Ecological Factors in Plants: Top 4 Types (With Diagram)

The following points highlight the four types of ecological factors in plants. The types are: (1) Climatic Factors (2) Edaphic Factors (3) Physiographic Factors and (4) Biotic Factors.

Ecological Factors in Plants

Type # I. Climatic Factors:

The climatic factors of the habitat are rainfall, dew (precipitation), atmospheric humidity, velocity of wind, temperature of the air and light.

The term microclimate or microenvironment refers to strictly local combination of atmospheric factors which owing to local variations in the climate, topography or other factors differ from the prevalent general climate of the region (macroclimate). These microenvironments (microclimates) may be arranged both vertically and horizontally within even a small area.

Within each area embraced by one particular macroclimate there may exist several microclimates, some of which may differ so much from the macroclimates as to be of considerable ecological importance; as for examples marked local climates on mountain summits, on steep slopes, in deep ravines, etc., which are relativity cool and damp and also in shallow basins.

Very limited cases of local climate are seen in conditions obtaining on the side away from the wind of a rock or even a large stone which may protect the plants in its immediate neighbourhood from strong light and wind, thus creating a microclimate. Similar conditions are also found in the sheltered side of a wood and in the lower strata of the woodland vegetation. In a rooted green plant, the root system is certainly in a different environment compared to the aerial parts. The microclimate or microenvironment of the roots in the soil is really very unlike the general atmospheric climatic (macroclimate) in which the shoots occur.

A formation of deciduous trees may establish itself on a wide range of soils if certain more or less definite combinations of climatic factors are prevalent in a parti­cular locality characterising definite types of climate. A particular type of climate pre­vailing over a region thus tends to produce in that region a definite, predominant and stable type of vegetation or, as it is ecologically termed, climax type of vegetation.

The climate of Western Europe is such that it generally results in deciduous forests as the natural climax vegetation. Where the deciduous forests are lacking in Western Europe, its non-occurrence can be explained by the local operation of some other interfering strong ecological factor, particularly of the soil (edaphic). The deciduous forests are then replaced by other formations adapted to the particular local soil conditions.

Rainfall:

In general rainfall is of little direct significance although rain is of tremendous importance and a factor of prime significance for plants as source of soil moisture. Some absorption of rain water also takes place through cutinised and suberized coverings of shoots. Most of the rain that falls on land results from the condensation of water vapour derived from the surface of the oceans and thus the amount of rain that falls on a parti­cular area depends at least to some extent on its proximity to an ocean or sea.

Mountain slopes usually have a relatively more wet side facing the sea from which the moisture is derived than the landward slope and as a result vegetation differs considerably on sea­ward and landward slopes.

Rainfall operates indirectly through the medium of other habitat factors, its direct importance being connected with its influence on the available water in the soil. Rain is also important in regard to its effect on humidity of the atmosphere. Dry mosses and lichens absorb moisture from a humid atmosphere and the abundance of these plants which grow on rock and bark surfaces is in direct ratio to the humidity of the climate.

Quite recently the probable importance of high humidity under exceptional environ­mental conditions has been further emphasised through the experimental proof that higher plants also can absorb water through their leaves from a saturated atmosphere and this together with contained solutes, can exude through the- roots into the soil which (soil) may thereby attain field capacity.

The claims that the presence of a forest causes an increase in rainfall over the amount which would fall on bare ground seem to have been based on insufficient data. Though the presence of forests certainly raises the humidity of the air near the ground level, it is certainly inadequate to cause any heavy rainfall, for only major mass move­ments of air actually bring about precipitation.

When rainfall increases the atmospheric humidity it exerts the same influence on the plant community as shade conditions (i.e., weak illumination). Structural pecu­liarities in plants which accompany the weak light climatic conditions are more of less the same as plants grown in full light but in a moisture saturated atmosphere.

The time of the year, when moisture is deficient, is always a critical factor. In areas where rainfall is restricted chiefly to winter months compared to the areas where most of the rain falls during spring and early summer, the floras generally have very few spe­cies in common. Nearly every difference in rainfall, either in the annual, total or in the seasonal distribution, brings about a significant difference in the natural plant popu­lations.

The character of a soil depends primarily on climate. A cool wet climate (Eastern Himalaya) means that a large amount of rain water falls on the soil while on the other hand evaporation is slow. The tendency of water will therefore be to move through the soil chiefly in the downward direction. A comparatively dry warm climate, on the other hand, may result in alternate upward and downward movement of soil-water derived from rainfall.

In the tropics, the low rainfall region (12.5 cm) is arid desert with an exceedingly sparse vegetation, highly adapted to scarcity of water. The high rainfall regions (200 cm or above), on the other hand, will bear tropical rain forests, the most complex and highly developed vegetation in the world. The soils at both the regions would be totally different though they may be derived from absolutely identical rock.

Heavy rainfall disintegrates the same rock, which remains largely bare in the deserts, to some depths only in high rainfall regions and the deep soil, is covered partly mixed with a layer of humus, which would constantly have a considerable amount of moisture. The nature of the soil, thus distinguished by the difference in rainfall, determines the nature of vegetation.

