On the lands of the earth is of prime concern to the geographer. Plants, as stationary objects possessing distinctive physical properties, are an element of the landscape just as fully as landforms, soils, and hydrographic features. That the physical forms of individual plants and of their assemblages vary in wine systematic way with latitude, elevation, and continental position excites the, interest of the geographer and causes him to inquire more closely into the interrelations of vegetation, soils, landforms, and climate. Plants as consumable and renewable sources of food, fuel, clothing, shelter and many other categories of life-essentials, form a great natural resource base essential to man. The ways in which man has used these resources to his advantage, or has been hindered by plants in his progress throughout human history, forms a vital part of historical geography; the present and future development and management of plant resources is a part of the fabric of many studies of economic and political geography.
Our concern here is with natural vegetation, that is, vegetation, which develops without appreciable interference and modification by man. The widespread activities of man in agriculture, herding, forest-cropping and urbanization guarantee that a considerable proportion of the earth's land surface does not bear a natural vegetation.
Floristic or structural approach?
Our selection of study topics is extremely limited in terms of the scope of the science of botany and even of plant geography as a specialized field within botany.
To be sure, where the trees of a forest consist largely of two or three dominant species, it will be desirable to name these species and to designate the forest accordingly. For example, one finds in the plateaus of Arizona and Utah a woodland in which the tree population consists almost exclusively of pinyon pine (Pinus edulis). Instead, the forest must be described according to its structure, that is, by the way in which the living parts of the plants are shaped and distributed in space. Generally, in the classification of the world's natural vegetation, the structural approach is most meaningful to follow.
Bioclimatology and ecology
Our study of plant geography will not be concerned with the historical I aspects. We will be greatly interested in the bioclimatological aspects of plant geography in which the responses of plants to light, heat, and moisture are studied and the knowledge of climate elements and their global distribution is brought into play. If we choose, the bioclimatological approach can be narrowed to the response of a single species to a single meteorological element.
A quite different approach to the question of a plants relation to its environmental influences is to consider together all the organisms-plant and animal-living us one place at one time in terms of their total environment and of their adjustments to both the environment and to each other. Such a whole dynamic system is referred to as an ecosystem; the study of an ecosystem is plant ecology. The ecosystem represents the combined use by plant and animals of the resources provided, in one place at one time, by the surroundings. Air, water, nutrients, heat and light are variously utilized, transformed, stored, and returned to the nonliving state.
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