Evidence of Plant Succession on Granite Outcrops of the Georgia Piedmont

Abstract

Successional changes are taking place in the vegetation of certain on granitic rock outcrops in the Lithonia, Georgia, area (Piedmont Plateau). The floristic composition and maximum soil depths of 34 communities, first studied in 1957, were re-examined in 1968 and 1976-1978. Maximum soil depth tended to increase over time. Sixteen of the 34 communities studied showed successional changes in floristic composition, confirming the earlier hypothesis that lichen-annual, annual-perennial and herb-shrub communities represent seral stages as soil depth increases. There are characteristic plants and maximum soil depths for each stage, with overlapping depth ranges. Diamorpha communities, which occur on the shallowest soils, have remained unchanged floristically and maximum soil depths have changed only slightly. With 1-12 cm increases in soil depths, three lichen-annual communities became annual-perennial communities, four annual-perennial communities progressed to an herb-shrub or shrub-tree stage, and one herb-shrub community became a shrubtree community. Shrub-tree communities may represent an edaphic climax subject to periodic regression to earlier seral stages. INTRODUCTION Plant communities on exposed granitic rock to the E and SE of Atlanta, Georgia, and the life histories of characteristic species of these communities have been the subject of scientific study for many years. McVaugh (1943) included the portion of the Piedmont near Atlanta in his comprehensive study of the vegetation of granitic flatrocks in the southeastern United States and suggested two types of succession on rock surfaces, mat formation and succession in soil-filled depressions in the exposed rock. In 1964, Burbanck and Platt published an analysis of the vegetation of 40 communities of plants located on three geologically similar granite outcrops. The communities were termed since each occurred on soil in depressions surrounded by exposed rock. These communities were classified into four types based on maximum soil depth and type of vegetation in each community. Burbanck and Platt suggested that the four types of communities represented seral stages related to the edaphic conditions; the greater the soil depth, the more advanced was the seral stage. Since the original observations were made in 1957, 1958 and 1959, the communities have been examined periodically and data collected pertaining to depth of soil and floristic changes to determine whether or not plant succession is taking place and whether the four types of communities represent seral stages. This paper presents some of the results of a 22-year study of communities on three different granite outcrops in the Georgia Piedmont near the town of Lithonia, SE of Atlanta, Georgia. MATERIALS AND METHODS Burbanck and Platt (1964) selected 40 communities for study in 1957: 17 on Arabia Mountain, 16 on Rock Chapel Mountain and seven on Mount Rollaway. These communities occurred in irregularly circular or elliptic depressions in exposed rock surfaces, ranged from 1.3 to 56.9 m2 in area but usually had a diam of 5 m or less, and the depth of soil ranged from 2-50 cm at the deepest part of each island community. Communities were chosen which had intact bottoms; that is, there was no evidence of cracks or crevices in the rock beneath the community. Four types of communities were recognized by Burbanck and Platt (1964) based on characteristic vegetation and maximum soil depth: diamorpha (a succulent annual) (2-9 cm); lichen-annual herb (7-15 cm); annual-perennial herb (14-39 cm); herb-shrub