Forest Stand Development Patterns in the Southern Appalachians

By Copenheaver, Carolyn A; Matthews, Jeff M; Showalter, Julia M; Auch, Walter E

Abstract –

Composition of southern Appalachian forests are influenced by disturbance and topography. This study examined six stands in southwestern Virginia. Within each stand, a 0.3-ha plot was established, and all trees and saplings were measured and aged. Burned stands had lower densities of saplings and small trees, but appeared to have greater Quercus regeneration. Ice damage from the 1994 ice storm was most evident in Pinus strobus saplings. A stand on old coal-mine slag appeared to be experiencing a slower rate of succession than other sites. A variety of stand development patterns were observed, but one common pattern was that oak-hickory overstories had different species in their understory, which may indicate future changes in species composition.

Introduction

In order to understand the past, present, and future composition of southern Appalachian forests, we must first understand the patterns and processes that drive succession (Clebsch and Busing 1989, Elliott et al. 1998, Phillips and Shure 1990). An understanding of disturbance as a factor in forested ecosystems and the identification of differing species’ responses can provide useful insights into the alteration of biological diversity on a community and landscape scale. Inherent in this type of understanding is an appreciation of the disturbances that determine the composition and structure of forested systems. Fire, ice storms, and human manipulation are the three primary disturbance agents in southern Appalachia. Fire and ice storms alter the rate and direction of forest succession (Abrams 1992, Boerner et al. 1988). Anthropogenic manipulation is driven by land-use objectives and includes agriculture, prescribed burning, fire suppression, logging, mining, and establishment of recreational areas (Leopold and Parker 1985, Richter et al. 2000). In addition to disturbance, site factors such as topography (slope position, slope, elevation, and aspect) influence the temperature, light, moisture, and soil, which subsequently affect vegetation recruitment and growth (Elliott et al. 1999b). A more complete appreciation of how disturbance and topography influence vegetation distribution in the southern Appalachians will allow us to better understand, predict, and model stand development spatially and temporally.

Three periods of fire activity have occurred in the southern Appalachians (Brose et al. 2001): infrequent low-intensity surface fires from pre-European settlement until the mid-1800s; high- intensity stand-replacing fires often associated with intensive logging and railroads from the mid-1800s to 1920; and an era of fire suppression after 1920 (Van Lear and Waldrop 1989). This pattern of fire history has allowed Quercus spp. (oaks) to dominate, creating even-aged oak-dominated hardwood stands. However, with the prolonged absence of fire during most of the twentieth century, oaks have been unable to establish in the shaded understory (Brose et al. 2001, Shuler and McClain 2003), and in their absence, many shade-tolerant and fire-intolerant species, such as Acer rubrum (red maple), Pinus strobus (eastern white pine), and Nyssa sylvatica (black gum) are succeeding. If fire suppression continues, oaks will decrease in dominance and shade-tolerant species will dominate in the Appalachians (Abrams 1992, Christensen 1977).

Direct mortality from ice damage in the southern Appalachians is rare. However, gaps created by ice damage can substantially alter forest-regeneration patterns and subsequent stand composition (Boerner et al. 1988, Rhoades 2002). The density of seedlings and sprouts, primarily shadeintolerant species such as Liriodendron tulipifera (yellow poplar), Pinus virginiana (Virginia pine), and white pine were significantly greater in icedamaged stands than stands unaffected by an ice storm (Rhoades 2002, Whitney and Johnson 1984). Although Warrillow and Mou (1999) did not find a difference in the density of seedlings found in ice-caused gaps compared to areas not damaged by the ice storm, the seedlings in gaps were taller than those under the intact canopy. Ice storms seem to have a greater impact on species composition on mesic sites than on xeric sites (Whitney and Johnson 1984). However, on mesic Virginia pine sites, extensive canopy damage enabled oak saplings to grow rapidly and accelerated successional transitions from pine to oak-dominated stands (Whitney and Johnson 1984).

Land-use history also influences species distribution in the southern Appalachian forests. The three most common historical land uses were agriculture, logging, and coal mining (Eller 1982). Areas that have been used for agriculture in the twentieth century and not experienced heavy erosion have increased fertility immediately after abandonment because of the effects of residual fertilizers (Richter et al. 2000). Conversely, agricultural sites that experienced substantial soil erosion have reduced soil fertility, lower plant community diversity, and slower tree growth (Lafon et al. 2000). Timber harvesting often results in changes in overstory species composition; e.g., successive clearcuts in southwestern North Carolina caused a shift from an oak-dominated forest to a mesic hardwood stand dominated by a yellow poplar and Betula lenta (black birch) overstory (Leopold and Parker 1985), and in Pennsylvania, oakdominated stands shifted to mesic hardwood stands dominated by red maple and Prunus serotina (black cherry) following logging (Abrams and Nowacki 1992, Ruffner and Arabas 2000). Areas that have been mined for coal have substantially slower rates of succession because the disturbance to the soil and bedrock often requires ecological restoration to make the land suitable for reforestation (Burger and Zipper 2002).

