Seasonal burning of juniper woodlands and spatial recovery of herbaceous vegetation

Decreased fire activity has been recognized as a main cause of expansion and infilling of North American woodlands. Piñon-juniper (Pinus-Juniperus) woodlands in the western United States have expanded in area 2–10-fold since the late 1800s. Woodland control measures using chainsaws, heavy equipment and prescribed fire are used to restore big sagebrush (Artemisia tridentata Nutt.) steppe plant communities and reduce woody fuel loading. Immediate objectives in the initial control of piñon-juniper are; (1) recovery of perennial herbaceous species to restore site composition, structure and processes (resilience) and resist invasion and dominance by invasive annual grasses (resistance) and (2) reducing woody fuel accumulations. Spanning a 7 year period (2006–2012), we compared herbaceous recovery following cutting and prescribed fire on three sites in mid and late succession western juniper (Juniperus occidentalis spp. 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 was measured in three zones; interspace, litter mats around tree stumps (stump), and beneath felled trees. In interspace zones of all treatments, comprising between 51% and 63% of site areas, perennial bunchgrasses dominated two sites and co-dominated with invasive annual grasses at one site after treatment. Burning in the JAN treatments, when fuel moisture and relative humidity 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 high fire severity in stump and felled tree zones. At two sites, these fires consumed all fuel up to the 1000-h fuel class, largely eliminated herbaceous perennials, and created islands within treatments that enhanced annual grass invasion and dominance. To maintain or boost site resilience and resistance following control of late successional woodlands, reducing piñon-juniper fuels by burning in winter provides managers with a low-impact option for conserving sagebrush steppe.