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- (a) Temperature. It has been experimented that plant requires at least a monthly average temperature of 6°C 'for its growth. If the monthly average falls below 6°C plant life comes to an end. Excessively high temperature is also fatal to plant and may put a limit to plant growth. High temperature raises the rate of evaporation and low temperature turns moisture into ice. Both are harmful to plant. Polar regions, where the growing season is short, the ground is often frozen, snow or ice-bound and the rainfall is very low and the deserts where the rainfall is scanty; atmospheric humidity is also low and where soils are too thin to support trees.
- (b) Rainfall. This is an important factor influencing the growth of a plant. Various types of plants grow in different areas having different amount of rainfall. Hydrophylous plants will grow in damp and humid climate-as they require much water. Xerophylous plant requires very little water and as such they grow in dry climate.
- (c) Soil. The chemical contents of the soil, viz., magnesium, nitrogen, calcium, potassium, aluminium, iron, etc. favor the growth of different plants. The forest does not make great demands on soil fertility, but it is more exacting in its moisture requirements than prairie grass or bush vegetation. Nitrogen requirements are largely obtained from air. Texture, porosity, permeability, and retentive capacity, etc. determine the growth of different types of vegetation.
However the high mountain parts of the world, especially the middle and higher latitudes are too cold or lacking in soil to support trees. Also, it is now thought that the natural vegetation of many grasslands has been reduced or eliminated. For example, savannas occupying as patches in forests or around the margins of tropical forests may have degenerated from the forests as a result of burning by pastoralists of shifting cultivators over many hundreds of years.
(d) Altitude. This factor affects plant growth as temperature falls with the rise in the altitude. Plant species differ with the change in the elevation.
(e) Wind. The areas over which wind flows violently have fewer trees. Excessive flow of wind restricts the growth of trees due to increase in the transpiration of trees. Strong winds, destructive in nature like tornado, typhoon, cyclone, etc. may destroy large number of trees.
(f) Aspect of slope. Mountain slopes function differently on the leeward and the windward sides determining the types of vegetation on the two sides of a mountain. Sloping land even if it is gently sloping favors the growth of certain types of plants, which grow in lands having less moisture retentive character.
Cultural Influences and Forests: With the gradual growth of civilization and maturing of culture, cultural forces began to receive greater importance over natural forces. Man took ages to understand that the relation between forests and their environment is not one-sided but mutual. It represents a nice balance of the right and obligation of all living organisms found in its confines. Nature strives towards ecological equilibrium. This ecological equilibrium had been very often disturbed by man. The laws governing the extent of forest cover were violated by man. The violation is a tragic example of man's folly in the nature's ordered system.
As pressure of population on land in Europe increased as a result of increase in migrants the forest was pushed back with axe and fire. Man in his greed for more land destroyed forests, for human settlement, for constructing roads and railways, for carrying on agriculture extensively or for setting up factories and township.
It is gratifying to note that man has, of late, appreciated the significance of forests and attempts are now being made in different countries of the world to preserve, develop and scientifically manage the forests.
Next: Temperate Forests