资源环境科技发展态势分析平台

Western juniper (Juniperus occidentalis spp. occidentalis Hook.) woodlands are replacing many lower elevation (< 2100 m) quaking aspen (Populus tremuloides Michx.) stands in the northern Great Basin. We evaluated two juniper removal treatments (Fall, Spring) to restore aspen woodlands in southeast Oregon, spanning a 15-year period. The Fall treatment involved cutting 1/3 of the juniper followed by a high severity broadcast burn one year later in October 2001. The Spring treatment involved cutting 2/3 of the juniper followed by a low severity broadcast burn 18 months later in April 2002. The cut trees increased the amount of dry fuels to carry fire through stands. We tested the effectiveness of treatments at removing juniper from seedlings to mature trees, assessed aspen ramet recruitment and development, and evaluated recovery of the shrub layer. In the Fall treatment, burning eliminated all remaining juniper trees and saplings, stimulated an 8-fold increase in aspen density (16,000 ha(-1)) and increased aspen cover 6-fold compared to the untreated controls. After 15 years, aspen density in the Spring treatment was about 1/3 of the Fall treatment, however, aspen cover did not differ from the Fall treatment. Because spring burning was less effective at removing juniper, leaving about 20% of the mature trees and 50% of the saplings, retreatment of conifers will be necessary to maintain the aspen community. In the Fall treatment, juniper began establishing within 15 years after conifer control indicating re treatment might be necessary earlier than expected. Total shrub cover and density in the Spring treatment was greater than the control and Fall treatments. Cover and density of sprouting shrub species, particularly western snowberry (Symphoricarpus oreophilus Gray), increased and were greater in the Spring treatments than the Fall treatment where they had declined. Shrubs that increased in the Fall treatment were species where seed germination is enhanced by fire, especially snowbrush (Cermothus velutinus Douglas ex Hook) and wax currant (Ribes cereum Dougl.). If an objective is to maintain or increase native understories the Spring treatment was more effective than the Fall treatment for recovering the shrub layer.