Changes in plant species composition along an elevation gradient in an old-growth bottomland hardwood-Pinus taeda forest in southern Arkansas1

Journal of the Torrey Botanical Society, Jan-Mar 2005 by Grell, Adrian G, Shelton, Michael G, Heitzman, Eric

GRELL, A. G. (Arkansas Forest Resources Center, School of Forest Resources, University of Arkansas-Monticello, Monticello, AR 71656-3468), M. G. SHELTON (USDA Forest Service, Southern Research Station, Monticello, AR 71656-3516), AND E. HEITZMAN (Arkansas Forest Resources Center, School of Forest Resources, University of Arkansas-Monticello, Monticello, AR 71656-3468). Changes in plant species composition along an elevation gradient in an old-growth bottomland hardwood-Pi'mis taeda forest in southern Arkansas. J. Torrey Bot. Soc. 132: 72-89. 2005.-Old-growth bottomland hardwood-P inus taeda L. forests are rare in Arkansas, and the complex relationships between plant communities and environmental conditions have not been well described in these forests. To investigate these relationships, a digital elevation model was developed for a 16.2 ha old-growth bottomland hardwood-Pinus taeda forest in southern Arkansas. Overstory trees, saplings, seedlings, and herbaceous plants were analyzed in three 0.5 m elevation classes and by using indirect gradient analysis. Information was also collected on site factors (canopy cover, forest floor litter cover, and elevation), soil physical factors (bulk density and soil texture), soil moisture, and soil chemical factors (pH, electrical conductivity, organic matter, N, P, K, Ca, Mg, S, Fe, Mn, Zn, Cu, and Na). Importance values for 35% of seedling species, 30% of overstory species, 22% of herbaceous species, and 8% of sapling species differed significantly by elevation class. Significant differences by elevation in species diversity, richness, and evenness were identified in the seedling stratum, while only sapling evenness differed by elevation. Seventy-four percent of the environmental characteristics produced significant differences by elevation except for soil texture (sand, silt, and clay), September soil moisture content, Mn, and Cu. Seventy percent of environmental variables were significantly correlated with elevation. Dominant environmental influences on species composition in all strata included elevation as well as many other correlated variables such as Fe, forest floor litter cover, bulk density, and Na. Results from this study suggest that differences in vegetation were primarily the result of subtle elevational variations. Restoration or management of these forests should carefully consider microtopographical influences.

Key words: direct gradient analysis, forest ecosystems, indirect gradient analysis, vegetation-microtopography relationships, ordination.

Researchers have given considerable attention to the distribution of plant communities within bottomland hardwood forests (BHF), which are undergoing a rapid reduction in area in the southern United States. Most studies that characterize the distribution of BHF vegetation along elevation gradients typically involve only woody vegetation (Gemborys and Hodgkins 1971, Bell and del Moral 1977, Bell 1980, Huenneke and Sharitz 1986, Nixon et al. 1987, Titus 1990), or only non-woody vegetation (Barnes 1978, Menges and Waller 1983). However, few studies have examined the effects of elevation and other environmental variables on the distributions of both woody and non-woody plant communities (Wikum and Wali 1974, Hutchinson et al. 1999), and even fewer have investigated these relationships in BHF old-growth forests (Robertson et al. 1978).

In Arkansas, older and relatively undisturbed BHF communities are rare. Where they occur, they provide excellent opportunities to observe and study plant distributions. One such community is the "Lost Forty", a 16.2 ha oldgrowth mixed bottomland hardwood-Pinus taeda L. forest located in southern Arkansas that is characterized by distinct microtopographical features (e.g., ridges, flats, and sloughs). Moreover, the Lost Forty is home to a diverse array of woody and non-woody plant species providing excellent opportunities for studying various influences of environmental conditions on plant distributions. This study examines if and how plant communities at the Lost Forty are influenced by changes in environmental conditions. Since considerable interest exists in the conservation and restoration of BHF, results of this research should serve as a reference for other small and fragmented southern old-growth BHF. Our objectives were: 1) to create a digital elevation model and quantify environmental characteristics of the Lost Forty, and 2) to characterize woody and non-woody vegetative communities and their relationships to environmental conditions of an old-growth BHF.

Methods. STUDY SITR. The study site is the Lost Forty, a 16.2 ha forested tract located in Calhoun County, Arkansas in the West Gulf Coastal Plain ecoregion (Fig. 1). Uncertainty surrounds how the area received its name; possibilities include its inaccessibility, uncertainty over ownership, or the fact that it was never harvested. Regardless, a sequence of owners have valued the undisturbed tract as a unique natural area and refrained from commercially harvesting it. The Lost Forty is located at 33° 22' 58'' North and 92° 23' 49'' West. The forest's stand structure, species composition, and age distribution have been described by Heitzman et al. (2004). Overstory vegetation consists primarily of Pinus taeda, Liquidambar styraciflua L., Quercus spp., and Carya spp. Some trees reach as tall as 46 m and are more than 200 years old. The understory and midstory are mainly Ostrya virginiana (Miller) K. Koch, Carpinus caroliniana Walter, Ilex opaca Alton, and Nyssa sylvatica Marsh. Species most common in the seedling layer include Quercus nigra (L.), Callicarpa americana L., Carpinus caroliniana, Quercus phellos L., Ilex opaca, Symplocos tinctoria (L.) L'Her., and Quercus michauxii Nuttall.