WESTERN JUNIPER CONTROL STUDIES: EOARC RESEARCH REPORT

Authors: Bates, Jonathan - Jon

Submitted to: Eastern Oregon Agricultural Research Center
Publication Type: Experiment Station
Publication Acceptance Date: 12/1/2007
Publication Date: 2/1/2007
Citation: Bates, J.D. 2007. Western juniper control studies: EOARC research report. Eastern Oregon Agricultural Research Center. 57 p.

Interpretive Summary: Western juniper encroachment into sagebrush steppe and other plant communities throughout the northern Great Basin is of serious ecological concern; resulting in reduced shrub-steppe productivity, increased erosion, altering of nutrient cycles, and causing decreases in wildlife habitat. This report provide results of 10 research studies conducted by ARS scientists located in Burns, Oregon and Boise, Idaho evaluating the effectiveness of juniper control methods in removing tree interference and restoring shrub-steppe and aspen plant communities. Fire and mechanical control treatments of western juniper were determined to be generally effective at restoring native plant communities and reducing soil erosion and runoff. However, there are potential risks for non-native weeds to increase after juniper treatment, especially after fire. To select appropriate treatment packages to control juniper and make restoration efforts more successful requires thorough knowledge of an area’s environmental conditions and species composition.

Technical Abstract: Western juniper encroachment into sagebrush steppe and other plant communities of the northern Great Basin is of serious ecological concern. Juniper now dominates millions of acres of rangeland in eastern Oregon, southwest Idaho, and along the northern border region of Nevada and California. Current estimates put juniper woodlands, in all phases of woodland invasion, at between 8-9 million acres. The purpose of this report is to provide results of 10 EOARC research studies evaluating; A) the effectiveness of juniper control methods; and B) the effects of control to plant succession, nutrient cycling, and surface hydrology. Fire studies indicate that the predictability of the plant community response after increases with site condition, site potential, and when the community is still in the early stages of juniper woodland development. The degree of predictability is lowest when a site is approaching or is at a threshold such as in fully developed woodlands. Post fire vegetation succession will have multiple entrance points with sites tending to be dominated by perennial and annual forbs the first five years following fire. However, treated early and mid-successional woodlands with a good understory composition tend to recover adequately and more speedily than late succession woodlands. Late successional woodlands are more susceptible to non-native weed dominance because of high mortality of native perennials. The results indicated that cutting 25% of mature trees in communities dominated by western juniper (juniper tree cover 40-80%) was sufficient to remove the majority of remaining live trees in stands with fall prescribed fire. Winter burning of cut trees in late succession woodlands should be considered to limit mortality of herbaceous perennials and speed recovery. Winter burning should be done when soil and ground litters are at field capacity and frozen (Nov.-March). Cutting treatments have a higher degree of control than fire applications. Cutters can select specific trees to cut and not to cut (e.g. old growth trees should probably be left on site). Unlike prescribed fire, cutting operations will stay within the boundaries of the treatment site and liability is thus reduced. Cutting can be conducted almost year round as long as access is not constrained by weather or road conditions. The only area of concern is that leaving cut trees on site can present a fuel load problem for several years following treatment. This is of particular concern in woodlands that abut forested plant communities such as Ponderosa pine forest. Recovery of native vegetation requires patience. It takes anywhere from 1 to several years before the understory fully responds to removal of tree interference especially when growing conditions are not favorable. Juniper treatments (cutting or fire) on drier sites with adequate perennial understory to recover will result is reduced erosion and runoff. The research results reported cannot account for every management contingency and woodland situation observed in the field. Western juniper is present in several ecological provinces and though many of the problems faced by juniper expansion are similar, each province has unique issues to address when managing their woodlands. For example, weedy annuals are of major concern in large portions of the John Day province while the High Desert and Humboldt provinces have tended to avoid significant weed problems when woodlands are treated. Despite the limitations of research several guiding principles emerge from the results which are conveyed in the study sections and the summary. These guiding principles are applicable in the field though implementation of juniper control projects at larger scales should remain flexible and adaptive and take into account site and woodland characteristics.