Woodland successional phase effects vegetation recovery after prescribed fire

Background/Question/Methods

Piñon-juniper (Pinus-Juniperus L.) woodlands have expanded into big sagebrush (Artemisia tridentata Beetle) steppe of the western United States primarily as a result of reduced fire disturbances. Prescribed fire in post-settlement piñon-juniper woodlands has been increasingly employed to restore big sagebrush steppe plant communities. We compared vegetation recovery following cutting-prescribed fire treatments on Phase 2 (mid expansion) and Phase 3 (late expansion) western juniper (Juniperus occidentalis Hook.) woodlands on Steens Mountain, Oregon.  In Phase II woodlands, trees co-dominate with shrub and herbaceous plants, and in Phase III woodlands trees are dominant and shrubs and herbaceous layers are much reduced. Twelve Phase 2 and nine Phase 3, 0.63 ha plots were established in May 2003 and all study sites were located in Mountain big sagebrush/Idaho fescue (A.t spp. vaseyana /Festuca idahoensis Elmer) associations.  Pre and post-fire tree, shrub and herbaceous cover and density were collected over a seven year period. Repeated measures analysis of variance for a completely randomized design was used to test for year, woodland phase, and year by phase interaction for vegetation response variables.

Results/Conclusions

Herbaceous vegetation on Phase II woodland sites was comprised of native perennial and annual vegetation before and after fire. Herbaceous vegetation in Phase 3 woodlands shifted from native species to dominance by cheatgrass (Bromus tectorum L.) and other invasive weeds after fire. Shrubs on Phase 2 sites were comprised of sprouting species and snowbrush (Ceanothus velutinus Dougl.) after fire. The shrub component on Phase 3 sites was dominated by snowbrush following fire. The results indicate that woodland phase influences post-fire vegetation recovery and composition.  This suggests that sagebrush steppe in Phase 1 or 2 woodland expansion stages are more likely to recover with their native vegetation components intact than Phase 3 woodland sites. The results also indicate that sites transitioning from Phase 2 to Phase 3 woodlands have crossed a recovery threshold where there is a greater potential for invasive weeds to dominate rather than native vegetation after fire.  Phase 3 woodlands that are burned by wildfire or prescribed fire are likely to require additional inputs, primarily seeding and weed control, for vegetation recovery goals to be accomplished.  Managers should consider applying alternative treatments in Phase 3 woodlands that would potentially improve sagebrush steppe recovery such as cutting or cutting followed by winter/spring burning.