Factors Controlling the Distribution of Douglas Fir in Semi‐Arid Regions of the Northwest

The aim of the present investigation was to obtain quantitative data on the factors which control the distribution of Douglas fir (Pseudotsuga taxifolia) in semi-arid regions of northwestern United States. It has been pointed out, as the result of observations in different localities, that Douglas fir is confined largely to north facing slopes, while south slopes are practically without forest trees except where out-cropping rocks help to furnish sufficient shelter and moisture for the establishment of seedlings (2, 7, IO, I2). In that part of Idaho adjacent to the Spokane region in Washington, included in Merriam's Arid Transition Area (5), the Douglas fir grows chiefly on the north and northeast slopes. These are considerably steeper than the southwest slope. The prevailing winds are from the southwest. METHODS