Age Structure and Successional Dynamics of a Colorado Subalpine Forest

Abstract

Population age structures of old-growth Engelmann spruce, subalpine fir and lodgepole pine are described in the Colorado Front Range subalpine forest (2850 and 3500 m). Among the populations of these three species, five types of age structures can be recognized; up to four different types of age structures were found in one species under different environmental or historical conditions and no species showed the same type of age structure throughout its entire range of occurrence. The age structures of populations are used to infer their climax or successional (selfreplacing or not self-replacing) status. From the combination of climax or successional populations in a stand and the distributions of these combinations on environmental gradients, three types of forest are recognized: (1) climax lodgepole; (2) climax sprucefir, which is pioneered after disturbance by lodgppole; and (3) climax spruce-fir which is pioneered primarily by spruce. The changes from one forest type to another follow a unidimensional gradient which begins with climax lodgepole at low elevation, dry sites, followed by a forest mosaic of successional lodgepole and climax spruce-fir at both higher elevations and intermediate moisture sites, and ends with climax spruce-fir in which lodgepole is not pioneer, at both high elevations and on the most mesic sites. INTRODUCTION Studies of the seral status of species are commonly based on size distributions and differences in sizes between species in a stand. However, significant errors may be introduced into these studies by differences in growth rates between species. In addition, the seral status of a species may not be reflected in its size distribution because of a tendency for gaps in the distribution to be filled by differential growth of individuals (Schmelz and Lindsey, 1965; Johnson and Bell, 1975). The result may be that the shape of the size distribution does not reflect whether a population is reproducing, i.e., successional or climax. Ideally, succession should be studied by observing an area over a period of time. In the case of long-lived species, such as trees, this is not practical. However, the age structures of tree populations may indicate whether they are successional -or climax, and successional pathways may be inferred from this information (Leak, 1975). Climax types and advanced seral sequences were investigated in the Colorado Front Range subalpine forest using tree population age structures of the three dominant species: Engelmann spruce (Picea engelmannii Parry), subalpine fir IAbies lasiocarpa (Hook.) Nutt.] and lodgepole pine (Pinus contorta Engem.) The objectives of this study are: (1) to describe the age structures of Engelmann spruce, subalpine fir and lodgepole pine; and (2) to infer seral and climax status from tree age distributions. The study area was the Fraser Experimental Forest (39?50'N, 105050'W), located 8 km (5 miles) SW of Fraser, Colorado. This area is in the southern part of the W flank of the Colorado Front Range, and was chosen because of its accessibility and undisturbed condition. LITERATURE There has been considerable debate over the boundaries of a subalpine zone in the Colorado Front Range and other nearby portions of the Rocky Mountains (Ramaley, 1907; Young, 1907; Ramaley and Robbins, 1909; Bruderlin, 1911; Present address: Department of Botany, Louisiana State University, Baton Rouge 70803.