Buds and Twigs

Buds and Twigs
by Robert Adrian

The branch of science concerned with the interrelationships of living organisms and their environment is called ecology. The ecological factors that in great part determine the general disribution of the various tree and shrub species are tempera, uo and moisture.

Particularly great are the differences in the temperature requirements of the various species; that is the principal factor limiting their occurrence and determining the yrs that can be cultivated in a given environment.

The area where shrubs grow naturally in the wild is called the area of natural distribution and may best be depicted on it map. When we compare the areas of distribution of the various species it is evident that some require specific temperature levels since they occur, for instance, only in southern Europe or only in coastal regions or only in a continental limate. On the other hand, some are very adaptable and grow throughout most of Europe from south to the far north, often beyond the Arctic Circle. Examples includes the dog rose, blackthorn, and water elder.

Other shrubs extend far to the north but do not occur in southern Europe with its warm climate. Such typical northern species are the European black currant and willow spiraea.

Still other species, e.g. the rhododendrons, green alder and rock currant, grow in the high mountain climate of central and western Europe but never occur in the north polar regions. Some shrubs grow mainly in the part of western Europe with a constant mild oceanic climate but are absent in the continental areas with severe winter frosts. These include the English holly, hawthorn and common broom.

Others are common shrubs of southern Europe but in central Europe are to be found only in warm, sheltered situations. In such localities they are often relics from the warmer period following the Ice Age when their area of natural distribution extended farther north, with only those growing in warmer situations surviving when the climate changed again. Examples are the cornclian cherry, barberry, bladder senna, box and traveller’s joy.

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