Longleaf pine seedling growth and survival: Effects of season and intensity of simulated prescribed burning

Abstract

Restoration of longleaf pine (Pinus palustris Mill.) forests is an important land management goal in the southern United States. To achieve this goal, longleaf pine is increasingly being planted across its range. Fire is an important ecological factor that has historically shaped and sustained longleaf pine forests. However, we have limited information on how fire impacts longleaf pine seedlings in newly established plantations. We conducted this study in Northwest Florida to examine the effects of fire intensity (low intensity ((LI), ∼100 °C), medium intensity ((MI), ∼300 °C), high intensity ((HI), ∼500 °C), and an untreated control) on survival, growth (root collar diameter (RCD) and height), physiological processes (net assimilation (A), stomatal conductance (gsw), and intrinsic water use efficiency (iWUE)), and biomass allocation of longleaf pine seedlings over a period of six months after fire was applied in four seasons (summer, fall, winter, spring). Within a given season, the survival of seedlings did not significantly differ at different fire intensities, except in the summer (α = 0.05). For the summer application, survival was lower (18%) in the HI treatment compared to the MI (98%), LI (93%), and control (93%) (p = 0.003). RCD growth was significantly affected by fire intensity treatment only for the summer application. For the summer application, RCD growth six months after treatment (MAT) was highest (7.3 mm) in the control compared to the LI (2.7 mm), MI (2.7 mm), and HI (2.0 mm) (p = 0.021). The height growth of seedlings did not significantly differ at different fire intensities, though their values were greater at 6 months compared to 3 months. The effect of fire temperature on percent biomass allocation was only significant for the fall foliage (p = 0.004). Neither root, stem, nor total shoot percent biomass significantly differed for any other season. Largely, A, gsw, and iWUE were not significantly affected by fire temperature. iWUE did differ for MAT (p = 0.003), however, no pattern could be discerned. Overall, our observations suggest that longleaf pine seedlings are highly resistant and resilient to fire intensity in different seasons, with some vulnerability to high-intensity summer fires which can adversely affect their survival and growth. Summer burning is still a viable option for seedling survival provided conditions on the ground favor low/medium intensity fire.