In the southern Appalachians, topography and soils also influence the distribution of tree species (Harrison et al. 1989, McEvoy et al. 1980), with terrain shape, elevation, and soil organic matter explaining 50% of the variation in species distribution in the Coweeta Basin, NC (Elliott et al. 1999). On Potts Mountain in southwestern Virginia, vegetation structure was strongly influenced by geomorphic characteristics. Xeric taxa, such as pine and ericaceous shrubs, dominated southwest slopes and high-elevation ridges, and mesic species, such as yellow poplar, Quercus alba (white oak), and black cherry, favored low-elevation sites and north/ northeast aspects (McEvoy et al. 1980). However, not all taxa distributions are determined by site characteristics; some tree species appear to be generalists. Elliott et al. (1999b) found that the distribution of red maple, Quercus prinus L. (chestnut oak), Oxydendrum arboreum (sourwood), and black gum showed no correlation to site characteristics. Harrison et al. (1989), in a study in Shenandoah National Park, found no correlation between site variables and chestnut oak or Quercus rubra (northern red oak); however, Quercus velutina (black oak) was correlated with east and southeast slopes.

Southern Appalachian forests are dynamic systems that have experienced substantial shifts in species composition and structure since European settlement. The objective of this paper was to reconstruct patterns of stand development in six stands in order to understand the importance of disturbance in creating successional patterns. These reconstructed stand histories provide an understanding of changes in forest composition which can be used to predict future forest-composition changes.

Field-Site Description

This study took place on the Fishburn Forest, a 486-ha mature mixed oak forest in southwestern Virginia. The forest is owned and managed by the Department of Forestry within Virginia Polytechnic Institute and State University (Virginia Tech) and is used for research, education, and timber harvests (Aust et al. 2003). Some of the land was previously owned by the Bessie Dunlap Coal Company, and several open-pit coal mines, which were abandoned in the 1930s, still exist at the base of Price Mountain. The most recent logging event in the areas where we worked was a high-grade in the 1960s (W.M. Aust, Forestry Department, Virginia Tech., Blacksburg, VA, pers. comm.). However, other sections of the Fishburn Forest have been harvested in the past few years as salvage from bark beetle infestations and as demonstration cuts for education and outreach.

The forest is within the Ridge and Valley physiographic province. The current overstory composition at the Fishburn Forest is 88% hardwood and 12% conifer. The dominant species include red maple, chestnut oak, white oak, and Quercus coccinea (scarlet oak) (Warrillow and Mou 1999). The soils are a mixture of Berks (Typic Dystrudepts) and Weikert (Lithic Dystrudepts) series with 25-65% slopes. These are very acidic, well-drained soils with low available- water holding capacity and low nutrient status. The Weikert soils (50% of the area) are shallow to bedrock (25-51 cm), and trees growing on this soil are prone to windthrow. By contrast, the Berks soils (30% of the area) are slightly deeper (51-102 cm), and trees on it are less susceptible to windthrow (Cregger et al. 1985). Susceptibility to windthrow combined with frequent ice storms has led to a high level of disturbance. The last major ic\e storm occurred in 1994 and deposited 2.5-2.9 cm of ice on all surfaces and caused extensive damage (Rhoades 2002), with ice damage heaviest on steeper slopes (Mou and Warrilow 2000).

Six mature stands representing the two topographic positions- ridge top and midslope-most common to oak-hickory forest were selected within the Fishburn Forest (Table 1). We selected stands with mature oak-hickory overstories that were at least 20 m from a road or trail and where harvesting had not occurred within the past 40 years. The disturbance history of the stands was selected because they reflected typical historical land uses (coal and logging) and/ or had been prescribed burned, an increasingly common management tool in this forest type (Elliott and Vose 2005, Elliot et al. 1999a, Hutchinson et al. 2005, Van Lear and Waldrop 1989). All stands sampled in this study had been influenced by the 1994 ice storm. The Viola stand (37 11’28”N, 8029’05”W) was sampled in the fall of 2001; the Moon Hollow stand (3711’41”N, 8028’38”W) was sampled in the fall of 2002; the Water Tower (3711’14”N, 8028’59” W) and Radio Tower (3711’10”N, 8028’56”W) stands were sampled in the fall of 2003; and the Stroubles Creek (3711’30”N, 8029’00”W) and Price Mountain (3711’11”N, 8028’49”W) stands were sampled in Spring 2004.

