Impacts of different fire return intervals on longleaf pine stand dynamics

Background/Question/Methods Longleaf pine (Pinus palustris Mill.) was the dominant tree species across an estimated 60% of the Southeastern USA landscape at the time of European settlement.  These ecosystems were maintained by frequent, low-intensity ground fires which eliminated woody competition and allowed longleaf pine to regenerate itself in canopy openings.  Several studies have examined the effects of different fire return intervals and season of burns on ecosystem components, i.e., understory and ground cover vegetation but impacts to longleaf pine have been virtually ignored.  A study was established in 1984 to determine the comparative impact of both winter and spring prescribed fires at several intervals on the growth of a longleaf pine overstory and development of a hardwood understory.  The study was established in young, naturally regenerated longleaf pine stands (trees were 9 years old) located in south-central Alabama, USA.  Treatments include both winter and spring burns repeated at intervals of 2, 3, or 5 years plus an unburned check.  Fire type, flame length, air temperature, relative humidity, fire-line intensity, and crown scorch were measured and recorded to provide data on the prescribed burns due to the variable nature of fire depending on the environmental and weather conditions of a particular day.

Results/Conclusions

For the first six years of the study there were no treatment effects on longleaf pine. The first treatment effects on longleaf pine growth appeared when the trees were 17-20 years old and have become more apparent with time.  Mortality has been minimal with longleaf pine survival running from 91% on the spring 2- and 3-year burns to 99% on the no burn, winter burns and spring 5-year burn treatment. While there have been no significant differences in DBH and total height, there has been a significant difference in basal area.  By 1994, the no burn and 5-year spring burn were different from the other burn treatments.  By 1999, the 3-year spring burn joined the no burn and 5-year spring burn as being significant from the other treatments.  Both season and fire return intervals had an early and significant effect on understory hardwoods.  There has been an increase in number, basal area, and volume in understory hardwoods with winter burns.  Hardwood volume and number of stems have declined over the previous 15 years with both 2- and 3-year spring burns and increased only slightly with the 5-year spring burn.