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Effect of Tree Spacing and Understory Vegetation on Water Use in a Pumice Soil¹

ABSTRACT

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).

¹ Contribution from Pacific Northwest Forest and Range Exp. Sta., Forest Service, USDA, in cooperation with Soils Department, Oregon State University. Presented before Div. S-7, Soil Science Society of America, Denver, Colo., Nov. 19, 1963.

² Research Forester and Professor of Forest Soils, respectively.

Received for publication November 20, 1964. Accepted for publication January 22, 1965.