Methods

Within each stand, a 0.3-ha plot was established and the diameter of all trees and saplings was measured at breast height. Increment cores or disks were removed from all trees and saplings at 0.5 m above the root collar. The trees (> 10-cm dbh) were classified as overstory, and the saplings (< 10-cm dbh, but taller than 1.5 m) were classified as understory. All disks were sanded on a belt sander with a 100-grit belt, and cores and disks were hand sanded with increasingly finer sandpaper until the cellular structure was visible under a microscope. The cores and disks from each stand were cross dated by species using the narrow rings as reference years (Yamaguchi 1991), which allowed an accurate determination of the year of origin. For cores where the pith was missed, a graphical technique was used to estimate the date of the pith (Villalba and Veblen 1997). During the cross-dating process, it was noted that many of the trees had an unusual amount of compression or reaction wood formed during 1994, 1995, and 1996. It was concluded that this was a result of the tree's response to the 1994 ice storm, and trees with visible reaction wood or compression wood were noted.

Within each plot, two bucket-auger holes were sampled to measure the depth of the A-horizon and depth to bedrock. Soil from the B- horizon was hand-textured with a number of tests such as the ribbon test, the shine test, and ball formation (Brady 1990, USDA Soil Conservation Service 1971). Slope and aspect were measured at several locations within each plot.

For each stand, importance values for all species were calculated by averaging the relative basal area and the relative density (# trees/ha). The importance-value calculations included both trees and saplings.

Results

Stand descriptions

At the Viola stand, trees and saplings with the highest importance values were white pine (23%), red maple (14%), red oak (12%), and scarlet oak (11%) (Table 2). The basal area for the stand was 13.9 m^sup 2^/ha. The overstory (trees greater than 10 cm in diameter) and understory (saplings with diameters of 1 to 10 cm) had different dominant species, with the overstory dominated by oaks with Virginia pine, red maple, and black gum as secondary species (Fig. 1). Most of the canopy dominants established from the 1890s to the 1940s. The understory was dominated by eastern white pine, which mostly established from the 1960s to the present. The peak period of eastern white pine establishment was in the 1970s. The increase in eastern white pine regeneration occurred concurrently with a decrease in successful establishment of other tree species. Oaks and Virginia pine showed the greatest decline in understory dominance, with no successful oak or Virginia pine establishment since the 1970s. Red maple has had a steady presence throughout the stand’s development. The age and diameter distributions indicate an unevenaged stand, although regeneration in the 1950s was less successful than other time periods. The Viola stand was growing on an abandoned coal slag heap with a very shallow O-horizon and no A- horizon (Table 3).

The most important tree and sapling species in the Moon Hollow stand were white pine (43%) and red maple (16%) (Table 2). These high importance values resulted from high densities rather than high basal area (white pine had 291 saplings/ha and 300 overstory trees/ ha, while red maple had 180 saplings/ha and 117 overstory trees/ ha). The basal area for the stand was 23.7 m^sup 2^/ha. The overstory was dominated by oaks, red maple, and black gum (Fig. 2). The understory was predominately eastern white pine, most of which established after 1960. Contrasting with the recent successful establishment of eastern white pine, oak establishment had decreased considerably in the last 30 years. Establishment of red maple and black gum remained constant, and the two species were minor species in the canopy and understory. Fagus grandifolia (American beech) established mostly in the 1960s. The age distribution most closely fits a single-age stand, the largest cohort established in the 1960s and 1970s, with several residual, older trees established before 1960 (Fig. 2). However, the diameter distribution is the typical reverse-J diameter distribution associated with uneven-aged stands (Smith 1986). The Moon Hollow stand was a xeric site with a southwest aspect, high rock-fragment content in the B-horizon, and the shallowest A-horizon (Table 3).

