Succession from Sagebrush to Mixed Conifer Forest in the Northern Rocky Mountains

Abstract

In northwestern Yellowstone National Park, lodgepole pine is invading sagebrush areas. The vegetational composition and environment of sagebrush, lodgepole pine, and intermediate sites were studied. Young lodgepole pine had little influence on the environment or vegetational composition of the sagebrush; however, when lodgepole pine was mature the shade-intolerant species, including sagebrush, were eliminated and shade-tolerant species developed. In the mixed conifer forest the environment was suitable for further development of climax conifers. Thus, lodgepole pine is not only a fire-successional species but part of a normal climatically caused sere originating with vegetation dominated by big sagebrush. INTRODUCTION The vegetation patterns in the Madison Range in Montana adjacent to the northwestern corner of Yellowstone National Park have been described as a complex mosaic (Patten, 1963). This area includes the typical Rocky Mountain montane and subalpine vegetation zones as described by Daubenmire (1943); however, Patten (1963) showed that a sagebrush type, not typical of high elevations in the Rocky Mountains, was prevalent in the area. Big sagebrush (Artemisia tridentata) 2 extends from the low broad valleys at 4500 ft (1360 m) elevation to near timberline at 9500 ft (2880 m) elevation. In addition to the excellent sagebrush development at the lower elevation, extensive stands of big sagebrush are found in the 6000 to 8000 ft (1820 to 2430 m) zone. The northwestern section of Yellowstone National Park includes many areas covered with sagebrush (Streeter, 1965); most are found on south-facing slopes or level topography, although big sagebrush can be found on some shallow slopes with a north aspect. Recently, I observed that areas once appearing to be pure sagebrush stands now support young lodgepole pine (Pinus contorta) equal in height to or slightly taller than the sagebrush while other sagebrush areas were found that had more mature lodgepole pine. Reconnaissance of the lodgepole pine forests of the region turned up a few sites that contained dead sagebrush as part of the understory. The history of lodgepole pine and its role in forest succession following a fire have been thoroughly described by Clements (1910) and Mason (1915). They concluded that lodgepole pine is the primary arboreal invader of heavily burned areas in the montane and subalpine zones of the Rocky Mountains; the resulting dense lodgepole forests I This study was supported by N.S.F. Grant GB-4683. 2 Nomenclature: herbs and shrubs from Booth (1950) and Booth and Wright (1959), trees from Little (1953).