These are the factors of habitat, which are concerned with and influenced by precipitation (rainfall, snow, dew etc), atmospheric humidity, temperature, light, velocity of wind etc. The behaviour, duration and intensity of those factors constitute the climate of a region or habitat. Some authors differentiate climatic factors into microclimate or microenvironment and macroclimate or macroenvironment.
The microclimate is strictly restricted to localised area. Hence microclimate refers to local combination of atmospheric factors, which due to local variations in the climate, topography etc differ from the prevalent general climate i.e., macroclimate of the region. Macroclimate is the general atmospheric condition of a region.
Climatic Factors Affecting Plant Growth
It determines the type of vegetation in particular areas. The main type of precipitation are rainfall, snow and dew. Of these, rainfall is most important as it affects the growth of plants, distribution of plants regarding types etc—so rainfall of a place has direct influence on vegetation.
With the increase of rainfall atmospheric humidity is increased. It acts indirectly through the medium of other factors like temperature and light. Mosses, ferns, lichens with evergreen trees abound in an area with heavy rainfall;
whereas tropical countries with low rainfall area reckoned as arid regions—there the vegetation is sparse. Rainfall with seasonal periodicity of restriction brings about more change in vegetation than constant rainfall in a particular area.
Seasonal restricted rainfall results in growth of few species whereas, constant rainfall with cloudy sky almost throughout the year favours the formation of tropical rain forest with abundance of species with lianes, ferns, epiphytes etc.
On the basis of seasonal distribution of railfall, there are three main types of vegetation viz., forest vegetation, grassland vegetation and desert vegetation.
In warm countries where annual rainfall is heavy although, only the formation of forest vegetation takes place there. Lighter rainfall (where rainfall is heavy during summer but low during winter) is responsible for the formation of grassland. Regions with very low rainfall both in summer and winter correspond with deserts.
According to Schimper, (1903, ’35), “the type of vegetation (both in tropical and temperate regions) is conditioned by the total amount and seasonal distribution of rainfall and by the humidity of the air’.
(b) Atmospheric humidity
Humidity of the air has a great influence on plant life, as it affects the water relations of plants. The rate of transpiration is directly influenced by atmospheric humidity.
Water vapour present in the air is responsible for humidity, and the concentration of water vapour is greatly controlled by temperature. Atmosphere i.e., air can hold maximum amount of water vapour at any temperature, and at that temperature the actual pressure of water vapour (absolute humidity) as expressed by percentage of the maximum is called relative humidity.
The difference between the absolute humidity and relative humidity expresses the saturation deficit of the air. Saturation deficit is directly related with the evaporating power of the air and thereby largely controls the rate of transpiration.
The rate of transpiration is higher if the relative humidity of the air becomes lower. Atmospheric humidity is increased owing to precipitation, soil evaporation and plant transpiration.
Hence in a dense vegetation, the air beneath the vegetation is more humid. In humid regions, moisture-loving delicate plants generally grow and such plants become more elongated with long internodes and thinner leaves.
It plays an intensive role in plant organisations, as vital activities in plant life are confined between maximum and minimum temperatures. Before maturation, each crop plant requires a certain number of “effective heat units”- this is called thermal constant which varies with different crop plants.
The temperature and the length of the vegetative season affect the physiognomy of the individual plants and also the whole of the vegetation. Temperature also exerts an influence on habitat, economy and struggles of plant communities.
In a country with low average temperature the vegetation cannot be similar to that of a country having an average higher temperature. so temperate and tropical flora differ in species, physiognomy etc. This is due to heat exerting an influence on habitat, growth, formation etc of a plant community.
In the equatorial region where high temperature is linked with humidity, evergreen tropical luxuriant vegetation develops with dense crowding of species.
In tropics where high temperature is associated with drought (i.e., dryness), dwarfish growth, shrubby heath-like formation result; control in the loss of water i.e. check of transpiration associated with sunken stomata, thick cuticularised leaves, reduction of leaves by spines etc. is found in plants growing in the region.
