Evaluation of dryland riparian restoration with cottonwood and willow using deep-planting and herbivore protection

Abstract

Degradation of dryland riparian ecosystems has been linked to the lowering of alluvial groundwater tables and reduced floodplain connectivity. Establishing riparian plants in dryland ecosystems with high water-stress and herbivore pressure presents major challenges for restoration practitioners. By planting at sufficient depths to reach lowered water tables, deep-planting provides direct access to water and encourages root development within hydrated soils. While deep-planting is a promising alternative to traditional supplemental irrigation in dryland areas affected by lowered water tables, few studies have evaluated deep-planting where planting depths must exceed one-meter to reach water tables and where herbivore protection is required. To evaluate deep-planting as an irrigation alternative where lowered water tables present a challenge to riparian restoration, we conducted experimental plantings along an incised stream within a semiarid watershed using deep-planting without supplemental irrigation in combination with several tree shelter designs. Our results indicate deep-planting cottonwood (Populus trichocarpa) and willow (Salix spp.) pole cuttings in augered holes that penetrated water tables up to 1.9 m below the surface significantly increased the probability of survival, with water table penetration significantly increasing the odds of survival by a factor of 7. Deep-planting with access to lowered water tables in combination with 0.9-m vented plastic tree shelters significantly increased the probability of survival, with over 50% higher survival after three years compared to unprotected and 1.-m circular fence caged plants that were also deepplanted with access to water. However, taller fence cages significantly reduced the probability of terminal bud loss from browsers with over 25% lower browse rates after three years. Therefore, we conducted additional experimental plantings to evaluate two taller plastic tree shelter designs to maximize survival while minimizing browsing. The results of our study indicate that deep-planting pole cuttings of cottonwood and willow with access to lowered water tables in combination with taller 1.8-m vented plastic tree shelters provided statistically similar survival as compared to the shorter 0.9-m vented plastic tree shelters after two years while significantly reducing browsing by approximately 75% two years after planting.