Site Quality for Douglas-Fir in Southwestern Washington and its Relationship to Precipitation, Elevation, and Physical Soil Properties

The objective of this study was to determine the effects of soil, climate, and topography on the growth of Douglas-fir (Pseudotsuga menziesii var. menziesii (Mirb.) Franco) in southwestern Washington. The results indicate the productive capacity of forest land in this area in terms of permanent mappable features of the forest environment. One hundred and fifty-five even-aged, second-growth stands of Douglas-fir were sampled. Total age and total height of dominant trees, slope, aspect, and topographic position were recorded. Descriptions of soil depth, soil consistence, elevation, and total annual precipitation were also made. Moisture equivalent and gravel content were determined for all soil horizons and in addition, the imbibitional water values of the substratum (C horizon) were measured. The physical soil values and observed stand characteristics were assembled in a multiple regression that expressed tree height in terms of tree age and associated quantitative environmental conditions. The results of this statistical analysis are as follows: 1. Site quality decreases with an increase in elevation and with an increase in the gravel content and compaction of the soil layers above the substratum. 2. Site quality increases with an increase in total annual precipitation and with an increase in depth to the substratum. Increased site quality also occurred with an increase in the product of moisture equivalent and gravel content of the soil layers above the substratum, thus indicating that the adverse effect of gravel is not as pronounced with fine textured soils. 3. The above relationships are expressed by means of regression equations calculated for each of five different soil groups. Site tables suitable for field use in the identification of Douglas-fir site quality may be prepared from these equations.