Composition and Structure of Two Old-growth Forest Ecosystem Types of Southeastern Ohio

Less than 1% of the pre-European settlement forest in Ohio currently remains, mostly as small and scattered woodlots. Consequently, few studies have been undertaken to quantify the composition and structure of Ohio’s old-growth forests using a landscape ecosystem perspective. We used an existing multifactor ecosystem classification system developed for the Wayne National Forest in southeastern Ohio to compare the composition and structure of two old-growth forest ecosystem types, located on contrasting north-facing and south-facing middle slopes. No differences in physiography were observed among the stands other than aspect; however, the north-facing old-growth ecosystem type had a greater A horizon thickness and a higher pH than the south-facing old-growth ecosystem type. Mixed-oaks dominate the south-facing ecosystem type, while sugar maple, American beech and northern red oak dominate the north-facing ecosystem type. No differences were detected in stand structural components. Similar trends were observed for the ground-flora layer; specifically, we observed differences in groundflora composition between the two ecosystem types but no differences in total percent cover or species richness. Finally, the composition and structure of coarse woody debris differed between the contrasting ecosystem types. Maple and oak snags and fallen logs dominate the north-facing ecosystem while oak standing snags and fallen stems are typically observed in the south-facing ecosystem. Few differences between the two ecosystem types were detected in coarse woody debris structure, except that snag density tends to be higher in the south-facing old-growth ecosystem and log density and volume tends to be higher in the north-facing ecosystem (P <0.10). Through the use of this ecosystem approach, we can begin to quantify the ecological factors regulating the composition and structure of old-growth communities, improving our ability to effectively manage and restore these rare ecosystems. OHIO J SCI 105 (2):8–16, 2005 Manuscript received 19 May 2003 and in revised form 2 October 2003 (#03-11). INTRODUCTION Although humans and forest ecosystems often interact in complex and synergistic ways, individual old-growth stands or forests typically represent an undisturbed condition where the influence of geomorphology, soils, and natural disturbances, in conjunction with plant reproductive processes and animals, constrain the development of plant communities (Rowe and Sheard 1981; Pregitzer and others 2001). Old-growth forests are generally considered to represent the final, stable phase of stand development and typically are recognized by the unique structural characteristics they share. For example, eastern old-growth forests are usually described as multi-aged stands with multiple structural layers, large amounts of coarse woody debris (both dead snags and fallen logs), undisturbed soils, and a diverse array of both plants and animals (Parker 1989; Leverett 1996). Ecosystem processes, including nutrient cycling, stability, and biodiversity, are also believed to remain undisturbed in old-growth forests (Leverett 1996; Meier and others 1996). In Ohio, as well as across the Central Hardwoods Region, the remaining isolated old-growth tracts have been the focus of old-growth preservation and recovery programs (Trombulak 1996). These remnant and isolated woodlots may be seen as analogous to museum archives, revealing little about the overall landscape or interactions among forest ecosystems at the time of European settlement. Additionally, many of these remnant old-growth stands are in transition. Land-use practices in the surrounding landscape, such as fire suppression, are resulting in compositional and structural changes in these old-growth forests (Goebel and Hix 1996, 1997). Because the composition and structure of individual old-growth stands is influenced strongly by the dispersal patterns of individual species, site history, and environmental factors, the focus of old-growth preservation must occur at the ecosystem level and focus on preserving the ‘natural’ processes of old-growth forests (Barnes 1989; Pickett and Parker 1994; Trombulak 1996). Ecosystem classification is a useful tool that facilitates the understanding of interrelationships among plant communities and the environment and how these factors influence ecosystem restoration decisions (Palik and others 2000). Ecosystem classifications define ecosystems hierarchically, as volumes of earth, air, and water with specific developmental histories in which plants and animals live and interact (Rowe and Barnes 1994; Barnes and others 1998). In Ohio, there has been some research published concerning the composition and structure of particular old-growth tracts (for example, McCarthy and others 1987; Cho and Boerner 1991; McCarthy and others 2001). However, very little is known about the compositional and structural variation among Ohio’s oldgrowth forest ecosystems in relation to the hierarchical OHIO JOURNAL OF SCIENCE 9 P. C. GOEBEL AND OTHERS factors regulating their composition and structure, especially physiography and soils. By applying the ecosystem classifications developed for the Wayne National Forest (Hix and Pearcy 1997; Hix and others 1997), old-growth conditions of individual forest ecosystems of southeastern Ohio can be described and compared, ultimately leading to improved programs to manage and restore these threatened ecosystems. Using the ecosystem classification developed for the Athens Unit of the Wayne National Forest as a framework, in this paper