Page:Climatic Cycles and Tree-Growth - 1919.djvu/19

 Rh The general country rock is Kaibab limestone in horizontal layers forming the plateau, surmounted by lavas over extensive areas near the mountain. The bedrock is covered by a thin soil, largely formed in place. The soil over the limestones is porous, while that over the lavas has much clay and holds water. There is no swampy ground and therefore no conservation of moisture from year to year. Consequently variations in moisture-supply are quickly felt by the trees. The pine forest is remarkable for the absence of other kinds of vegetation. It covers all parts of the plateau from about 5,000 feet in elevation to about 9,000. At the lower edge of the pine forest a belt of cedars, smaller than the pines and round in shape and with dark-green, thick foliage, makes an attractive landscape.

Climate and seasonal conditions.—The climate follows naturally from the latitude and altitude and the distance from the ocean. In the winters there may be from 1 to 6 feet of snow on the ground at one time. The storms are of the characteristic temperate-zone cyclonic types, but on account of the altitude the preliminary south or east winds are rarely observed. Storms come from the Pacific coast and rain occurs about a day later than in southern California. Spring and autumn are the dry seasons, and the warmest time of year is usually in June, just before the summer rains begin. The summer rains occur in July and August and often come in "spells" that last a week or two, with thunderstorms in the afternoons or at night, followed by clear mornings. Unlike the winter storms, the summer rains are local and apt to be torrential in character, with heavy run-off.

Meteorological records in northern Arizona are necessarily meager, yet not so deficient as might be expected. The country was first settled in the "fifties," when gold was discovered in Arizona as well as in California, and lines of travel were established from Santa Fe westward across the plateau. The "blazings" on the pine trees marking the earlier roads are still to be distinguished. Soon after the opening of the country the government located military camps at various places, and from that time records of rainfall and temperature were kept. The record at Whipple Barracks, near Prescott, begun in 1867, has been continued at Prescott to the present time. It is the longest consecutive record in the pine forest and is therefore used below.

The extreme range in temperature observed in Flagstaff is from about 20° F. below zero to about 100° F. above. But the town is in a peculiarly sheltered position and exhibits much lower night extremes than the "mesas" 200 to 400 feet above it. I have observed a difference of 26° F. between the top and bottom of the hill west of town at sunrise on a winter morning. During the early years of the Lowell Observatory, which is located on the mesa 350 feet above the town, the lower minima were about 5° F. These figures show the conditions to which the trees are subjected.