Effect of Tree Spacing and Understory Vegetation on Water Use in a Pumice Soil

Water use in a pumice soil in central Oregon increased significantly with increased density of a sapling ponderosa pine ( Pinus ponderosa Laws.) stand. Study plots were thinned to 62, 125, 250, 500, and 1,000 trees/acre and replicated six times. Soil moisture measurements were begun 2 years after thinning and continued for three successive growing seasons. Soil moisture was measured with a neutron probe at three permanent stations on each plot at 3‐week intervals during the summer. Total amount of water use was 1.6 times greater on plots containing 1,000 trees/acre than on plots containing 62 trees/acre. Moisture use on plots where understory vegetation was allowed to develop normally was 45% greater than on plots where vegetation was removed. Soil in the area is a Regosol developed in dacite pumice, approximately 7,300 years old, from the eruption of Mount Mazama (Crater Lake).