytes, sweetgum (Liquidambar styraciflua) and red maple (Acer rubrum). This study was conducted in two fire-prone landscapes: Bankhead National Forest, AL, USA, a mixed pine-hardwood forest, and Tall Timbers Research Station, FL, USA, a loblolly (Pinus taeda) and shortleaf pine (P. echinata) woodland. Our hypothesis is supported by preliminary results, with sweetgum showing lower specific leaf area (SLA) and higher leaf dry matter content (LDMC) in burned sites, while southern red oak showed higher SLA in unburned sites compared to burned sites. These findings indicate trait plasticity in both species and potential convergence of resource investment and use strategies in response to fire frequency. Trait plasticity might help explain the persistence of sweetgum across the landscape regardless of fire frequency, as well as the limited recruitment of southern red oak in fire-excluded areas. This understanding of species' response mechanisms to fire can help the development of better forest and fire management strategies that integrate other tools and practices to effectively restore fire-tolerant vegetation. Further analyses are planned to expand the set of functional traits examined, including belowground traits, and to explore the responses of red maple to fire frequency.