Structure and Productivity of Juniperus occidentalis in Central Oregon

Abstract

Western juniper ecosystems are well-adapted to the arid environments of central Oregon. In the stand examined, trees rarely exceeded 8 m in height and were uniformly spaced. Although foliage biomass averaged 4315.0 kg ha-', total stand leaf area was only 2.0 ha ha-'. Total aboveground biomass averaged 21,161.4 kg ha-'. Aboveground net primary production of the juniper was estimated at 1097 kg ha-'r1. Juniper forests have a higher proportion of bark and a much lower stem water-storage capacity than other coniferous forests in the Pacific Northwest. The individual trees examined had leaf areas per unit of stem water-conducting tissue that were less than for fir species on more mesic sites but similar to those for two western pine species. Double sampling provided reliable estimates of means and confidence intervals for juniper biomass and leaf area. INTRODUCTION Western juniper (Juniperus occidentalis Hook. subsp. occidentalis) occupies the ,driest of all coniferous forest sites in the Pacific Northwest. In the dry forest zone of central Oregon and Washington (Franklin and Dyrness, 1973), stands of juniper merge with ponderosa pine (Pinus ponderosa) forests on the moister sites and border plains of big sagebrush (Artemisia tridentata) throughout the region. In the past, fire has controlled the spread of juniper into the adjacent shrub/steppe (Burkhardt and Tisdale, 1976), and the practice of suppressing range fires-widespread during this century-has apparently allowed juniper to invade these recently nonjuniper -ommunities. Because juniper can compete successfully with more palatable forbs and grasses, range managers generally regard the species as a pest. Furthermore, many of its apparent physical adaptations to this harsh environment, such as a stubby growth Form with severe taper, make juniper undesirable for large-scale commercial exploitation by the forest products industry. As a result of this management status, methods to eradicate juniper, usually to release grazable grasses and forbs (Bedell and Bunch, 1978), have been extensively researched (Martin, 1978). However, little work has focused on juniper's commercial prospects, even as a fuel, and only preliminary work has examined its role as habitat for small mammals, birds and other game animals (Maser and Gashwiler, 1978). Nothing is known of its place in the hydrology, nutrient cycles or production relations of the dry forest zone. This study provides data from one western juniper habitat for: (1) the most important aboveground structural features of the juniper stand and the reliability with which they can be assessed; (2) the aboveground net primary production of the tree strata, and (3) the specific structural adaptations to the arid environment of the study area. STUDY AREA Located in central Oregon at 1356-m elevation along a NE-facing slope at the summit of Horse Ridge, the study site lies in the rain shadow of the Cascade Moun1 Paper 1256, Forest Research Laboratory, School of Forestry, Oregon State Univ., Corvallis, and contribution number 355 of the 'Coniferous Forest Biome. 2Present address: School of Forest Resources and Conservation, Univ. Florida, Gainesville 32611.