Effects of patch clearcutting on water yield improvement and on timber production in an Arizona mixed conifer watershed

Abstract

Southwestern mixed conifer forests cover approximately 2.5 million acres in Arizona and New Mexico, and provide a wide range of commercial and noncommercial products. The problem is to develop a management prescription which will benefit the greatest mix of resources. An alternatives analysis predicted that a prescription that included small patch clearcutting, in addition to other stand modifications, would meet this criteria. The two Thomas Creek watersheds, in eastern Arizona, were used to validate and test the responses of the forest resources to the preferred prescription, and to increase the understanding of the mixed conifer forest system. The actual harvest created 63 small patch clearcut and group selection openings, averaging 1-2 acres, over 13% of the South Fork watershed. Overall stand density was reduced 34% to 132 square feet per acre. The harvest resulted in significant hydrological changes. Average annual streamflow increased by about 45%, or 1.72 inches, mostly because of increased winter runoff. A greater proportion of the snowmelt generated streamflow occurred earlier in the spring, while annual peak flows were increased by an average of 66%, or about 2.60 cubic feet per second per square mile. The number of days without flow decreased. Average watershed maximum snow water equivalents remained unchanged. The primary causes of the increases were reduced evapotranspiration and increased snow accumulation in the openings; however, it appears that the partially cut stand also contributed to the increases. The treatment benefitted the timber resource. Diameter growth on the South Fork increased for most species compared to the unharvested stand on North Fork. Stand gross growth remained unchanged, but the same volume was being added to fewer trees. The stand, including most openings, is well stocked with adequate numbers of natural and advance regeneration. The Thomas Creek prescription, after 8 years of evaluation, has achieved its objectives of increasing water yields and stand growth while insuring adequate regeneration. It has also benefitted many wildlife species as well as livestock. A similar prescription should increase water yields, by about 15,000 acre-feet annually, from the Upper Black River Basin without adversely impacting other forest resources.