620 FUNDAMENTALS OF FRUIT PRODUCTION 



known as the Alleghanian, extends from the Atlantic seaboard approxi- 

 mately to the 100° meridian, which runs through central North and South 

 Dakota, Nebraska and Texas. To the west of this is a central arid area 

 extending to the Sierra Nevada-Cascade mountain range. West of this 

 is the Pacific coast humid area, very humid at the north but toward the 

 south gradually merging into the conditions presented by the central 

 arid area. Generally speaking, the same fruit species thrive in all of these 

 areas, though they cannot be grown without irrigation in the central 

 arid section. However, though the same fruit species are grown in all 

 three areas the same varieties are not equally successful; consequently 

 each area has a more or less distinctive variety flora. 



Upper Austral Zone. — The Upper Austral region includes a compara- 

 tively narrow belt of territory in the central Atlantic States but widens 

 out to include a comparatively large part of the corn belt area in the 

 middle west and like the Boreal and Transition regions it includes many 

 irregularly shaped areas from the Canadian line to far below the Mexican 

 border. According to Merriam: "Upper Austral species require a total 

 quantity of heat of at least 6,400°C. (11,500°F.), but apparently cannot 

 endure a summer temperature the mean of which for the six hottest 

 consecutive weeks exceeds 26°C. (78.8°F.). The northern boundary of 

 the Upper Austral zone, therefore, is marked by the isotherm showing a 

 sum of normal positive temperatures of 6400°C. (11,511°F.) while its 

 southern boundary agrees very closely with the isotherm of 26°C. 

 (78.8°F.) for the six hottest weeks. "*^ The eastern half of this zone, 

 known as the Carolinian area, has a humid climate; the western half, 

 known as the Upper Sonoran area, is comparatively arid. The walnut, 

 hickory, sassafras, sycamore, red bud and papaw are typical native 

 trees of the Carohnian area; the sage brush, grease wood and juniper 

 characterize the Upper Sonoran. Within this zone the peach, the 

 Japanese plum, the persimmon and many varieties of the apple, pear, 

 cherry and grape attain their highest development. 



Lower Austral or Subtropic Zone. — The Lower Austral zone lies 

 between the Upper Austral and Tropical regions. On the east it includes 

 most of the south Atlantic seaboard and in the Mississippi valley it 

 extends north into southern Missouri, Ilhnois and Indiana; in the west 

 it includes most of southern California and much of the Sacramento and 

 San Joaquin valleys. Merriam states: "Lower Austral species require a 

 total quantity of heat. of at least 10,000°C. (18,000°F.).'"»5 Like the 

 Upper Austral zone, its eastern half has a humid and its western half an 

 arid climate. The eastern half is known as the Austroriparian area, the 

 western half as the Lower Sonoran. The former is characterized by such 

 native vegetation as the long-leaf and loblolly pines, the magnolia, the 

 Hve oak and the pecan. It is a rich agricultural area producing cotton, 

 rice, sugar cane and many other warm season crops. The distinctive 



