Submitted to: Forest Ecology and Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/29/2015
Publication Date: 11/18/2015
Publication URL: https://handle.nal.usda.gov/10113/61968
Citation: Bates, J.D., Davies, K.W. 2015. Seasonal burning of juniper woodlands and spatial recovery of herbaceous vegetation. Forest Ecology and Management. 361:117-130. doi: 10.1016/j.foreco.2015.10.045.
Interpretive Summary: The decrease in fire activity has been recognized as a main cause of expansion and infilling of western North American conifer woodlands into native sagebrush-grass rangelands. Woodland treatments are designed to recover sagebrush-grass plant communities for livestock forage and wildlife conservation and to reduce woody fuel loads to lower wildfire risk. We evaluated fuel reduction and sagebrush grassland recovery after western juniper tree cutting followed by prescribed fire applied in early fall, winter and spring. Burning in early fall and spring reduced woody fuel loading more completely than winter burning, however, recovery of sagebrush-grassland was less complete than winter burning and invasive plant species cover and biomass was enhanced. Burning felled juniper in winter resulted in recovery of native forage grasses and forbs and invasive species presence was minimal to none.
Technical Abstract: The decrease in fire activity has been recognized as a main cause of expansion and infilling of North American woodlands. Piñon-juniper (Pinus-Juniperus L.) woodlands in the western United States have expanded in area 2 to 10-fold since the late 1800’s. Woodland control measures using chainsaws, heavy equipment and prescribed fire are used to restore sagebrush steppe plant communities. An immediate objective following control of piñon-juniper woodlands is recovery of perennial herbaceous species to; 1) restore site composition, structure and processes (resilience) and 2) resist invasion and dominance by invasive annual grasses (resistance). Over a 6 year period, we compared herbaceous recovery following cutting and prescribed fire on three sites in mid and late succession western juniper (Juniperus occidentalis Hook.) woodlands in southeast Oregon. Treatments were untreated controls, partial cutting followed by fall broadcast burning (SEP), clear-cut and leave (CUT), and clear-cut and burn in winter (JAN), and spring (APR). Cover of herbaceous species were measured in three zones; interspace, litter mats around stumps (stump), and beneath felled trees. In interspace zones of all treatments, comprising between 51 to 63 % of site areas, perennial bunchgrasses dominated two sites and co-dominated with invasive annual grasses at one site. Burning in the JAN treatments, when fuel moisture and relative humidity’s were high and temperatures cooler, reduced disturbance severity in stump and felled tree zones, which maintained perennial herbaceous understories and prevented or limited the presence of invasive annuals. Burning felled juniper in SEP and APR treatments resulted in moderate to severe disturbance in stump and felled tree zones. At two sites, severe disturbance consumed all fuel up to the 1000-hr fuel class and largely eliminated herbaceous perennials, resulting in invasive annual dominance of stump and felled tree zones. High disturbance areas, produced by burning where juniper fuels are concentrated (stump and felled tree zones) in early fall and spring, reduces or eradicates native perennials and create islands within treatments that enhance invasive annual encroachment and dominance. To maintain or enhance site resilience and resistant following control of late successional woodlands, reducing piñon-juniper fuels by burning in winter provides managers with a low-impact option for conserving these sagebrush steppe sites.