Demography and Fire History of a Western Juniper Stand

Abstract

The age, density, and fire history of western juniper (Juniperus occidentalis Hook.) trees growing on range sites of contrasting potentials were investigated. The l,OOO-ha study area consisted of 65% big sagebrush [Artemisia tridentata Nutt. subsp. wyomingensis (Rybd.) Beetle] and 30% low sagebrush (A. arbuscula Nutt.) plant communities. Density of western juniper trees was 150 and 28 trees/ha on the big and low sagebrush sites, respectively. The oldest western juniper found growing in the big sagebrush communities became established in 1855, and 84% of the existing trees became established between 1890 and 1920. The oldest trees on the low sagebrush sites had established by 1600, and most of the existing trees established before 1800. At the beginning of the 20th century, the western juniper populations on big sagebrush sites were doubling in density every 3 years. The rate of establishment on these sites has slowed until 1,370 years would now be required to double the population size. The rate of population growth on low sagebrush sites has varied from decade to decade with a trend to double the population every 200 years and trees that become senescent at about 400 years of age. About 0.4%~ of western juniper on the low sagebrush sites had fire scars, some of which indicated the occurrence of multiple fires. These fire scars indicated that since 1600 there were periods of up to 90 years when no fires scarred the trees. Changes in the frequency of wildfires appear to be the most logical explanation for the sudden invasion of trees into big sagebrush communities, but current technologies for reconstructing fire chronologies are woefully inadequate in this environment. During the last century there has been a pronounced change in the distribution, density, and age structure of virtually all juniper woodlands in the western United States. For the southwestern United States such changes have been related to the influences of grazing animals and fire suppression (Johnson 1962, Arnold et al. 1964). Western juniper (Juniperus occidentalis Hook.) woodlands are a northwestern extension of the extensive woodlands of the central Great Basin. There are two distinct subspecies of J. Occidentalis: J. Occidentalis Hook. subsp. australis Vasek, the Sierra juniper, which occurs from Lassen County, California, south through the Sierra Nevada mountains to the San Bernardino mountains of southern California; and J. occidentalis Hook. subsp. occidentalis, which occurs in northern California and adjacent Nevada, southwestern Idaho, and southeastern Washington, and reaches its greatest development in central Oregon east of the Cascade Mountains (Vasek 1966). The distribution of this subspecies generally follows that of the Columbia River Basalts (King 1959). In the words of a land manager, “Where they grow, stands of western juniper are generally accepted as a characteristic part of the landscape. They appear to be well established and form a logical transition between the open plains and the pine timber. The authors are range scientists, U.S. Dep. Agr., Agricultural Research Service Renewable Resource Center, Univ. of Nevada, 920 Valley Road, Reno, 89512. This study is a contribution from U.S. Dep. Agr., Agricultural Research Service and the Agr. Exp. Sta., Univ. of Nevada, Reno, Journal Series No. 457. Manuscript received March 31, 1980. JOURNAL OF RANGE MANAGEMENT 34(6), November 1961 They look like they belong. A closer look raises some doubts.‘* (Caraher 1978). The doubts of many observers have been raised by the observation of even-aged stands with no senescent trees and no reproduction (Adams 1975). The soils of many of these stands appear to be adapted to support communities of big sagebrush (Artemisia tridentata Nutt.)/ bluebunch wheatgrass [Agropyron spicatum (Pursh) Scribn. & Sm.]. At the same time there are stands of western juniper growing on relatively shallow or rocky soils or steep slopes, with all age classes represented, from seedling to