Every plant on Earth occurs in a certain region that can be marked out on a map. This region is termed its area of natural distribution. Knowledge of these areas is of great importance, because the woody plant grows for hundreds, and even thousands, of years in the given area, and is adapted to the climatic conditions of the locality, thus making it possible to determine whether it prefers a coastal or inland, lowland or alpine climate. The study of other data reveals what temperatures it is able to withstand in winter, as well as its soil and moisture requirements.
In the individual parts of its range, where it is exposed to varying conditions, the woody plant varies in appearance as well as in certain other characteristics. This is especially true of species with a wide area of distribution, where one will find several local races, called ecotypes. Thus, for instance, the pines of northern Sweden, Finland and Lapland have long, slender, tapering crowns which are better equipped to bear the heavy load of snow usual in northern winters, and make the most of the rays of the low-lying sun. Pines from the lower, dry elevations of central Europe, on the other hand, have broad crowns to shade their site as much as possible. Similarly, mountain spruces growing at high elevations, or in cold valleys, have slender, narrow crowns. Variations in climate are also reflected in the onset of budding and in other aspects of the annual cycle of growth. Such characteristics that have developed as a result of mutations which have given an environmental advantage over a period of several generations are hereditary and passed on to the offspring.
The boundaries of the area of natural distribution, however, do not constitute a line of demarcation outside which plants would not prosper. It is necessary to realize that plants, includeing trees and shrubs, migrated in the wake of the retreating ice sheet in the period following the Ice Age and thus established new ranges. Some species have not as yet occupied the whole of the area that would suit their needs. Other species came up against insurmountable obstacles such as mountain ranges, and broad river valleys, though beyond these they would again have found areas where they could thrive. In Europe, such obstacles are the Alps and Carpathians, which blocked the further northward spread of the Austrian pine (Pintts nigra), sweet chestnut (Castanea saliva), and Turkey oak ( Qiiercus cerris). These would otherwise have found favourable conditions in the warm Rhineland and other regions north of the Alps. The general pattern of distribution can lead us to deduce that species found beyond the Arctic Circle, such as birch, pine, aspen, alder and spruce are frost resistant, whereas species growing mainly in southern Europe will probably be more sensitive to cold; in more northern areas only warm and sheltered sites will be favourable for their growth. When introducing new exotics, a thorough study of their natural environment is the main condition of efficient selection and subsequent, vigorous growth.
Let us take a brief look at the purposes of, and problems associated with, the introduction of exotics. The main reasons for the introduction of new trees from other regions or continents is for their fruit, to increase the yield of timber, or to enrich the assortment of ornamentals. The introduction of new plants dates from long ago; its beginnings can be traced to the cultured peoples of ancient times, who concentrated primarily on the introduction of fruit trees that were important as a source of food. This practice has continued with greater or lesser intensity up to the present day, reaching a peak during the past two centuries when man was settling the vast expanses of the Americas, Africa and Asia. At first, these plants were imported as novelties for parks and landscape gardens, but, later, economic reasons also prevailed, and exotics were planted in forestry plantations.
Unfortunately, most of these trees and shrubs were set out without any plan, and without sufficient knowledge of their requirements, so that the greater part of these attempts ended in failure. Some successful plantings, however, e.g. the eastern cottonwood, Weymouth pine and Douglas fir, showed how great the economic importance of imported forest trees could be after several decades. The turn of the nineteenth and twentieth centuries thus marked the beginning of a new stage in the planting of imported trees in forests, with due attention being paid to their needs. The results appear to be very promising. For example, the North American giant fir (Abies grandis Lindl.) attains an annual increment of 25 to 28 cubic metres of merchantable wood per hectare. In Belgium and France its annual increment is also more than 20 cubic metres per hectare. Similarly, in England, France and Germany the Douglas fir (Pseudotsuga menzicsii Franco) gives a 40 to 60 per cent higher yield than native species. The largest areas planted with exotic trees and shrubs in woodlands are to be found today in Europe in those countries that have insufficient supplies of their own wood, such as Great Britain and Spain, where there were areas totalling about 250 thousand hectares in 1950. Good results and economic considerations are today forcing the further introduction and plaining of exotic trees and shrubs, even in those European countries with large areas of productive forests. A further application has been found for these* exotics as ornamental plants in parks and city streets.
The introduction of exotic trees and shrubs, however, has attracted much opposition. The main reason for this opposition is the fear of new diseases, and the fear of changing the characteristic aspect of the landscape. The first can be prevented by systematic checking of imported seeds and plants and of pests on the new plantings. The second can be avoided, if new plants are set out wisely and with forethought. Naturally, imported trees and shrubs should not be planted in the immediate vicinity of nature reserves and in important landscape areas Most of them differ very little from native species at first glance, and the layman is hard put to distinguish the Sitka spruce and Douglas fir from the Norway spruce or giant fir and the Caucasian fir from the European silver fir. Furthermore, the Ice Age in Europe greatly limited the assortment of species. Most of the genera found in America grew also in Europe at one time, e.g. Sequoia, Tsuga, Carya, Juglans, Liriodendron, etc., so that their rank as exotics in this area is of only recent date. The augmentation of Europe’s assortment of trees and shrubs is, therefore, only the remedying of the results of natural catastrophes, and it is not totally contrary to the natural scheme of things. The main mountain ranges in Europe, the Pyrenees, Alps and Carpathians, run from west to east. During the Ice Age, many trees were blocked by these mountains in their southward retreat before the advancing ice sheet. In America, where the main mountain ranges run from north to south, there was no such barrier, and all the trees that moved south were able to return again to the more northerly regions in the period following the Ice Age. That is why trees that perished in Europe survived in America and also why genera occurring on both continents are much more numerous in the New World.
Exotic trees and shrubs can also be put to good use in parks and city streets. They not only brighten such places and make them more varied, but are, in addition, a source of instruction, enabling nature lovers to expand their knowledge of the plants of other lands. The well-planned introduction of exotics, therefore, is in no way harmful; on the contrary, it can be very rewarding, both from the economic and the cultural aspect.