The Forest Floor Developed Under Conditions of Summer Rainfall Deficiency in a Californian Pine and Fir Forest

There Is a scarcity of detailed information concerning the soils of forests growing under conditions of a regular summer deficiency of rainfall. Such forested regions are limited on this continent to California and parts of Oregon. They are particularly widespread in California. The present results deal with soil taken from near Westwood, in the Lassen National Forest in northeastern California, at elevations of 5000 to 6750 feet where the forest consists of western yellow pine (Pinus ponderosa), Jeffrey pin«(P. ponderosa var. Jeffrey!) and white fir (Abies concolor). These trees form an open forest with an understory of scattered shrubs, the chief of which are Ceanothus velutinus, C. prostratus and Arctostaphylos patulal The forest is interspersed with areas of sagebrush and grass at the lower, and with brushfields at the higher, elevations. In the higher elevations and elsewhere locally the fir tends to replace the pine. In order to determine what differences, if any, exist in the soil beneath pine and fir, soil was collected from three sample areas beneath mature stands of pine and from three more beneath mature stands of fir. Comparisons have been made between these, and in the present report attention is directed particularly to the forest floor, although frequent reference is made to the mineral section of the profiles. The areas are considered fairly typical of the forest in the arid transition region of the Interior plateau. Climatological data have been obtained from an 11-year record (12) kept at Westwood, about 25 miles south of the sample areas. The normal annual precipitation la 21.7 inches and the normal mean temperature 45.6° F. Much of the precipitation falls as snow. The march of the precipitation effectiveness, by months, has been calculated for Westwood and other points lying along a line drawn through Westwood and intersecting the CascadeSierran axis approximately at right angles. Thorathwaite's methods (11) have been used for this purpose, to whose paper the reader is referred for details. The results of the calculations are shown in figure 1. Red Bluff and Redding in the Sacramento Valley are situated in Pacific grassland and broad-leafed woodland of the Sonoran zone. Downieville, Insklp, Mineral and Westwood in the northern Sierras and Sierra-Cascade Junction are situated in western pine forest; the first two in the transition zone, the latter two in the Canadian and arid transition zone, respectively, Susanvllle in the Great Basin represents the edge of the northern desert-shrub type in the lower Sonorandesert zone. It will be observed that Westwood is the least humid of the forest regions intersected. It lies in a partial rain shadow and has a precipitation efficiency only slightly superior to that at Susanville in the northern desertshrub type. It is characterized by warm summers, low annual mean temperature, and an index of temperature effectiveness (11) lower than any between the Sacramento Valley and the Great Basin. July and August are the only frost-free months recorded, although it is probable that the ground is seldom frozen at any time for more than a few inches from the surface. The condition at Westwood, where high temperatures coincide with the period of negligible precipitation, is in distinct contrast to the association of maximum temperatures with maximum precipitation which is normal for the northeastern, eastern and southeastern forests of the continent. The sample areas lie within the Great Basin Platform, which is formed chiefly of Tertiary and Quaternary lava flows. The lava is gray basalt both with and without olivine, which has