In arctic and alpine climates where temperature is very low and the favourable period for blossoming is very brief, annual plants are rare. there perennial herbs with long resting period and varieties of forms are common.
There are different types of plants which can withstand different temperatures. According to some authors entire geographical distribution of plants depends on varying temperatures e.g., tropical plants require constant high temperature for their maximum growth, the alpine plants require very low temperature and are adapted for short summer period for flowering, fruiting etc.
Light is one of the most important factors from the stand point of physiological processes. Light is directly concerned with the growth and development of plants as it helps in the formation of growth hormones, chlorophyll etc.
Synthesis of food by green plants and the process of transpiration also depends upon light. Light affects plants by its intensity, quality and duration.
Light intensity is influenced by altitude and humidity – e.g., light intensity increases with altitude while humidity reduces light intensity. Intensity and duration of light play important roles in the construction of vegetative shapes of plants—this is seen in the different shapes of sun (heliophytes or photophilic) and shade (sciophytes or photophobic) plants.
The characteristics of sun and shade plants are as follows :
(i) Intense or strong light retards the growth of shoot.
(ii) Sun plants have short internodes and small narrow leaves while shade plants have long internodes and large leaves.
(iii) Sun plants are shorter and stockier than shade plants.
(iv) Sun plants have thick and small leaves; leaves are often stiff and coriaceous with distinct veins. Cuticle of leaves is very thick and waxy; palisade tissue and vascular tissue are well developed, spongy cells are more or less poorly developed.
Shade plants have soft and succulent leaves, veins are small. their leaves are thinner but larger and broader, and without well developed cuticle. Palisade tissue is practically lacking, spongy cells are well developed.
(v) In sun plants lignified tissues are well developed; cells are smaller with less intercellular spaces. In shade plants lignified tissues are poorly developed, cells are larger with intercellular spaces.
(vi) Sun plants have dense covering of hairs, while shade plants are less hairy.
(vii) Shade plants are delicate and without any well developed mechanical tissues. Sun plants are not delicate, their mechanical tissues are well differentiated.
(viii) Plants growing in shade become etiolated, their stems are often weak and pale yellow, while sun plants have stout green stems.
(ix) The stems of shade plants are taller and more branched than that of sun plants.
Formation of flowers, fruits and seeds is favoured by intense light. Duration of light period, i.e., photopheriodism has marked influence on growth, flowering and fruiting of plants.
In nature, duration of light period varies from less than twelve hours or twelve hours (on the equator) to more than twelve hours during summer (in Arctic regions).
According to duration of light there are three groups, like (i) Short day plants – The plants produce flowers when the duration of light per day is less than 12 hours e.g., dahlia, tobacco, etc. (ii) Long day plants – The plants produce flowers when the duration of light per day is more than 12 hours e.g., spinach, radish, etc. (iii) Day neutral plants – Plants grow either in short day or long day conditions i.e. they have little response to length of day light period e.g., cotton, tomato etc.
The quality of light e.g. red, blue, ultra-violet rays of light also influences the growth and development of plants. Red rays generally induce the greater development of tissues and thereby cause the cells to enlarge. Blue-violet rays, on the other hand, inhibit the growth and enlargement phases of cells. Ultraviolet rays have injurious effect upon the growth of plants.
(e) Velocity of wind
It has both direct and indirect effects on plant life. The direct effect of wind is mechanical; strong wind is responsible for uprooting trees and breaking off branches, twigs etc.
Wind also exerts influence on structure, strong wind coming from one direction affects tree growth and form; those trees, subjected to one-sided high wind, are low with bent trunks and short shoots which irregularly branch.
Wind also causes erosion of soil. The indirect effect of wind is physiological. Owing to violent wind, desication due to transpiration takes place; so the growth of the axes and leaves is hampered or decreased.