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The change that is presented in the animal and plant forms in space is brought abont largely by two factors: temperature and moisture. Of these temperature is more important than moisture, but moistnre greatly affects the plant and animal forms. There is no qnestion but that life varies greatly as one moves from the tropics to the arcties and as one moves from a region of warmth to a region of cold. It was left for Dr. C. Hart Merriam1 to reduce this succession of life in space to a critical ajnd scientific basis. He subdivided North America into Life Zones, and divided the north temperate zone into areas much less in extent than the entire zone itself. He premised these zonal divisions on a basis of temperature range through the growing and productive seasons of the year. He divided each zone into a humid eastern and a drier western portion. One would expect life zone boundaries to extend east and west, since they are based principally on a temperature range, and so they do, but very irregularly and erratically. The irregularity is brought about by the proximity of the oceans and the elevation of the lands, and the great expanse of the interior plains. The Far West presents the life zones in their most dramatic form, and to the Far West this discussion will be restricted. In the West, because of lessened rainfall, Dr. Merriam called the Lower Austral region and the Upper Austral region the Lower Sonoran and the Upper Sonoran. The Lower Sonoran Life Zone extencds into southern Arizona, occupies nearly all of Lower California, and extensive areas in Mexico and California. In California it occupies the deserts and a long strip north and south through the seasonally hot Sacramento-San Joaquin Valley. The Upper Sonoran Life Zone in the Far West occurs in many of the valleys of the coast ranges of California, occupies the foothills around the great Sacramento-San Joaquin Valley, most of the coast range mountains, and occurs rather extensively in the dry region of eastern Oregon and Washington. Life zone boundaries which should run in an east-west direction in the Far West, run chiefly north and south because of the modifications of conditions brought about by the mountains, the Great Valley, and the Pacific Ocean. In no place in North America are temperature ranges as erratic as in the Far VVest. In this region the great Pacific Ocean modifies the tenmperature; the mountain ranges enclose the valleys so that these become hot in the growingseason; and the mountains themselves present differences in temperature as elevations are attained. As a consequence, the Transition Life Zone in California has been divided by Dr. Grinnell2 into 1 MERRIAM, C. HART. Life Zones and Crop Zones of the United States. Dept. of Agri. Bulletii, no. 10. 2 GRINNELL, JOSEPH. A Revised Life Zone Map of California. Uniiv. of Calif. Press. |
