Tree crops of the different climatic regions of the world
Mrose, Mary Emma
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The purpose of this thesis has been to present the economically important tree crops of the different climatic regions of the world in order to show man's present dependence upon them for his welfare and comfort. The tree crops taken under consideration consisted of fruits, nuts, beans, berries, flower buds, seeds, leaves, bark, latex, and juices of trees which grow in native forests and in a cultivated state in plantations and orchards. The geographical basis chosen for this study was climate, since climate is fundamentally responsible for the geographical distribution of trees. The climatic regions were analyzed as to the conditions favorable for the growth of crop-yielding trees; then within each climatic region the tree crops have been presented under headings indicative of man's major use of them: beverages, dyes and pigments, drugs and medicinal products, fibers, fruits, gums and resins, latex products, masticatories, oils and waxes, spices and flavoring materials, tree legumes and edible nuts, sugar and starches, tanning materials, and miscellaneous. Each tree crop is discussed as to geographical distribution, geographic factors underlying its production, its culture and preparation for market, and its economic importance. A number of geographic factors combine to determine the distribution and production of tree crops, economic, physiographic, political, and climatic factors. The economic factors influencing tree crop production are supply and demand, labor, and the time factors of gestation period, age commercial bearing begins, and the duration of yield. Physiographic factors which may restrict the localities of tree crop production are location—-in respect to land and sea, in respect to main trade routes, in respect to the labor supply—-soil, and topography. Existing political conditions may either foster or restrict tree crop production in a region despite the fact that climate favors its distribution there. Climatic factors fundamentally responsible for the distribution of tree crops include rainfall and humidity, temperature, wind, and sunlight. Of all the geographical factors, climate more than any other determines the possible areas where the tree crop may be produced; but the actual economic exploitation and production may be further limited by economic, physiographic, and political factors. Therefore, in this geographic study of tree crops the basis of subdivision is climate. Of all the climatic regions of the world three are conspicuous by the total absence of tree crops,--the Polar Continental, the Subpolar, and the Polar Ice Cap climates. In each of these three temperature is the forbidding factor in tree growth, In all other climatic regions tree crops are present in varying numbers, depending on the combination of temperature, rainfall and humidity, wind, and sunlight. Regions under the influence of the Tropical Rainy climate are the richest in crop-yielding trees. In general, optimum atmospheric conditions of temperature and rainfall exist in this climatic regime for the growth of trees and tree crops. Every major type of tree crop is to be found in this climate; numerous varieties of each type contribute to the length of the list. Many of these tree crops come directly from the tropical rainforests; others are cultivated on plantations or native farms. The Monsoon Tropical (Tropical Savanna) climate has only seven tree crops of commercial importance since the number and species of trees possible in this climatic type are limited to those which are adapted in response to the distinct rainy and pronounced dry seasons which exist there. There is but one tree crop of prominence in the Cool Tropical Highland climate-—coffee. Though certain temperate fruits can be grown in regions having this climate, they are not of economic importance since their quality is poor due to the lack of influence of winter conditions which are needed in order for them to do their best. Since the Semi-arid Tropical regions are lands of scanty rain which occurs only from one to three months of the year, few trees are able to exist there. Only five tree crops typical of this climate have attained commercial importance. One tree crop-—the date-—is representative of oases in the Arid Tropical regions of the world. The Arid Tropical climate is unfavorable to tree growth except where underground sources of water are available to make their growth possible in oases. Though other tree fruits, as pomegranate, apricot, olives, and even peach, may be found along with dates in this climatic type, they are only of minor importance here, having been introduced from the subtropical regions. The date is the dominant tree in the oases of tropical deserts and is "king of tree crops" there. The Humid Subtropical climate is an important contributor of tree crop products. The prevailing climatic conditions of abundant rainfall, somewhat evenly distributed throughout the year, and moderate temperatures favor the growth of a wide variety of trees and so furnish man with a large number and variety of tree crops. Regions under the influence of the Mediterranean Subtropical climate are rich in the number and variety of economically important tree crops which they produce. Those found within this climate are mainly of two types: the dry land type—-those which are native to this climatic regime and so are naturally adapted to the prevailing summer drought of the environment; and the intensive irrigated type—-those moisture-loving crops which have been introduced here from humid regions. Climatic conditions are adverse to tree growth in regions having a Dry Subtropical climate; but because of their adaptability to arid and semi-arid conditions such as exist in regions under the influence of this climate two tree crops—-mesquite and piñon nuts—-are to be found. The two Humid Continental climates—-the Long Summer Phase and Short Summer Phase—-are similar in all climatic aspects with one exception—-their length of summers. Therefore, the kind of trees and, naturally, the kind of tree crops in both are for the most part the same. Both of these climactic types are rich in the number of crop-yielding trees which exist within their bounds and provide man with a large variety of tree crops. The Marine West-Coast climate is not a large contributor of commercially important tree crops. Though the vegetation of regions having this climate is or has been essentially one of heavy forest growth, only three tree crops of any note are derived directly from this source. This type of climate with its rainy, cloudy, cool weather, and its continually moist soils is not favorable for maturing and harvesting fruit crops; therefore, only a few of the hardier tree fruits are grown in any appreciable quantities. The variety of tree crops and the many uses for which they are employed are more extensive than the average person realizes. From a study of this thesis it may be concluded that tree crops constitute a basic part of man's daily life. Without them he would bo deprived of a great variety of products which add to his welfare and comfort.
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