Changes in vegetation structure and composition in response to fuel reduction treatments in the South Carolina Piedmont

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Abstract

Fuel reduction treatments are helpful to restore ecosystem structure and function to forests that historically sustained frequent, low-intensity fires. But the impacts of these treatments on Piedmont forests are not well understood. We examined the effects of prescribed burning, thinning, and a combination of burning and thinning on community composition of Pinus taeda/Pinus echinata forests in the South Carolina Piedmont to identify changes in community structure and species composition. Overstory basal area was reduced across all treatments. The combination of thinning and burning resulted in a substantial increase in sapling density, whereas the burn-only decreased slightly after 3 years. Seedling density for all tree species increased across all treatment units during the same time span. In addition, cover of grasses and forbs increased in the burn-only and thin + burn treatments. Treatments appeared to affect understory life forms differently with the burn-only treatment encouraging forb cover while the thin + burn treatment promoted shrub and graminoid abundance. Non-metric multidimensional scaling (NMS) indicated rapid changes in understory composition for the burn-only and thin + burn treatments, whereas the thin-only treatment showed a more gradual shift over time.

Introduction

The southeastern Piedmont is a unique transitional region between the pine forests of the Coastal Plain and the hardwood forests of the Appalachian Mountains. In the past, fire functioned as an integral part of ecosystem development in the southern United States as Native Americans used burning to hunt, maintain prairies and grasslands, improve wildlife habitat, and clear land for agriculture (Van Lear and Waldrop, 1989, Williams, 1989, Silver, 1990, DeVivo, 1991, Carroll et al., 2002). The forests were characterized by an open, herbaceous understory with widely spaced trees and fire was observed throughout the landscape (Silver, 1990). Fire continued to be used as a tool for land management up until the 1900s when the federal government enacted the Clarke–McNary Act, which encouraged fire suppression by providing funding for such activities (Williams, 1989, Wade et al., 2000). Although prescribed fire was touted as a method to help reduce fuels in southern forests following major wildfires in the 1930s and 1950s (Stanturf et al., 2002), most forest land in the southeastern Piedmont is owned by non-industrial private landowners (Bechtold and Ruark, 1988) and remains unmanaged. Relatively little information is available on the effects of prescribed fire and fuel reduction treatments on vegetation in the southeastern Piedmont region.

Disruption of the fire cycle as well as other factors (e.g., farming and subsequent land abandonment, timber harvesting) has led to forests with less spatial heterogeneity, greater stem densities, and therefore, increased fuel loads. In South Carolina approximately 5000–6000 fires occur each year burning an average of 12,000 ha (South Carolina Forestry Commission, http://www.state.sc.us/forest/fire.htm). Since 1970, catastrophic wildfires (those over 400 ha) have occurred at the rate of one per year in South Carolina. Because of the high degree of urban/wildland interface in the region, fires of this size usually destroy homes, businesses, or other private property.

Fire-sensitive, shade-tolerant species are becoming established changing community composition (Halls and Homesley, 1966, Cowell, 1998) and altering nutrient cycling and decomposition rates in addition to other ecosystem functions (Lockaby et al., 1995). Fuel reduction techniques have been proposed to help restore stand structure and function to forests that traditionally experienced low-intensity fires with short return intervals (Mutch, 1994, Covington, 1995, Moore et al., 1999). Previous studies have documented the effects of silvicultural treatments on different vegetative components of Pinus taeda and Pinus echinata communities, but none have examined the effects of several different fuel reduction treatments on the entire vegetation community over time.

This work is a part of the National Fire and Fire Surrogate (FFS) study (http://www.fs.fed.us/ffs), which is evaluating the long-term effects of fuel reduction treatments (prescribed burning, mechanical treatment, prescribed burning + mechanical) on ecosystem structure and function at 13 sites across the U.S. (Weatherspoon, 2000). The study sites represent a variety of forest types that historically sustained frequent low-intensity surface fires which currently have greater stem densities and increased fuel loading as a result of long-term fire suppression. At the Southeastern Piedmont site the primary goals were to re-establish stand structure and composition characteristic of fire-adapted communities. We compared the effectiveness of dormant-season prescribed burns, thinning from below, and the combination of thinning and prescribed burning to achieve these goals while supporting stand management for timber production, wildlife habitat, and recreation.

We hypothesized that the combination of thinning and burning would have the greatest impact on stand structure and composition by reducing basal area and density for the overstory and midstory, and thus elicit the greatest increase in understory abundance. Intermediate levels of stand change and understory response were expected for the burn-only and thin-only treatments.

Section snippets

Study site

The study site is located on the Clemson Experimental Forest in Anderson, Oconee, and Pickens Counties, South Carolina. The forest, essentially reclaimed farm land, supported subsistence agriculture until the 1930s, which greatly reduced the land's productivity, as most of the topsoil was removed. Reforestation programs begun during the Great Depression and harvesting since that time have resulted in second- or third-growth timber on most of the forest. The dominant forest type is P. taeda and

Overstory

Overstory basal area (all P-values <0.0010) and stem densities (all P-values <0.0001) decreased across all treatments with the largest reductions occurring in thin + burn (Fig. 2). The thin-only treatment was not as intense as anticipated as harvesting only reduced basal area to 21.3 m2/ha (1st year post-treatment), whereas the burn-only and thin + burn treatments achieved our target residual basal area with 18.6 and 16.7 m2/ha (as measured immediately following treatment), respectively. Large

Discussion

Fuel reduction techniques used in this study significantly changed stand structure and composition across all treatments. The thin-only treatment reduced overstory density and basal area, but stimulated hardwood sprouting, resulting in a greater number of saplings. Changes in the understory were evident as seedling regeneration was greater and abundance of shrubs, graminoids, and forbs increased, but these results were delayed until the 3rd year after treatment. Incidence of southern pine

Acknowledgements

This paper is contribution number 161 of the national Fire and Fire Surrogate study. Funding for this research was provided by the Joint Fire Science Program. The authors thank Todd Hutchinson and Barry Clinton for their reviews and comments to improve the quality of this manuscript. We also express our appreciation to those who helped with data collection especially Gregg Chapman, Chuck Flint, Helen Mohr, in addition to the numerous field technicians. This work would not have been possible

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