UNDERSTORY PLANT COMMUNITY RESPONSE TO SEASON OF BURN IN NATURAL LONGLEAF PINE FORESTS John S. Kush and Ralph S. Meldahl School of Forestry, 108 M. White Smith Hall, Auburn University, AL 36849 William D. Boyer U.S. Department of Agriculture, Forest Service, Southern Research Station, 520 Devall Street, Auburn, AL 36849 ABSTRACT A season of burn study· was initiated in 1973 on the EscambiaExperimental Forest, near Brewton, Alabama. All study plots were established in l4-year-old longleaf pine (Pinus palustris) stands. Treatments conSisted of biennial burns in winter, spring, and summer, plus a no-burn check. Objectives of the current study were to determine composition and structure of understory plant communities after 22 years of seasonal burning, identify changes since last sampling in 1982, arid assess the structure of the communities that stabilized under each treatment regime. There were 114 species on biennial winter~burned plots, compared to 104 on spring- and summer-burned and 84 with no burning. The woody understory biomass «1 centimeter diameter at breast height) increased with all treatments compared with 1982. Grass and legume biomass increased with winter and spring burning. Forb biomass decreased across treatments. keywords: biomass, longleaf pine, Pinus palustris, plant response, prescribed fire, south Alabama, understory. Citation: Kush, 1.S., R$. Meldahl, and W.D. Boyer. 2000. Understory plant community response to season of burn in natural longleaf pine forests. Pages 32-39 inW Keith Moser and Cynthia F. Moser (eds.). Fire and forest ecology: innovative silviculture and vegetation management. Tall Timbers Fire Ecology Conference Proceedings, No. 21. Tall Timbers Research Station, Tallahassee, FL. INTRODUCTION bins and Meyers (1992) concluded that most lightning­ ignited fires occurred during the growing season. How­ Prior to the arrival of European settlers in the Unit­ ever, much of the prescribed burning done in the ed States, natural communities dominated by longleaf Southeast is conducted during the winter. Among the pine and maintained by periodic fire occurred through­ reasons are the cooler temperatures associated with out most of the southern Coastal Plain (Frost 1993). that time of year, more predictable winds, and poten­ Bartram (1791), an early traveler through the South­ tially less damage to the trees and regeneration. east, wrote about these communities: "This plain is Streng et al. (1993) provided an excellent literature mostly a forest of the great long-leaved pine (P. pal­ review of research studies evaluating the effects of ustris Linn.), the earth covered with grass, interspersed season of bum in longleaf pine forests. One of these with an infinite variety of herbaceous plants, and em­ studies came from research conducted on long-term bellished with extensive savannahs, always green, burning plots located in south Alabama (Boyer 1983, sparkling with ponds of water ... " 1987)•. Research was begun in 1973 to determine the The landscape that Bartram and early settlers en­ effects of understory hardwood control treatments on countered was largely the result of frequent fire. Low­ the growth of overstory longleaf pine and development intensity, non-lethal fires swept through the presettle­ of understory vegetation. One of the original study ob­ ment longleaf savannas at intervals ranging from 1- jectives was to follow understory plant succession un­ 10 years (Mattoon 1922, Chapman 1932, Christensen der different seasonal burning regimes. Boyer (1995) 1981). These fires were ignited by a combination of reported on responses of ground cover before treat­ lightning strikes (Komarek 1974) and aboriginal in­ ment and 7 and 9 years after treatments. Although cendiarism (Robbins and Myers 1992). The frequent treatments have continued, lack of resources prevented fires had an incredible impact on the flora of the long­ sampling understory vegetation after 1982. This study, leaf landscape. A mesic longleaf woodland may con­ a continuation of Boyer's earlier work, examines ef­ tain 140 vascular species per 1,000 square meters, the fects of 23 years of different seasonal biennial bums largest values reported for the temperate Western (or. no bum) on ground cover vegetation in natural Hemisphere (Peet and Allard 1993). longleaf pine forests. It has been well documented that in the absence of frequent burning, the diverse ground cover of the longleaf landscape is replaced by hardwood trees and STUDY AREA shrubs (Christensen 1981, Streng and Harcombe The study was conducted at the Escambia Exper­ 1982). The importance of frequent burning in these imental Forest in south central Escambia County, Al­ systems has been realized, but questions have arisen abama (31 0 01'N; 87°04'W). The forest is maintained concerning season of bum. Komarek (1964) and Rob- by the U.S. Department of Agriculture, Forest Service, 32 UNDERSTORY PLANT COMMUNITY RESPONSE TO SEASON OF BURN 33 Table 1. Ground cover biomass changes with time (all plots) from a 23-year-old season of bum study in south Alabama. Stand Basal BIOMASS Area. (square (kilograms per hectare) meters per Year hectare) Woody Herbaceous Total Organic litter Total 1973 6.9 508.5 422.2 930.7 8,310.4 9,241.1 1980 13.8 698.9 483.8 1,182.7 10,759.8 11,942.5 1982 17.2 636.6 387.5 1,023.7 9,255.7 10,279.4 1996 22.2 966.6 99.3 1,065.9 20,333.9 21,399.8 Southern Research Station, in cooperation with T.R. 1996, the spring in May 1995, and the summer in July Miller Mill C<;>mpany. 