we: 1) examine the physiographic and edaphic factors that regulate overstory and ground-flora vegetation of two old-growth forest ecosystems in southeastern Ohio; and 2) examine the physiographic constraints on coarse woody debris (CWD) composition and structure between the two old-growth forest ecosystems. MATERIALS AND METHODS Study Area The study area is located in the Western Hocking Plateau Subsection (221Ef) of the Southern Unglaciated Allegheny Plateau Section (221E) in the Eastern Broadleaf Forest Province (Keys and others 1995). The Subsection is described as a maturely dissected plateau with moderate to steep slopes, narrow ridgetops, rock outcrops, and narrow stream valleys with elevations ranging from 195 to 322 m above sea level. Geology of the study area consists of inter-bedded sedimentary bedrock of shale, siltstone, limestone, and coal that was laid down in the shallow seas of the Mississippian, Pennsylvanian, or Permian periods in an anticline that dips eastward to the Appalachian Geosyncline (Rypma 1961; Keys and others 1995). In general, the soils are moderately acidic with surface layers that are moderately drained to well-drained loams or silt loams, and with subsoils comprised of silty clays, loamy clays, or clays. The climate of the area is humid continental with a mean annual temperature of 9° C (Lucht and others 1985). Winters are relatively cold, while summers are generally warm with a mean July maximum temperature of 32.2° C and a mean January minimum temperature of 6.9° C (Athens weather station; Lucht and others 1985). Average annual precipitation is 98 cm, half of which falls from May to October (Lucht and others 1985). The topographic variability associated with the study area is responsible for significant differences in microclimate, which are common. A ridge system oriented from northwest to southeast occurs over most of the study area. This results in southerly-facing slopes that receive higher levels of solar radiation and, consequently, have higher air and soil temperatures, lower relative humidity, and lower soil moisture than their northerly-facing counterparts. Field Methods Eight old-growth stands (defined as stands >150 year old; see Goebel and Hix 1996; Olivero and Hix 1998 for information on how these stands were identified) were selected within two contrasting ecosystems using a multifactor ecological classification system (ECS) based on climate, physiography, soils, and vegetation developed recently for the Athens Unit of the Wayne National Forest in southeastern Ohio (Table 1). These included: 1) northfacing mesic slopes (ELTP 42 – mesic middle slopes), and 2) south-facing dry slopes (ELTP 32 – dry upper to middle slopes). Two sample plots were then established randomly on a transect that roughly bisected the stand along the contour. The first plot was located randomly 20 to 30 m from the boundary, and the second plot was installed randomly at least 40 to 50 m from the first plot. Each sample plot consisted of a circular 500-m plot and eight rectangular 1.0 m × 2.0 m quadrats. The centers of the quadrats were located 7.0 m from the center of the 500-m plots in eight directions (N, NE, E, SE, S, SW, W, NW). At the center of each plot the following physiographic features were observed or measured: aspect (azimuth in degrees), slope steepness (%), slope shape (concave, linear, or convex), length of slope, distance to nearest surface water, and the distance to the ridgetop. The percentage of the distance to the ridgetop (PDR) was calculated by dividing the distance to the ridgetop by the total length of the slope. The elevation of each plot was determined from a topographic map. Surface soil characteristics were also measured on each plot. Thickness and texture (determined by feel in the field) of the A horizon was estimated by averaging eight push-tube samples randomly located across the plot. Push-tube samples for each plot were placed in sample bags and pH of the A horizon determined in the lab using the calcium chloride method (McLean 1982). On each 500-m plot, the species, dbh (diameter at breast height; 1.37 m), and crown class (dominant, codominant, intermediate, and overtopped; compare Smith 1986) of all living overstory trees >10.0 cm dbh was recorded. Dead snags >10.0 cm dbh were also tallied by species and dbh on each 500-m plot. Heights of the snags to the nearest meter were recorded using a clinometer. Data on the fallen trees >10.0 cm mid-diameter included species and length. Although not all snags and fallen trees were determinable to species, it was possible to determine the genus of each snag and fallen tree. Ground-flora vegetation (vascular plants <1 m tall, including pteridophytes, graminoids, forbs, woody vines, and shrubs) was sampled in each of the eight 1.0 × 2.0 m quadrats on each plot. Percent coverage was estimated visually for each ground-flora species in a quadrat using the following cover class codes: 1, <1%; 2, 1-5%; 3, 610%; 4, 11-20%; 5, 21-40%; 6, 41-70%; 7, 71-100%. Data Analyses Importance values (IV) were calculated for overstory trees as the summation of relative density and relative dominance (as expressed by basal area) divided by 2. Mean cover for each ground-flora species by plot was calculated by averaging cover class values from the eight quadrats. Mean diameter, height (m), density (stems/ha), basal area (m/ha) and volume (m/ha) of each standing dead species (snags) were computed for each plot. Similarly, the average mid-diameter, density, and volume of fallen dead stems (CWD) were also calculated. Canonical correspondence analysis (CCA) was used to 10 VOL. 105 SOUTHEASTERN OHIO OLD-GROWTH FORESTS