Forty Years Of Prescribed Burning On The Santee Fire Plots: Effects On Understory Vegetation

The effects of 43 years of repeated prescribed burning on crown cover, species composition, species richness, and diversity in the lower understory strata of the Santee Fire Plots were examined. Five study treatments, installed in 1946, include an unburned control, periodic winter and summer bums, and annual winter and summer bums. Understory cover has not changed in the past 20 years except in the annual winter bum plots where cover of trees I 1.5 m in height declined and grass cover increased. Detrended correspondence analysis indentitied four distinct understory plant communitites corresponding to season and frequency of bum. Distribution of understory species across a tire disturbance gradient is discussed in terms of varying plant adaptations to tire. Species richness, when separated into herbaceous and woody species groups, and Shannon’s diversity index varied significantly across treatments. INTRODUCTION The Santee Fire Plot (SFP) study in the Francis Marion National Forest provides a unique opportunity to examine the response of understory vegetation to long-term use of several combinations of season and frequency of burning. Several studies have examined the effects of single or repeated prescribed fires on understory vegetation (Abrahamson 1984; Conde and others 1983; Cushwa and others 1966, 1969; DeSelm and others 1974; Fox and Fox 1986; Gilliam and Christensen 1986; Grano 1970; Grelen 1975; Hodgkins 1958; Lemon 1949, 1967), but none of these studies was conducted over a period as long as the period of the SFP study. Prescribed burning in loblolly pine stands on the SFP was initiated in 1946 and continued without interruption until 1989, when the overstory pines were destroyed by Hurricane Hugo. Previous SFP studies focused on the effect of prescribed fire on understory vegetation (Langdon 1971, 1981; Lewis and Harshbarger 1976; Lotti 1955, 1956; Lotti and others 1960), benefits to wildlife (Lewis and Harshbarger 1976) and soil chemical changes (Wells 1971; McKee 1982). Waldrop and others (1987) summarized the effects of the various treatments on the growth of overstory pines after 40 years. Lewis and Harshbarger (1976) reported the effects of prescribed fire on shrub and herbaceous vegetation in the plots after 20 years. On the basis of information developed by Lewis and Harshbarger (1976), Langdon (1981), Waldrop and others (1987), and Waldrop and Lloyd (1991), the following generalizations can be made regarding the effects of long-term use of prescribed fire on understory vegetation in the SFP: (1) the unburned control plots were dominated by several size classes of shrub and hardwood species and contained only ‘Ecologist, Southeastern Forest Experiment Station, Clemson, SC; Research Forester, Southeastern Forest Experiment Station, Clemson, SC; and Assistant Professor, Department of Forest Resources, Clemson University, Clemson, SC. small numbers of grasses and virtually no forbs; (2) plots that were burned periodically contained two distinct size classes of understory hardwoods (> 15 cm and <5 cm d.b.h.) and herbaceous species, most of which were grasses; (3) annual winter and biennial summer bums resulted in large numbers of woody stems < 1 m taLl and many grasses and forbs; and (4) annual summer burning virtually eliminated understory woody vegetation, and produced an understory dominated by grasses and forbs. This paper describes differences among plant communities in the Santee Fire Plots after 43 years of prescribed burning. More specifically, we compare the understory plant communities in the context of plant species composition, species richness, and diversity. We also sought to determine whether there have been any changes in understory species composition since year 20 (1967).