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In southwestern Wisconsin, close to the southern limit of its range, eastern white pine (Pinus strobus L.) grows in isolated stands on steep slopes of exposed Ordovician St. Peter sandstone and Cambrian sandstone of the Tunnel City group. On a larger geographic scale, white pine is associated with a humid, cool climate, which is typical of the northern two-thirds of the state of Wisconsin, where the species grows best on deep loams or sandy loams (Curtis, 1959). In southwestern Wisconsin, south of its typical range, white pine also grows from cracks in sandstone cliffs (Curtis, 1959). On sandstone outcrops, the establishment of hardwood competitors is impeded by low water retention of the weakly developed soils (Wilson and McQuilkin, 1965). Thus, white pine located on steep sandstone bluffs in southwestern Wisconsin has an advantage over species with less tolerance of xeric sites with minimal soil horizon development.
Fossil pollen studies show that white pine has been regionally abundant in southwestern Wisconsin since at least 10,480 [+ or -] 100 radiocarbon years ago (Davis, 1977), when vegetation zones were located S of where they are today due to the cooler climate (Hansen, 1939; Curtis, 1958, 1959; Davis, 1977; Webb et al., 1983). Although palynological evidence that these pine relicts have persisted in the Driftless Area of southwestern Wisconsin from the late Pleistocene through the Holocene indicates successful adaptation of the species to the local environment, several authors have documented the different degrees to which white pine is maintaining itself. Curtis (1959) reported that "although these relic stands are considerably removed from the area of their climatic optimum, there is no indication that they are in a retreating or moribund condition" (p. 216). In a study of the forest vegetation of the Driftless Area in NE Iowa, Cahayla-Wynne and Glenn-Lewin (1978) observed that white pine grows in relict communities primarily on the drier sites, but that white pine seedlings were not a significant component of the herb layer, suggesting that the species was not reproducing well. In 1948, McIntosh completed a taxonomic survey of species composition of 22 sites in six counties in southwestern Wisconsin where white pine, red pine (Pinus resinosa Ait.) or jack pine (Pinus banksiana Lamb.), all species with northern affinities, were abundant. He found representatives of diverse plant communities, including numerous species characteristic of northern pine stands. McIntosh (1948, 1950) concluded that in a few of these stands white pine was reproducing enough to persist in this area. His study also indicated that in several other stands white pine was not reproducing well, as marked by the absence of seedlings and saplings, with a trend toward local extinction.
Species distributions change constantly over long and short time scales in response to changing environmental conditions (Hengeveld, 1990) and interactions with other plant species (Vale, 1982). The potential for population growth of a given species is greatest in the center of its geographic range (Hengeveld, 1990); vigor in plant growth and reproduction decreases toward the margins of the range. Species are more vulnerable to environmental change and competition with other species at the limits of their range. Northern conifer species, in general, are limited at the southern margin of their distribution not by minimum temperature, but by competitive interactions with hardwood species (Woodward, 1987). One might expect, then, that white pine at the southern extent of its range would grow better than hardwood species on sites where the soils are drought-prone and nutrient-poor (Wendel and Smith, 1990). Davis (1977) noted that the mosaic character of the prairie-deciduous forest ecotone in the upper Midwest corresponds with topographic variation, and that relicts of past climatic conditions persist in favorable habitats. I hypothesized, therefore, that local site conditions are important in determining where stands of white pine are located and are regenerating at the southern margin of its range in southwestern Wisconsin. I predicted that white pine reproduces best on exposed slopes of steep sandstone cliffs, less vigorously on upper slopes of intermediate steepness and least successfully on the slopes of ravines, where better-developed soils would likely support fewer conifers and more hardwoods (Vankat, 1979).
This study investigates the contemporary dynamics of white pine in a selection of the stands studied by McIntosh (1948, 1950) more than 45 yr ago. The approach is two-tiered. First, by comparison with McIntosh's results, it evaluates qualitatively how stand dynamics have changed in the past 45 yr, and, more specifically, determines whether white pine in certain stands still grows and reproduces vigorously. Second, it analyzes the species composition of …