Fire season differentially affects resprouting vigor of pyrophytic and mesophytic hardwoods in a southeastern U.S. pine savanna

Abstract

Like many savannas worldwide, pine savannas of the southeastern U.S. contain pyrophytic (fire-adapted) broadleaf woody plants (hardwoods), as well as mesophytic (fire-sensitive) hardwoods that persist through cycles of top-killing and resprouting. The persistence of mesophytes may be facilitated by anthropogenic fire regimes that top-kill hardwoods when they have higher carbohydrate reserves in roots, and the effects of fire timing may interact with herbivory. We investigated the resprouting response of pyrophytic and mesophytic hardwoods to fire in the four seasons by measuring the change in above-ground woody biomass between two fire-free intervals as a relative growth rate (RGR), with half of the plants protected from browsing by white-tailed deer (Odocoileus virginianus) to assess potential interactions with herbivores. Spring fires similarly reduced RGR of both pyrophytes and mesophytes (difference between functional groups [Δ] = 0.02, p = 0.86), the summer (Δ = 0.66, p < 0.0001) and fall (Δ = 0.51, p < 0.0001) fires disproportionately disadvantaged mesophytes, and winter fires resulted in relatively high resprouting vigor for both groups (Δ = 0.31, p = 0.06). Similar patterns were shown by the biomass to which plants were predicted to equilibrate under a given fire regime (the persistence equilibrium). Herbivore access did not influence resprouting. Our results indicate that growing season fires, which correspond to historic fire regimes, inhibit the relative growth of mesophytes more effectively than dormant season fires. We recommend that fire season be considered in the restoration and maintenance of historic woody plant composition and structure in pine savanna ecosystems.