Evaluating Effects of Season of Bum in Longleaf Pine Forests: A Critical Literature Review and Some Results from an Ongoing Long-Term Study

Southeastern pine forests have traditionally been burned during the winter, at least in part because growing-season burning is thought to be harmful to pine .silviculture. In contrast, most natural fires are thought to have occurred during the spring and summer, when the chance of ignition by lightning is greatest. Recently, conservationists have argued that growing-season burning is critical to restoration and maintenance of natural groundcover vegetation in southeastern pine forests and not overly harmful to pine trees. Despite considerable feelings on both sides of the argument, there is surprisely little scientific evidence with which to evaluate the effects of season of burn on either trees or groundcover vegetation. Furthermore, much of the evidence that does exist appears con'tradictory or inconsistent. Many season-of-burn studies are plagued by statistical and design problems, including "pseudoreplication;' inadequate statistical analyses, and failure to control for important confounding variables (e.g., fire intensity or time between burning and sampling). These problems may be especially significant for short-term studies, or for studies with only short-term records of vegetation responses. In this paper, we illustrate some problems from previous work and present some of our own results from a long-term study of season of burn in longleaf pine forests in north Florida. We found, as have others, that early growing-season burns result in high rates of topkill and complete-kill for midstoryoaks. In contrast to some earlier studies, however, we found I.itlle effect of burning season on pine dynamics. In addition, we found almost no effect of season of burn on the abundance of species in the groundcover. We did, however, find a large positive effect of growing-season burning on flowering and presumably seed production of dominant grasses and some forbs. We suggest that recruitment into the groundcover from seed occurs only infrequently in these habitats, and thus, changes in groundcover species composition due to season of burn will be apparent only after long periods of repeated burning during the same season.