Factors Affecting Grade Change of Ponderosa Pine Lumber in the Desert Southwest

Dimension lumber marketed in arid climates continues drying after it is kiln-dried and graded. This additional drying causes some pieces to change to a lower grade (degrade) primarily due to dimensional instability. Measurements were taken on all the dimensional lumber from 99 young-growth ponderosa pine trees with a diameter at breast height of 12 inches or less and a control sample of dimension lumber from larger trees. Lumber was sawn and graded in the fall at a sawmill marketing dimension lumber in the desert Southwest. Bundles of lumber were opened and regraded either that winter or the following summer. Each board was classified by three criteria: originated from small or large trees, initial grade, and regraded in winter or summer. Percentage of boards that degraded differed from small and large trees (small 20 percent, large 7 percent), by initial lumber grade (Standard and Better 12 percent, Utility 22 percent, and Economy 7 percent), and by time of regrading (winter 11 percent, summer 15 percent). Factors apparently associated with degrade in the lumber from young-growth trees were the presence of pith and the amount of crook in the board when it was initially graded.

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