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

Abstract

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 ( 2,7,10,12). 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. METHODSParadise Ridge and Tomer's Butte in Latah County, Idaho, four and five miles distant from Moscow, were selected as suitable stations for measuring the differences between the north-northeast and the south-southwest slopes. The forest condition on these two hills are fairly representative of the. Spokane region in eastern Washington and adjacent Idaho.The vegetation of the southwest slopes is sparse. An examination of the burned trunks of trees and of fallen logs. reveals the fact thl).t the previous forest was yellow pine ( Pinits ponderosa). This is practically the only forest tree at the present time, and furnishes the small amount of reproduction that is taking place. A limited growth of Douglas fir may be found in ravines. These locations are protected to a certain extent from the prevailing winds, and receive considerable quantities of drifting snow. Thus the ravines present more favorable conditions for tree growth.The vegetation on the northeast slope is often quite dense. An examination similar to that made on the south and southwest slopes reveals the fact that the previous forest was made up of a mixture of yellow pine and Douglas fir. The present forest is composed of a few trees of yellow pine in the less protected places. The greater part, however, is made up of dense stands of Douglas fir. Some larch ( Larix occidentalis) may also be found in favorable locations.The stations werP. visited once each week, and the following physical 281

Ecological Photometry and a New Instrument for Measuring Light

“…The method which has been most widely used (Wiesner, 1895 and1907;Clements, 1905;Pearson, 1910;Gordon, 1912;Vouk, 1913;Grafe, 1914;Salisbury, 1916;McLean, 1917;Gail, 1921;Loftfield, 1921;Tansley, 1923;Plitt & Pessin, 1924;Pessin, 1925) is the tinting of silver chloride paper (P. 0. P.) to a standard tint and noting the time taken to reach that tint.…”

Soil Moisture Relations in the Southern California Chaparral

“…losses. Causal factors implicated have been lower insolation (Shanks and Norris 1950, Cantlon 1953, Ayyad and Dix 1964, lower air and soil temperatures (Weaver 1917, Gail 1921, Miller 1947, Parker 1952, Cooper 1961, and higher air humidity and lower wind speeds (Weaver 1917, Gail 1921, Cottle 1932, Parker 1952 on the north-than on the south-facing slopes.…”

Vegetation and Microclimates on North and South Slopes of Cushetunk Mountain, New Jersey

“…Various environmental differences have been found associated with exposure-induced differences in vegetation in the United States (Turesson 1914;Gail 1921;Shreve 1922Shreve , 1924Shreve , 1927aBates 1923;Platt 1951;Parker 1952, among others). In general, it has been found that north facing slopes may differ from adjoining south facing slopes in soil and air temperature, soil and atmospheric moisture, light intensity and wind velocity.…”