Vegetation response varies by season of burning in pine woodlands across the southeastern US

Background Open pine woodlands occur throughout the southeastern United States. Thinning and prescribed fire commonly are used to establish and manage pine woodlands for multiple objectives, often including timber production and wildlife habitat. Although fire effects in pine woodlands have been summarized widely, the effects of fire during all seasons of the year are not well understood. We implemented 72 burns at 9 sites throughout the southeastern US, 2020–2023, to evaluate how fire treatment during the dormant, early growing-, mid-growing-, and late growing-season on a 2-year fire-return interval may affect understory composition, structure, and species diversity indices. Results Fire intensity and burn coverage were greatest in the dormant-season treatment and least in the mid-growing-season treatment. Coverage of semi-woody and woody plants in the understory was less in all treatments compared to control. However, after two fire events, coverage of semi-woody and woody understory plants increased in the dormant-season treatment and woody understory plants increased in the mid-growing-season treatment, whereas neither increased in the early- and late growing-season treatments. These results indicate growing-season fire sets-back semi-woody and woody vegetation better than dormant-season fire if intensity is adequate to top-kill the plants. Forb coverage increased following all seasons of burning, but the increase was greatest in the late growing-season treatment. Grass coverage decreased in control and in the mid- and late growing-season treatments, partially because of fire timing and because understory sunlight was reduced from an average of 54% to 39% over 4 years as overstory tree crowns expanded following thinning. Visual obstruction was least following early growing-season fire. Understory species richness increased in all treatments as well as control. Conclusion We documented changes in plant composition and structure as related to fire seasonality and intensity after 2 fire events on a 2-year fire-return interval. All fire treatments changed understory composition, and each produced different effects that could allow managers to better meet objectives in systems dominated by Southern yellow pines. Growing-season fire offers more flexibility throughout the year to accomplish objectives, including wildlife habitat management, beyond the traditional dormant-season burn window.