Effects of burning season and frequency on saw palmetto (Serenoa repens) flowering and fruiting

Medicinal use of saw palmetto (Serenoa repens) fruits in treating benign prostatic hypertrophy has driven a recent sharp increase in fruit harvesting. At the same time, saw palmetto often is considered a keystone species, serving as habitat or a food source for several hundred mammal, bird, reptile, amphibian and insect species. Due to harvesting pressure on this ecologically important, non-cultivated species, land management practices that produce environmentally sustainable harvests are needed. As part of research focusing on use of controlled burning as a management tool, we investigated effects of burning season and frequency on saw palmetto flowering and fruiting in 32 pine flatwoods sites in central and southwest Florida from 1997 to 1999. The study was conducted as two repeated measures experiments, each involving 16 sites. Each experiment included two among-subjects factors - burning season and past burning frequency - and one within-subjects factor -year- with four replications of each treatment combination. For both experiments, growing season (i.e., April-July) burns had high flowering levels the first year after burning (mean proportion of saw palmettos flowering = 0.49, n = 16 sites), but decreased the second year after burning (mean proportion flowering = 0.15, n = 16 sites), while winter (i.e., November-February) burns had intermediate levels of flowering during both the first and second years after burning (mean proportions flowering = 0.21 and 0.28 for first and second years after burning, n = 16 sites). Although most flowering occurred during April and May, flowering occurred 2-5 months after winter burns, but did not occur until 9-12 months after growing season burns. During the 1997-1998 experiment, fruit infection by the fungus Colletotrichum gloeosporioides caused a fruiting failure in all sites. During the 1998-1999 experiment, fruit yields were higher in growing season burns only in infrequently burned (i.e., burned less often than every 5 years) sites, and only in 1998 (mean fruit yields = 325 kg/ha and 0.25 kg/ha for growing season, infrequently burned sites and winter, infrequently burned sites; n = 4 sites in each group). Frequently burned (i.e., burned every 2-3 years) sites generally had low fruit yields (mean fruit yields = 163 kg/ha and 242 kg/ha for frequently burned sites in 1998 and 1999; n = 8 sites). Although controlled burning can be an effective management tool in producing economically sustainable saw palmetto fruit harvests, land managers also must consider ecological needs of other plant and animal species when planning environmentally sustainable burning regimes. (c) 2006 Elsevier B.V. All rights reserved.