Heavy rainfall must be one of the determining factors in forming the evergreen forests of tropical regions and the deciduous forests in Western Europe where rainfall, though heavy, is restricted to a very short period only. In rainy climates, above average rainfall is sometimes associated with reduced plant vigour and crop yield.

Atmospheric Humidity:

This invisible water vapour content of the air is usually expressed as relative humi­dity. It is really the actual moisture content of a volume of air expressed as a percen­tage of the maximum quantity that the air can hold at the prevailing temperature.

Moisture in the atmosphere intercepts much of the radiant energy before it reaches the earth’s surface. Even then, the radiant energy received at the earth’s surface, though greatly reduced by such atmospheric absorption, is much more than normal for a great many plants. The detrimental effect of full sunlight upon a vegetation is consequently more pronounced in dry than in humid atmosphere.

On the other hand, deficiency of light and heat under shade becomes more critical in humid atmosphere than in dry. Moist air favours the growth of many fungi which readily become serious pests on higher plants. A series of rain showers, within a short period or even cloudy weather (high humidity conditions) favours the growth of rusts and other parasites on plants, com­monly the field crops.

Velocity of Wind:

The action of wind on plant formation as an ecological factor may be direct or indirect. If the wind velocity is high, the plant communities are frequently in danger of being uprooted and blown away by strong winds. The result is that in those regions, the vegetation is largely composed of species which have prostrate habit of growth and a very tenacious underground root or rhizomatous stem.

The occasional nor’ wasters and gales which sweep through our country cause considerable mechanical damage in breaking off twigs or branches of plants, particularly in tall trees, thus prevent them from growing above a certain height.

A cover of low herbaceous plants strongly reduces the velocity of wind along the ground and a forest cover reduces the velocity still further. By means of this action in slowing down wind currents near the surface of the earth, a plant cover prevents wind- erosion of soil. A windbreak is generally a densely planted strip of tall trees usually about a thirty metre wide and placed at right angles to the direction of the prevailing wind, in order to reduce its velocity and this may be a good protection for fields and orchards. A well planned windbreak definitely increases total crop production.

Plants developing under the influence of violent drying winds never attain the degree of hydration essential for the expansion of the maturing cells to normal size. As a result, in general all organs are dwarfed. This is very commonly observed on an approach to sea coast or arctic regions, where the stature of the trees is greatly reduced.

Trees vary greatly in their response to the force of strong wind. Many trees such as pines and oaks often grow flattened against the ground while other trees in the same habitat remain erect. Tree branches often develop only on the side away from the wind. This may simply be a pressure effect. Asymmetrical crowns of trees sometimes result from the death of all buds on the windward surface. Constant high wind velocity may cause woody plants to sway chiefly in one direction, causing an adaptational response which consists in flattening of the trunk branches and roots in a plane parallel to the direction of the wind.

Lodging, a form of wind injury, caused usually by violent winds, peculiar to plants like wheat, maize, sugarcane, etc., frequently flatten the plants against the ground. The prostrated plants, if the stems are not too mature, may become partially erect once more. This adaptation is due to differential growth at still meristematic lower nodes of the plants.

When strong wind carries particles of ice or soil, it is a very effective abrasive force. Bark and buds are sometimes eroded away from the windward sides of woody stems if they are situated in exposed locality. Crops grown in sandy soils in windy climates are very often damaged in this way.

A plant cover is very effective in preventing the blowing away of soil by wind (soil erosion). But if the plant cover is thinned or destroyed even at one point, the wind may scoop out the soil to such an extent as to expose the roots of the adjacent plants, bringing about death and devastation over a wide area.

The eroded and blown sand may be deposited over wide areas on other plants near their roots on the ground, bringing about a sharp reduction in aeration about their roots following deposition of new soil upon the old. Very few plants can tolerate inadequate aeration and as a result many perish. Those that survive this change adapt themselves by developing adventitious roots at successively higher levels on the stem as sand deposition, caused by wind, occurs.

Along the sea coasts, the salt sprays of the waves and breakers are carried ashore by wind and certain sensitive species of plants are sometimes injured considerably by these sea wind-borne salt sprays. Fortunately, air borne salt content diminishes with increa­sing distance from the shore and it has been found that only those species, which are tolerant of salt, grow nearest the sea. Salt damage from violent storms has been seen as far as 80 kilometres or so inland.

In areas of uneven topography snow is swept from the windward slopes of the peaks and deposited on the away slopes and sometimes in hollows and cracks, winter after winter. Vegetation types on slopes where accumulation of snow is little and those where it is excessive are fundamentally different and their boundaries very sharp.

The indirect general effects of wind in increasing the rate or cuticular transpiration in plants by constantly bringing unsaturated air into contact with the leaves is well known. Wind velocity increases with height above the surface of the soil, so the nearer the leaves of a plant are to the ground, the less certainly will be the danger of excessive transpiration caused by wind.