In the Stroubles Creek stand, tree and sapling species with the highest importance values were white pine (34%), red maple (12%), white oak (12%), and chestnut oak (10%) (Table 2). The basal area for the stand was 20.3 m^sup 2^/ha. Oaks and red maple dominated the overstory while eastern white pine with a moderate presence of red maple, oak, Tsuga canadensis (hemlock), and black gum dominated the understory (Fig. 3). The overstory established between 1920-1950, and some older individuals (especially white oaks) originated in the late 1800s. The eastern white pine in the understory originated in the late 1960s and early 1970s. This stand experienced moderately successful oak recruitment into the 1990s. The stand’s diameter distribution follows a reverse-J shape with a greater proportion of small-diameter individuals. This stand was on a steep northern aspect, yet had the deepest A-horizon of any of the stands (Tables 1 and 3).

The most important tree and sapling species at the Water Tower stand were chestnut oak (28%), white oak (20%), sourwood (20%), and red maple (12%) (Table 2). The basal area for the stand was 22.4 mVha. The overstory consisted predominately of oaks established prior to the 1930s (Fig. 4). The stand was uneven-aged, with a lower rate of successful regeneration occurring in the 1950s and 1960s than in other decades. The understory was comprised of sourwood, black gum, and maple. Since the 1970s, sourwood dominated the successful regeneration at the site and makes up 47% of the stand’s saplings. Successful oak regeneration peaked in the 1930s and has declined ever since. Carya spp. (hickories) have not successfully regenerated on the site since 1950. Also since 1950, a fairly low level of red maple recruitment has occurred. The overall lack of significant oak regeneration has been replaced by the successful regeneration of species with greater shade tolerance such as black gum and sourwood.

The trees and saplings with the highest importance values at the Radio Tower stand were chestnut oak (20%), white oak (15%), red maple (13%), and black oak (12%) (Table 2). The basal area for the stand was 12.8 m^sup 2^/ ha. The overstory (Fig. 5) was dominated by white oak, chestnut oak, black oak, Carya glabra (pignut hickory), and Carya tomentosa (mockernut hickory). The understory was dominated by red maple, sourwood, and pignut hickory. The presence of a few shade-intolerant Pinus rigida (pitch pines) and Virginia pines in the overstory indicate that this area was fairly open when stand initiation began. The stand had a controlled burn in 2002, which disproportionately killed smaller trees. The 1960s was a decade of particularly low successful tree establishment and also marked a decrease in successful oak regeneration.

The most important tree and sapling species at the Price Mountain stand were red maple (24%), black gum (24%), and chestnut oak (19%) (Table 2). The basal area for the stand was 16.7 m^sup 2^/ha. The overstory was dominated by oaks and black gum (Fig. 6). In recent years, the number of black gum stems successfully establishing has decreased slightly. After 1940, there was very little successful oak establishment. Red maple appears to have replaced oak, and it constituted a major proportion of the trees established post-1960. Eastern white pine, sourwood, and hickory were minor species in both the overstory and the understory, with no major changes in establishment rate across time. In contrast with an increase in establishment during the 1920s and 1930s, the 1950s represented a decade of unprecedented low regeneration. There were a number of disks with fire scars that dated to two years: 1986 and 1972. The site had a surprisingly deep soil for a ridge site, with an average depth to bedrock of 64 cm (Table 3).

Discussion

Patterns of regeneration and vegetation distribution

We identified a period of low tree recruitment in the 1950s, which may have coincided with the droughts of the 1950s and 1960s experienced in the eastern United States. Reductions in radial growth of overstory trees during the 1950s and 1960s have been attributed to these droughts (Jenkins an\d Pallardy 1995, McClenahen and Dochinger 1985, Orwig and Abrams 1997, Rubino and McCarthy 2000), but this study indicates an equally important influence on regeneration patterns. The drought stress appears to have affected ridge and mid-slope sites equally, thus providing a contrast to physiological results from oaks that demonstrated more drought stress at mesic sites than xeric sites (Abrams 1994). Although the lack of regeneration could have been attributed to other factors, such as grazing or stand-development patterns, grazing was never a dominant land use at the Fishburn Forest, and the age structure of the stand does not indicate that the stands would have been in a period of stem exclusion.

Although the relationship between species and topographic position was confounded by disturbance history, our study indicated that eastern white pine was a specialist species found mostly at the mid-slope sites (Viola, Moon Hollow, and Stroubles Creek), and chestnut oak, although present at the midslope sites, was much more dominant at the ridge-top sites (Water Tower, Radio Tower, and Price Mt.). Red maple, identified as a generalist in other southern Appalachian stands (Elliott et al. 1999b), functioned as a generalist in our stands and occurred at all sites regardless of topographic position.

