Temperature, another of the important climatic factors in plant ecology, acts directly upon plants through its influenced upon the rates at which the physiological processes take place. In general, we can say that each plant species has an optimum temperature associated with each of its function, such as photosynthesis, flowering, fruiting, or seed germination, and that there exist some overall optimum yearly temperature conditions for its growth in terms of size and numbers of individuals. There are also limiting lower and upper temperature for the individual functions of the plant as well as for its total survival. Temperature acts as an indirect factor in many other ways. Higher air temperatures increase the water-vapour capacity of the air and thus induce greater plant transpiration as well as greater rates of evaporative loss of moisture from the soil.
In general, the colder the climate, the fewer numbers of species that is capable of surviving. A large number of tropical plant species cannot survive below-freezing temperature. In the severely cold arctic and alpine environments of high latitudes and high altitudes only a few species can survive. Application of this principle explain why a forest in the equatorial zone has many species of trees, whereas a forest of the sub arctic zone may be dominantly of one, two or three tree species. Tolerance to cold is closely tied up with the ability of the plant to withstand the physical disruption that accompanies the freezing of water. If the plant has no means of disposing of the excess water in its tissues, the freezing of that water will endanger the cell tissue.
On the basis of preference for, or tolerance to, temperatures, plant geographers distinguish the following: megatherms, plants favoring warm regions; microtherms, favoring cold regions: and mesotherms, favoring regions of intermediate temperatures.
It is a law of bioclimatology that there is a critical level of climatic stress beyond which a plant species cannot survive; hence that there will exist a geographical boundary that will mark the limits of this distribution. Such a boundary may also be referred to as a frontier. Although the frontier is determined by a complex of climatic elements, it is sometimes possible to single out one climatological element that coincides with the plant frontier. Another example is seen in the distribution of the yellow pine (Pinus ponderosa) of western North America. In this mountainous region annual rainfall varies sharply with elevation. The 20 in (50 cm) isohyet of annual total precipitation enclose most of the upland areas having the yellow pine. It is the parallelism of the isohyet with forest boundary that is significant, rather than actual degree of coincidence.
Wind is seen as an important environmental factor in the structure of vegetation in highly exposed position. Close to timber line in high mountains and along the northern limits of tree growth in the arctic zone trees will be found to be deformed so that the branches project from the lee side of the trunk only (flag shape), or the trunk and branches are bent to near-horizontal attitude facing away from the prevailing wind direction.
In such habitats the effect of wind is to cause excessive drying on the exposed side of the plant. The tree limit on mountain-sides thus varies in elevation with degree of exposure of the slope to strong prevailing winds and will extend higher on lee slopes and in sheltered pockets.
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