Longleaf and shortleaf pine seedling fire tolerance is more sensitive to shade than encroaching hardwood species

Abstract

Traits enabling fire tolerance are essential for plant persistence in fire-prone ecosystems. Yet, fire tolerance alone fails to account for other pre-existing forms of stress that could modify species tolerance of fire. Shade is an increasingly important form of stress on the contemporary forested landscape, as widespread fire exclusion has hastened the transition of many open woodlands into closed canopy forests. To investigate the interaction of fire and shade in the longleaf pine ecosystem, we established a shade house experiment examining seedling mortality and recovery of six native subtropical tree species from prescribed fire applied under four levels of shade (unshaded, 63 %, 73 %, 90 %) in northwest Florida, USA. Longleaf and shortleaf pine mortality consistently increased with shade and were best predicted by shade intensity and degree of crown scorch. Resprouting became the primary survival mechanism of longleaf and shortleaf pine under increasing shade. Among encroaching species, mockernut hickory maintained the highest pre-fire non-structural carbohydrates (NSC) and experienced the lowest post-fire mortality; loblolly pine maintained the lowest pre-fire NSC and suffered the highest mortality. Survival of encroaching hardwood species was entirely dependent on resprouting across shade levels. Post-fire NSC recovery of northern red oak and mockernut hickory increased with shade intensity, while shortleaf pine NSC recovery declined with shade. Collectively, these results demonstrate that reducing pre-fire shade below 63 % will minimize longleaf and shortleaf pine mortality and accelerate post-fire NSC recovery compared to encroaching hardwood species.