Submitted to: Oregon State University Agricultural Experiment Station
Publication Type: Experiment Station
Publication Acceptance Date: 11/1/2004
Publication Date: 6/1/2005
Citation: Miller, R.F., Bates, J.D., Svejcar, A.J., Pierson Jr, F.B., Eddleman, L.E. 2005. Biology, ecology, and management of western juniper (juniperus occidentalis). Oregon State University Agricultural Experiment Station. 77 p.
Interpretive Summary: This document represents a synthesis of what is known about the history, biology, ecology, and management of western juniper. Western juniper is occupies 7.9 million acres primarily in central and eastern Oregon, northeastern California, southwestern Idaho, and northwestern Nevada. The expansion of western juniper during the past 130 years has resulted in increased soil erosion, diminished wildlife habitat, reduced forage production, and reduced biodiversity in northern Great Basin plant communities. About 95% of the woodlands are less than 120 years old. Old growth woodlands make up the other 5% and are primarily found on fire safe sites. Old trees can reach ages well over 1000 years. Historically juniper expansion was kept in check by frequent summer range fires. With fire suppression these woodlands have expanded rapidly since settlement of the region. Since the 1960's juniper woodlands have been controlled, primarily by cutting with chainsaws and with prescribed fire, with the goal of restoring sagebrush grassland, aspen, and riparian communities. These control practices have in most cases been successful at restoring the historic plant communities and reducing soil erosion on these rangelands. However, weeds introduced from Eurasia and Eastern Europe can pose serious challenges to rehabilitating native rangelands. The threat of introduced weeds is one of the most important considerations when applying control practices in juniper woodlands. A large variety of wildlife species use early transitional states of woodlands that still contain an understory of shrubs and herbs. However, once juniper dominates an area and shrubs and understory species decline or die out, wildlife abundance and diversity decline substantially
Technical Abstract: The rapid expansion of western juniper into neighboring plant communities during the past 130 years has caused considerable concern because of increased soil erosion; reduced forage production; altered wildlife habitat; changes in plant community composition, structure, and biodiversity. However, the impacts of post-settlement woodland expansion are not always clear or consistent across sites and have lead to debate and legal challenges over juniper control projects and management plans for western juniper. Western juniper occupies 7.9 million acres in central and eastern Oregon, northeastern California, southwestern Idaho, and northwestern Nevada, and occurs in a few outlying stands in southern Washington. Evidence supporting rapid post-settlement expansion is derived from old surveys, photographs, the distribution of relict presettlement woodlands, and tree-ring chronologies. Western juniper represents the northwestern portion of the piñon and juniper region in the Intermountain West. Seeds are dormant and germination potential is greatly enhanced by prolonged cool-moist stratification, which is cumulative from year to year. Seed dispersal of western juniper occurs through gravity, overland flow, and animals. At least 12 species of birds feed on the fruits and as a group are the most important disseminator's of western juniper seed. Western juniper grows on a wide variety of parent materials and soils including materials derived from aeolian, sedimentary, and igneous sources. Soil textures range from clay to sandy and soil temperature regimes from mesic to frigid. Western juniper communities may be separated into presettlement (old-growth) or post-settlement (expansion) communities. We suggest 1870 as a cut-off to separate the two age classes. Western juniper is a long-lived species (more than 1,000 years). Old-growth represents only a small proportion of the population throughout most of its range with the exception of the Mazama Ecological Province. Old-growth trees and stands can easily be separated from post-settlement stands based on morphological and stand structure characteristics. The majority of post-settlement communities are still in a state of transition. The stage of woodland succession (defined in this paper as Phases I, II, and III) directly affects plant community structure, composition, seed pools, wildlife habitat, and ecological processes including hydrologic and nutrient cycles. The phase of woodland development also affects the selection of management treatment, response following treatment, follow-up management, and treatment cost. As the tree layer increases in dominance the shrub and herb layer decline. The degree that the herb layer is depleted is dependent upon soil depth to a restrictive layer. The minimum time for the tree overstory to begin suppressing the understory is 45-50 years and to approach stand closure 70-90 years on cool wet sites and 120-170 on dry warm sites. Western juniper expansion into sagebrush grassland can affect the spatial distribution of soil organic matter, carbon, and nutrients. The loss of nutrients will also increase if woodland development results in accelerated erosion. Changes in hydrologic processes and water balance as tree abundance and dominance increase are not well understood. Evidence suggests that juniper can impact infiltration rates, sediment loss, and soil water storage and depletion rates. Accelerated soil water depletion rates in western juniper-dominated stands can decrease the length of the understory growing season by as much as 4-6 weeks. A large variety of wildlife species use early transitional states of woodlands that still contain an understory of shrubs and herbs. However, as structural diversity declines with increasing tree dominance, wildlife abundance and diversity also decline. Western juniper has significantly in