1995. The climate is humid and mild with plentiful rain­ Information on understory composition and bio­ fall well. distributed thr<;>ughoutthe year. The warmest mass by components was compared to the earlier re­ months are July and August with average daily max­ sults from this study. The number of species, species imum and minimum temperatures of 33° and 20°C, frequency and biomass, and percent biomass were ex­ respectively. The coldest months are December and amined among the different season of burns. January with average daily temperatures of 18° and 3°C, respectively. The growing season is 250 days. Annual precipitation averages 156 centimeters. Octo­ RESULTS AND DISCUSSION ber is the driest month. The predominant soil series on this coastal plain Ground cover biomass was highly variable and site was the Troup (Grossarenic Paleudult, loamy si­ changed substantially over the last 23 years. Estimates liceous thermic) with some Wagram, Dothan, and Fu­ of biomass by year and component (type) are given in quay represented. These soils formed in unconsolidat­ Table 1 (this is an update of Table 1 from Boyer 1995). ed marine sediments of loamy sands, sandy loams, and In terms of average weights, as stand age and basal sandy clay 10ams. They are very low in natural fertility area increased over time, the organic litter component and organic-matter content; increased' greatly, the woody component increased slightly, and herbaceous component declined. We hy­ pothesize this large increase in organic litter may be METHODS due to 2 major factors. The first factor concerns the decrease between 1980 and 1982. The lower 1982 lit­ Boyer (1987, 1991, 1993, 1994, 1995) described ter weight might have been the result of a hotter than the establishment, methods, and treatment regimes for normal fire or lower than normal litter fall prior to this study. Treatments consisted of biennial bums in sampling. The dramatic increase in litter weight from winter, spring, and summer, plus a no-bum check. 1982 to 1996 might also be due to a major increase in Each burning treatment was combined with 3 supple­ the amount of woody debris added to the organic litter mental hardwood control treatments: chemical injec­ over the last 14 years of the study. Some of the debris tion of all hardwood stems in 1973, hand-clearing of may have been added during the 1990 thinning oper­ all hardwood stems >1.3 meters in height in 1973 and ation, The analysis of raw biomass weights indicated periodically thereafter as needed, and an untreated that after 23 years of constant burning regimes, equi­ check. The study area had not been bUrned since 1962, librium had not been reached on the plot!). so all season of bum plots were first given a winter An examination· of the biomass data by burning conditioning bum in January 1974. Boyer reported on treatment suggested some consistent trends from effects of treatments on growth of the longleaf pine 1982-1996 (Table 2). Woody, total green, and organic overstory (1987, 1994); the effects of a single chem­ litter components increased for all burning treatments ical treatment, with and without fire, on development while the forb component uniformly decreased. Le­ of woody vegetation (1991); and development of hard­ gumes; a potentially important factor for wildlife food woods in relation to season of biennial bums (1993). and maintaining or improving site quality, decreased In late September-early October 1996, living ma­ on the no-bum treatments. Legume biomass increased terial < 1 centimeter diameter at breast height (DBH) with the winter burning treatment, remained the same was destructively sampled from 9 0.893-square meter with the spring burning treatment, but decreased with sample' plots per trea.tment plot to coincide with the summer burning where low values in 1982 declined sampling done in 1982. The vegetation was sorted by further in 1996. Grass biomass did not show any con­ species using the taxonomy of several authorities (Gre­ sistent trends. Organic litter was the largest component len and Duvall 1966, Radford et al. 1968, Clewell of understory biomass and increased substantially for 1985, Godfrey 1988, Kartesz 1994). It was oven-dried all burning treatments. at 70°C for 72 hours and weighed. Organic litter was One hundred forty-three species were identified in collected from 1 30.5-square centimeter subplot within ground cover in the early autumn sampling (Table 3).