STRUCTURAL PATTERN IN ALPINE TUNDRA VEGETATION

Several aspects of the spatial pattern of plants were investigated in six very diffierent stands in the alpine tundra of the Colorado Front Range. Pattern was related to the internal structure of the stands, to interactions among the species, and to microhabitats within the stands. Evidence from species-species associations and species ordinations clearly divided the stands into two groups. The first was characterized by thin, coarse, unstable soils, low plant cover, and nurse-plant establishment. The second group was characterized by deeper, more mature, and more stable soils, higher plant cover, and more competition between species. Pattern evidence allowed some species to be categorized as competitors, ruderals, or stresstolerators in the context of the alpine vegetation. The two groups of stands also diffiered in the relative importance of species with these "strategies." THIS STUDY concerned spatial pattern as a structural characteristic of alpine tundra communities in the CColorado Front Range. The objectives were to understand internal structure and competitive interactions in several tundra community types by the use of pattern analyses, and to compare several methods of pattern study. In addition, the results of the pattern techniques were combined with the results of species vrdinations and species-species associations to yield a coherent description of the structure of the vegetation. It is widely recognized that the spatial distribution of plants is important in competition between them. As Harper (1977), and Turkington and Harper (1979a, b) have emphasized, competition occurs between neighbors. In communities composed of several strata, this is complicated by such processes as the shading of understory plants by those in the canopy, and in communities with longer growing seasons, by phenological differentiation, which effiectively separates neighbors in time and thus reduces competition between them. In many ways the alpine tundra presents the investigator with an excellent field laboratory for the study of pattern and its implications in such ecological processes as competition. Since few plants in the tundra attain heights greater than a few centimeters the problems associated with strata are minor or absent. In addition, the short growing season reduces the opportunities for phenological differentiation and the problems it presents in the interpretation of the results of pattern studies. t Received for publication 27 January 1984 accepted 21 June 1984. The author thanks Prof. Richard Ward and Dr. William Slauson for their invaluable assistance in performing this research and in preparing this publication. SITE DESCRIPTION-The study area lies about one-half km inside the north-central boundary of Rocky Mountain National Park, and is in T7N, R74W, Sec. 36, 6th PM, Larimer County, Colorado. The elevation of the site is approximately 3,475 m, some 275 m above local timberline. The area is just over half a km northeast of Fall Mountain, the closest named peak. Prior to the designation of the park in 1915, some grazing may have occurred on the site. Since then, however, the area has been little disturbed. A trail passes through the area, but the stands chosen for study are away from it and from all evidence of disturbance by riders or hikers. The six stands chosen were selected because they were the subject of an earlier study (Eddlemann, 1967), which provided useful background information on their species compositions and relationships, and their soils and microclimates, and because they represent very diffierent and important segments along the vegetational gradients present in this area of alpine tundra. The six stands chosen, the dominant species, and physical descriptions of each are presented in Table 1. MATERIALS AND METHODS-Pattern has generally been studied by two classes of methods (Mead, 1974). One class includes plotless, or nearest-neighbor, sampling. As traditionally employed, this class of methods presents many difficulties of interpretation and usually detects pattern only at the smallest scale present. The other class of methods uses quadrats to sample the vegetation. This class has been derived from the early tests for nonrandomness of distribution, and has progressed toward quantitative measures of scale and intensity of pattern. Prominent in the development of this latter