1994 Proceedings
North American Conference on Savannas and Barrens

In the presettlement southeastern United States, fire dependent communities, such as those dominated by Pinus palustris (longleaf pine), covered large areas. It is only recently that the detrimental effects of fire suppression on these communities has been recognized. Fire suppression, conversion to agricultural use and conventional forestry practices have reduced the original extent of longleaf pine communities from 24 million to 2 million ha (Dennington and Farrar 1983). Currently, considerable effort is being expended to reestablish fire regimes in order to benefit rare species and maintain these unique and threatened ecosystems (Frost et al. 1986).

The Zuni Pine Barrens in Isle of Wight County, Virginia is a fire dependent community with an open, P. palustris-Quercus laevis (turkey oak)-Pinus taeda (loblolly pine) overstory, a dense understory of low, ericaceous shrubs and frequent openings dominated by herbaceous species, lichens and mosses (Frost and Musselman 1987). The area is unique in that only a very few pine barrens remain in the state, several plant species of southern pine savannahs reach their northern limit here, and the site may represent a community type distinct from both southern (North Carolina) and northern (New Jersey) pine barrens.

The goals of the present study were to examine the effects of fire on plant demography, population dynamics (mortality and regeneration), shifts in dominance and species composition in two adjacent parcels, each containing xeric longleaf pine barrens and wetter communities, and to compare these effects between the barrens and adjacent mesic and hydric forests.

STUDY AREA

The study area is located in Isle of Wight County, Virginia; 7.3 km southwest of the town of Zuni (36°49' N, 76°51' W). The first area burned (BA1) is 21.1 ha and contains 7.5 ha of xeric, P. palustris-Q. laevis barrens, 9.1 ha of mesic, Pinus serotina-P. taeda forest and 4.5 ha of Acer rubrum-P. serotina swamp. BA1 was burned in February, 1986 and July, 1987. The second area burned (BA2) is 11.6 ha, borders the northern edge of BA1, and contains 4.8 ha of xeric, P. serotina-P. palustris-Q. laevis barrens and 6.8 ha of mesic, P. serotina-P. taeda forest. BA2 was burned in February, 1988. All areas were clear-cut and burned between 1955 and 1957, and allowed to naturally regenerate (Union-Camp Corp. stand records). The forests were found to be 32 years old and even-aged at the time of the study.

METHODS

Vegetation was quantified using stratified regular sampling. Over-story species (_ 10 cm DBH) and saplings/large shrubs (< 10 cm and > 1.25 cm DBH) were sampled in 100 m2 circular plots. Understory species (_ 1 m tall) were sampled in 1 m2 quadrats located at the center of the 100 m2 plots. All plant species were sampled as numbers of individuals (stems) by species per plot. In BA1, 10 pine barrens plots, 12 mesic forest plots and 6 swamp plots were sampled. In BA2, 6 pine barrens plots and 9 mesic forest plots were sampled. Plots were sampled annually, at the end of the growing season (August and September). In BA1, plots were sampled in 1985 (before burning), 1986, 1988 and 1989. In BA2, plots were sampled in 1986 (before burning), 1988 and 1989. Barren understory plots were again sampled in 1991.

Fire induced mortality in the overstory and sapling/large shrub layers were analyzed. For the dominant species, paired one-tailed T-tests were conducted to determine which species decreased in abundance due to fire. In order to determine whether there were differences in stem numbers between species over time, one-way Anovas were conducted using species as treatments and differences in stem number between years as independent variables.

In the understories, species' fire related population dynamics were studied. For the dominant species, two-tailed paired T-tests were conducted to determine whether changes in numbers between years occurred. Multi-variate based repeated measures Anovas were used to determine whether there were differences in numbers of individuals per plot due to time, and which species differed from the mean over time. In order to overcome possible lack of independence between species, Canononical Discriminant Analyses were conducted in which numbers of individuals by species were used as dependent variables and times (years) were independent variables. This conservative technique determines whether separation between years exists and whether this separation can be described in terms of some combination of species and their densities. Paired two-tailed T-tests were used to determine whether tree, shrub or herbaceous understory components changed in number between years.

RESULTS AND DISCUSSION

Fire resulted in 40% aboveground mortality in the overstory, 80% in the sapling/large shrub layer and, and nearly 100% in the understory, followed by a 3.3 to 10.6 fold increase in understory density. Two to four fold increases in understory species diversity were exhibited. Several species noted from Fernald's 1936 survey but since extirpated from the site, reappeared. In the barrens, Vaccineum crassifolium and Aristida virgata and in the mesic areas, Sarracenia purpurea and Habenaria blephariglottis were noted. Regeneration was predominantly by vegetative means. Platt et al. (In Press) in Florida pine savannahs, and Reiners (1965) in the Long Island (NY) pine barrens also found post-fire regeneration to be almost entirely due to vegetative sprouting. Shrub species accounted for greater than 90% of the post-fire density and 80% of the increase in density.

In the wetter areas, tree and herbaceous species made up a larger percentage of post-fire understory density than was the case in drier areas, and species diversity also was greater. Frost et al. (1986) and McCormick (1979), in southern pine savannahs and New Jersey pine barrens, respectively, both reported greater species diversity and a larger herbaceous component on wetter sites. In drier areas, the same species which dominated the understory before the fire continued to do so afterward, and these same species, Gaylussacia baccata and Kalmia angustifolia, increased in number significantly more than other species. In the wetter areas, the majority of pre-fire dominant understory species were no longer important components of the communities after the fire, and these species either failed to increase significantly or even decreased in number. Acer rubrum, Pteridium aquilum and Gaylussacia frondosa, species that were not important components of the wetter communities before the fire, increased significantly more than others and were among the dominant post-fire understory species (Table 1). In the drier areas, all significant changes in density occurred in the first year following the fire, while in the wetter areas, significant changes occurred in the second year as well. This may indicate that drier areas are historically more fire-prone and the understory species are fire adapted and able to respond more quickly. In wetter areas, fire is not a common occurrence and understory species are not fire adapted. Thus, the response to fire is delayed and species that were previously a minor component or absent from the community are the ones that exhibit the greatest increase.

LITERATURE CITED

Dennington, R., and R. Farrar. 1983. Longleaf pine management. USDA Forest Service Research Report R8-FR3. 17p.

Frost, C. and L. Musselman. 1987. History and Vegetation of the Blackwater Ecologic Preserve. Castanea 52: 16-46.

Frost, C., J. Walker and R. Peet. 1986. Fire dependent prairies and savannahs of the southeast: Original extent, preservation status and management problems. In: D. Kulhavey and R. Connor, eds. Wilderness and Natural Areas in the Eastern United States. Center for Applied Studies, School of Forestry. S. F. Austin State Univ. Nacogdoches, TX.

McCormick, J. 1979. Vegetation of the New Jersey Pine Barrens. In: R. Forman, ed. Pine Barrens: Ecosystem and Landscape. Academic Press, NY.

Platt, W., G. Evans and M. Davis. Effects of fire on season of flowering of forbs and shrubs in longleaf pine forests. Oecologia. In Press.

Reiners, W. 1965. Ecology of a heath-shrub synusia in the pine barrens of Long Island, New York. Bulletin Torrey Botanical Club 92: 448-464.

Table 1: Overstory, sapling and understory species exhibiting statistically significant population changes in Burn Areas 1 and 2. An asterisk (*) indicates a statistically significant population increase (Ps .005).