Species Diversity in Forests established before and after 1950 in a Southern Appalachian Watershed

Scott M. Pearson¹, Kristy Pyatte¹, Alan B. Smith¹, Monica G. Turner², and Paul B. Bolstad³

¹ Biology Department, Mars Hill College, Mars Hill NC
² Department of Zoology, University of Wisconsin, Madison WI
³ Forest Resources Department, University of Minnesota, St. Paul MN

Land use is the leading cause of landscape change. Both worldwide and in the United States, land cover is altered principally by direct human use--through agriculture, pasture, forestry, and development (Meyer and Turner 1992). Land use patterns influence biodiversity for several reasons (Turner et al. 1998). First, land-use activities may alter the relative abundances of natural habitats and result in the establishment of new land-cover types. Species richness may be enhanced by the addition of new cover types, but natural habitats are often reduced, leaving less area available for native species (Walker 1992). Exotic species may become established and outcompete the native biota. Second, the spatial pattern of habitats may be altered, often resulting in fragmentation of once-continuous habitat. Species using a particular habitat may respond individualistically to these changes. Species that differ in their patterns of survival, fecundity, and dispersal, as well as in habitat needs, will likely differ in their response to habitat loss and fragmentation (Pearson et al. 1996). Species having specialized habitat needs, requiring a large area for home range, or having limited vagility, will be impacted more than generalist species that disperse well and can live in small isolated patches (Terborgh 1992, Dale et al. 1994, Matlack 1994).

The Blue Ridge Mountains of western North Carolina host a great deal of biodiversity. Spatial patterns of diversity are influenced both by natural patterns of heterogeneity produced by topographic variation and by historical patterns of land use. This landscape currently has a high percentage of forest cover (approximately 70% forested, SAMAB 1996). The overall trend in land use for this area is a decrease in agriculture, an increase in forest cover and suburban cover types. Wear and Bolstad (1998) have documented land cover change in this region by comparing maps representing land-cover at periods between 1950 and 1990. They documented the unlikely combination of increasing human population size accompanied by increasing forest cover since 1950. The present-day landscape consists of a mosaic of forest patches interspersed with agricultural and suburban land uses (Figure 1). The spatial pattern of forest cover in the present-day landscape is correlated with species diversity of forest patches (Pearson et al. 1998). Moreover, there is a great deal of heterogeneity within these forests due to differing land-use histories. The effects of land-use history and landscape patterns needs to be better understood.

Our research sought to document differences in diversity and community structure in forest stands of two age classes: established before and established after 1950.

Specifically,

How do pre-1950 and post-1950 forests compare with respect to:

1. the species diversity and percent coverage of forest herbs?
2. the species diversity and size of trees?

Paired study plots were selected using maps such as shown in Figure 1. Each pair of plots had one member in a pre-1950 forest stand and one member in a post 1950 forest stand. Plots were located as to minimize topographic and edaphic differences between the two members of the pair. Twenty-four pairs of plots were sampled in June 1996.

Study plots were 0.10 ha in area. The diversity and abundance of forest herb and shrub species were recorded in nine 0.5 m² quadrats. The percent coverage (of the ground) was visually estimated for each species in each quadrat. All trees occurring in a 20 x 20 m area within the plot were recorded. Diameter at breast height (dbh) was recorded for individuals >10 cm dbh.

Species richness and total coverage for all herb species combined was compared for the two forest ages. Herb species were divided into three groups (see Table 1) to determine if the diversity and abundance of species with differing habitat requirements and life histories varied between the two forest ages.

For trees, species richness and size distributions were compared for the two forest ages. Tree species were grouped into two categories that reflect successional trends (see Table 2).

Herbs
There was considerable overlap between the vegetation communities of the two types of forest. Almost 60% of the species recorded were common to forest of both age classes (Figure 2). There were several genera that were found only in one of the age classes.

Pre-1950 forest stands had higher herb species richness for all species combined, generalist species, and weed species (Table 2, Figure 3a). There were no differences in the percent coverage for all species combined and generalists between the two stand ages (Figure 3b). Weeds and dispersal-limited species did differ among the two forest ages. Weeds had higher species richness and coverage in the younger forests. Dispersal-limited species had higher species richness and coverage in older forests.

Herb diversity was correlated with tree diversity and size (Tables 3-4). After accounting for forest age, weeds were less abundant in stands with more tree species. Dispersal-limited species were more abundant in stands with larger trees.

Trees
Although the younger forests tended to have more tree species, there was no significant difference between the number of tree species in pre- and post-1950 forest stands (Figure 4). Pre-1950 stands had a higher abundance of late-successional tree species. These species attained larger sizes in these older forests (Figure 5a,b). Post-1950 stands had a greater abundance and greater total dbh of early-successional species. These stands had lower abundance and lower total dbh of late-successional species (Figure 5a,b).

Landscape Context
These results clearly reflect successional trends. However, the differences between pre- and post-1950 forests depended strongly on the edaphic characteristics and landscape context of specific sites (Table 3). Post-1950 stands in landscapes with a larger percentage of pre-1950 forest had higher species richness (Figure 6). Young stands surrounded by older forest had more generalist and late-successional herbs than stands surrounded by more recovering forest.

Conclusions

• Forest stands established after 1950 (young stands) had higher herbaceous species richness than stands established before 1950 (old stands). These results were due to increased abundance of generalist and weed species in young stands.
• Younger forests had lower richness and abundance of dispersal-limited species characteristic of older forests.
• Herb species diversity was correlated with tree size and diversity.
• Landscape context affected species richness of post-1950 forests. Young stands surrounded by older forests had higher species richness due to increased rates of colonization from older stands.

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