English

Abstract

The survival and growth of trees and shrubs in arid regions of the western U.S. is often limited by inadequate soil moisture availability during establishment. This four year study investigated plant growth and water conservation benefits associated with sub- surface water delivery via PVC pipes (sub-irrigation tubes) on two sites. The first test site was maintained fallow, and the second site with a cover of thickspike wheatgrass Elymus lanceolatus. Four species of woody plants (bur oak Quercus macrocarpa; green ash Fraxinus pennsylvanica; ponderosa pine Pinus ponderosa; and Rocky Mountain juniper Juniperus scopulorum) were tested in randomized complete block designs. The effects of water delivery method (surface-applied versus sub-irrigation tube) and herbaceous competition (fallow versus vegetated) on plant survival, height growth, and vigor rating were compared. In year one, sub-irrigation tubes did not result in significant improvements (ANOVA, LSD Separation, p=0.05) in survival or height growth of any species on either site, but did result in significantly better vigor rating in bur oak on the vegetated site. In year two of the study, only green ash on the fallow site demonstrated significantly better height growth and vigor rating where sub-irrigation tubes were used. In year three on the fallow site, bur oak had significantly greater height growth and green ash had significantly better vigor rating where sub-irrigation tubes were used. Additionally in year three, ponderosa pine on the vegetated site had significantly better height growth where tubes were used. In year four on the fallow site, green ash had significantly better vigor rating where tubes were used. There were no significant differences in survival of any species on either site as a result of water delivery method. With the exception of ponderosa pine in year one, each species by treatment had equal or greater survival, height growth, and vigor rating on the fallow versus the vegetated site. Results strongly support clean cultivation to increase the functional success of tree and shrub conservation plantings. In addition, sub-irrigation tubes may prove effective for some species depending on proximity to the seedlings root system, severity of drought, and other management practices.