Methods for Successful Establishment of Cottonwood and Willow Along an Incised Stream in Semiarid Eastern Oregon, USA

Abstract

Restoration of riparian plant communities in the dry interior Columbia River basin and other semiarid regions where channel incision has altered hydrological connections is a major challenge. Establishing riparian vegetation within incised systems has the potential to facilitate the aggradation of the stream bed by increasing bank stabilization and providing large woody debris inputs. We tested an alternative approach to irrigation in a heavily incised subwatershed of the dry interior Columbia River Basin by using motorized augers to penetrate lowered water tables. By planting dormant pole cuttings of black cottonwood (Populus trichocarpa) and willow (Salix spp.) in auger holes within terraces up to 2 m above the incised streambed elevation, we hypothesized that riparian vegetation could be established without the aid of irrigation. We also tested the efficacy of vented plastic tree shelters and circular fence cages with respect to survival, growth, and browsing after 15 months. Survival and average growth was significantly higher for pole plantings protected with vented plastic tree shelters and planted in auger holes where lowered water tables were penetrated (89% and 95 cm, respectively). Survival and average growth was significantly lower for unprotected pole cuttings planted in augur holes where lowered water tables were not penetrated (5% and 33 cm, respectively). Our results suggest that survival of willow and cottonwood pole plantings along incised streams can be greatly enhanced if they are: 1) placed in auger holes that reach the water table via deep-planting; and 2) protected with vented plastic tree shelters.