Prescribed fire every two to five years is necessary for longleaf pine (Pinus palustris Mill.) ecosystem function and is an important part of longleaf pine restoration in the southeastern United States. Sustained longleaf pine growth, despite crown scorch, is attributed to gas exchange and carbohydrate mobilization responses to fire that reestablish foliage mass. An upward shift in crown volume also occurs over several cycles of prescribed fire which reduces future lags in foliage mass due to scorch. These means of sustained growth are likely to occur in frequently burned Pinus forests regardless of dominant tree species. Water deficit reduces gas exchange by a decrease in stomatal function. Thus, when drought occurs in the year of prescribed fire, carbon fixation by surviving foliage may not be sufficient to regrow scorched foliage. Drought effects on carbon fixation may also limit carbohydrate pools that are later mobilized to support tree growth processes. Two studies on the Kisatchie National Forest in central Louisiana, USA were conducted to explore longleaf pine adaptations to repeated fire. Co-occurrence of drought during each study expanded our objective to include an assessment of how Pinus responds to interaction between drought and prescribed fire.
Results/Conclusions
In both studies, May fire was applied by prescription when stand conditions met specifications for operational fire application. In study 1, May prescribed fire was applied midway through a precipitation deficit between April and June. In this study, annual stem growth was reduced by May fire and stem growth loss was correlated with scorch level and tree size. By the end of the growing season after May fire, post-scorch foliage recovery failed to achieve a normal level of foliage mass and annual stem growth was reduced among larger trees. In study 2, May prescribed fire was applied in the tenth month of a 20-month drought. By the end of the growing season after May fire, foliage recovery and stem growth did not differ between trees that were and were not prescribed burned. However, in the second year after May fire, annual stem growth was reduced and this loss was correlated with tree size. Where crown scorch is likely, prescribed fire may be detrimental to Pinus vigor when fire coincides with water deficits that reduce carbon fixation. Precipitation records in the vicinity of these studies will be used to assess the likelihood of a similar Pinus growth loss due to interaction between drought and prescribed fire.