New Methods and Results of Growth Measurement in Douglas Fir

Abstract Practically all foresters who have had anything to do with the preparation of normal yield tables have recognized the fact that understocked stands tend to approach normality with the passage of time. Evidence supporting this fact is found in the permanent sample plot and forest growth survey records throughout the country. Nevertheless, the usual instructions given by the authors of normal yield tables state that the proper method to follow in predicting growth is to determine the ratio of existing stocking to normal stocking and to compute increment in the existing stand by applying this same ratio to normal growth. The recognized error in this procedure has frequently been considered to introduce a slight and perhaps desirable conservatism into the growth estimates, and for fairly well-stocked or for understocked stands of poor spacing such has probably been the case. But where the proportion of understocked stands of good spacing is high, as it is in many sections of the country, this method may result in estimates 50 to 100 per cent below measured growth. On the other hand, recent analyses in the Douglas fir region have indicated strongly that the conventional method may result in overestimates of growth when applied to areas well stocked with second growth of advanced age. Confronted with the problem of estimating growth of all forest stands the nation-wide forest survey has attempted to devise simple techniques equally adequate for understocked and for well-stocked areas. One such method is described below, followed by a preliminary analysis of results obtained during the summer of 1941 in the Douglas fir region.