Influence of controlled burns on regeneration patterns

The prescribed fires conducted at the Radio Tower and Water Tower stands were low-intensity, surface fires. Although other studies in the southern Appalachians identified a substantial decrease in understory density in burned stands (Barden and Woods 1976, Kuddes- Fischer and Arthur 2002, Wendel and Smith 1986) and an increase in red maple and other mesic hardwood species following fire (McGee 1980), we did not identify a peak in post-fire recruitment of mesic hardwoods at either of the burned stands. Perhaps we sampled too soon after the fire to witness this pulse in recruitment. However, we did find that oak regeneration constituted a higher proportion of the total successful post-fire regeneration in the burned stands. The success of oak regeneration following fire has been modeled in several ecosystems (Abrams 1992, Elliott et al. 1999a, Kirwan and Shugart 2000).

Patterns of eastern white pine recruitment

Although eastern white pine has been identified as a recent invader of oak stands following fire suppression at ridge-top sites in the southern Appalachians (Abella and Shelburne 2003, Blankenship and Arthur 1999), we did not find evidence for this pattern of eastern white pine invasion at Price Mountain. The relationship between eastern white pine and fire may vary across its range. For example, in the Lake States and New England, eastern white pine has been shown to survive low-intensity fires (Beverly and Martell 2003, Engelmark et al. 2000) but be excluded from areas with high- intensity fires (Engelmark et al. 2000). At the Fishburn Forest, eastern white pine invasion appears to be a function of stand age and slope position rather than fire history because recruitment of eastern white pine was higher on the younger, mid-slope stands (Fig. 1, 2, and 3; Table 2) but absent on the older ridge site (Fig. 6). Higher rates of eastern white pine establishment in younger, more open-canopy stands has been identified in other forest types as well (Lutz and McComb 1935, Sharik et al. 1989).

Influence of ice storms on stand development

Previous studies have found that ice storms in the southern Appalachians have a substantial effect on stand density, regeneration patterns, and understory species composition (Bragg et al. 2003, Rebertus et al. 1997, Rhoades 2002). In this study, the species with the strongest tree-ring record of the ice storm (as indicated by the compression wood formed in the years following the ice storm) was white pine. The white pine was most common at the mid- slope sites (Table 1), indicating that the mid-slope sites may also have been substantially impacted by the ice storm. Most other studies have found ridge sites to be more susceptible to ice-storm damage because of their greater exposure (Lafon et al. 1999, Millward and Kraft 2004, Warrillow and Mou 1999). One of the common consequences of ice damage is the creation of canopy gaps from branch damage to overstory trees, which allows higher rates of sapling and shrub establishment (Rebertus et al. 1997). However, the presence of compression wood in the white pine implies that these trees were bent during the ice storm rather than broken. The compression wood would have formed in the subsequent years as the trees began to straighten themselves. Although Rhoades (2002) attributed the increase in eastern white pine in the understory at the Fishburn Forest to the 1994 ice storm, when we dated these eastern white pine saplings, we found that most recruited in the 1960s, with no pulses in recruitment following the storm and therefore, the results of this project agree with Mou and Warrillow’s (2000) study that found minimal change in the species composition of understory recruitment as a result of ice damage.

Land-use history

The Viola stand, which was located on a coal-mine slag heap, had the second-lowest basal area (13.9 m^sup 2^/ha) for all of the stands and appeared to be at an earlier stage in stand development, as indicated by the dominance of such early successional species such as Virginia pine and pitch pine, compared to the other studies. It may be that the poor soils from the coal mining resulted in a slower rate of stand development (Holl 2002).

One consistent pattern at all six stands was that current oak- hickory stands all had different species in their understories. Thus, future development within these stands may include a shift in overstory species composition, and it appears that the influence of disturbances, such as fire and land-use history, may have the potential to alter the understory composition and thus determine the composition of future stands.

Acknowledgments

We greatly appreciate the excellent field assistance of Noah Adams, Christine Shook, Kris Jenson, Judd Pittman, Mary Yonce, Cristina Issem, and the students in Forest Ecology and Silvics (FOR 3314). We thank two anonymous reviewers for their comments on an earlier draft of this paper. The project was supported by grants from the Center for Excellence in Undergraduate Teaching, the Forestry Department at Virginia Tech, and the USDA Forest Service Southern Research Station through agreement No. SRS 05-CA-11330134- 251.

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Carolyn A. Copenheaver1,*, Jeff M. Matthews1, Julia M. Showalter1, and Walter E. Auch1

1 Department of Forestry, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061. * Corresponding author – ccopenhe@vt.edu.

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