Fire effects on a fire-adapted species: response of grass stage longleaf pine seedlings to experimental burning

Background: Longleaf pine (Pinus palustris Mill.) seedlings have a morphological grass stage that is considered to be an adaptation to frequent surface fire regimes. However, fire can kill longleaf pine seedlings and thus may play an important role in longleaf pine regeneration dynamics. We used a prescribed burn simulation tool designed to treat individual grass stage longleaf pine seedlings with controlled delivery of fire treatments and then measured survival and growth responses through two growing seasons. Naturally regenerated grass stage longleaf pine seedlings were randomly selected from three size classes and each assigned one of four treatments (Control, no treatment; Clip, mechanical needle removal; LB, a low-temperature burn treatment; or HB, a high-temperature burn treatment) in both the dormant season (January) and the growing season (May). Results: Seedlings greater than 15 mm root collar diameter had greater than 0.5 probability of survival after the first growing season in the HB treatment, regardless of the season of treatment application, and seedlings across all sizes had greater than 0.6 probability of survival in the LB treatment after the first growing season. The growing season treatment application resulted in additional mortality during the second growing season, across all seedling size classes, which was not observed in the dormant season application. Burning reduced root collar growth through two growing seasons, likely due to needle mortality and the subsequent prioritization of growth to needle production rather than to root or stem growth. Conclusions: Our results suggest that the interplay between seedling size and fire intensity likely contributes to the success of longleaf pine natural regeneration and that seedling size should be considered when scheduling the first burn following planting of longleaf pine seedlings.