The wind velocity seems to determine the limit of the height to which a particular plant may grow, for theoretically the higher the plant is, the greater will be the rates of transpiration and consequently a plant can only attain such a height where the loss of water from the leaves is lower than the absorption of water from the soil. The adaptive response is seen in many tall trees where the height is limited by the decay of those upper shoots which are above the critical level where transpiration is in excess of absorption.

Temperature:

The effect of temperature on plants, as in all living organisms, becomes obvious when we compare arctic with tropical vegetation which shows large morphological and anatomical differences. The active growth and reproduction in summer in plants is in marked contrast with the dormant state in winter, particularly in plants of the tem­perate regions. Such marked differences in the kinds of vegetation and their seasonal relations are brought about only at narrow range of temperatures, between 0-50°C, for there is little living-activity below 0° or above 50°C.

All physiological activities of living cells including reproduction are greatly in­fluenced by temperature and as a result temperature as an uniformly operated ecolo­gical factor significantly affects the vegetation of a locality.

The landward migration of plants from their ancestral environment of the open seas certainly necessitated marked adaptations for enduring wide variations in tem­perature that characterised the newer environment. It is true that as there is no region on earth which is so cold or so hot that some plants cannot live there, adaptations acquired by plants are not so perfectly adjusted and as a result extremes of temperature frequently cause permanent injury or even death.

Considering the vegetation of the earth as a whole it is possible to correlate to some extent certain vegetation characters with annual or seasonal temperatures. It must be borne in mind that a consideration of temperature as a separate habitat factor is diffi­cult for action of wind and atmospheric humidity (more definite and particular in their operation) cannot be separated from the temperature effect.

Some investigators believe that temperature, i.e., geographical latitude is probably of the greatest importance in determining distribution of plants on the earth.

According to them the entire vegetation of the earth can be divided into four following classes:

(i) Megatherms—plants which require more or less constant high temperature throughout the year for their optimal growth and development. These plants occur mainly in the tropical zones.

(ii) Mesotherms—these plants are capable of enduring considerably lower tempera­ture during some period of the year, such as winter months. Many plants of the tropical and subtropical regions of the world can be included in this class.

(iii) Microtherms—plants of the temperate regions of the earth demand much lower temperature for their growth and development. These plants are incapable of enduring high temperatures, even for some part of the year, such as summer months. All high altitude plants (up to about 3,600 metres) of the tropical and subtropical regions can also be included in this group.

(iv) Hekistotherms—these plants are restricted only to arctic and to alpine regions, above 4,800 metres in tropics and about 3,600 metres in the temperate zone regions of the world. The hekistotherms have the lowest temperature demand of all plants and they are also adapted to short summer which prevails in the extreme temperate regions of the world. These plants seem to endure long and extremely cold winter months without any permanent injury.

Temporary protoplasmic adaptation affording a certain measure of immunity to low temperature injury is called low temperature-hardening and the perennial vegeta­tion of cold climates regularly develops this particular hardiness in autumn. In spring this hardiness seems to be lost and it is not uncommon in summer conditions that the same plants are killed by temperatures far above those which they endure easily in winter.

Considering high temperature injury of plants, aside from its role in desiccation and in bringing about disparity in the balance of respiration and photosynthesis, (res­piration is greatly in excess of photosynthesis) high temperature injures and kills proto­plasm.

The principal adaptational features which protect plants against high tempera­ture injury are:

(a) High rate of transpiration which may cool the leaves, at least to some extent,

(b) A vertical orientation of the leaf blades compared to the leaves at right angles to the sun’s rays,

(c) White incrustation on the surface of the leaves which reflects rays that would otherwise be absorbed increasing the temperature of the leaf surface,

(d) A covering of dead hairs shading living cells,

(e) A bark insulating phloem and cambium and

(f) Protoplasm with a low moisture content.

Stems are more frequently injured by high temperature than leaves or roots. Stem girdle (presence of discoloured and shrunk bands of tissue in stems) is due to scorching of tender stems at the soil surface when the soil temperature becomes very high. Seed­lings and herbs may be killed by contact with such hot soil.

As we have seen before, interaction between various factors, climatic, edaphic or biotic, frequently prevents a plant from attaining its theoretical maximum growth with respect to temperature.

It is well known that when we consider equal areas, the number of plant species increased in an equatorial direction from both north and south poles, although other factors such as stable climate and the land continents play important parts in bringing about such distribution. The longitudinal variations in temperature may have greatly influenced the rate of evolution, for the rate of mutation has generally been found to increase directly with increase in temperature.

Light:

The energy necessary to sustain life on earth is derived from sunlight directly by green plants or indirectly by other heterotrophic organisms which must eventually depend upon various organic compounds synthesise by the green plants only. The essential connecting link between living organisms is provided by the ability of chloro­phyll to absorb radiant energy and convert it into chemical energy of carbohydrates.

Light also exerts much stimulating influence upon plants, especially on the differentia­tion of tissues and organs. It can be truly said that with the exception of water, light is unrivalled in its influence upon the morphological and anatomical characteristics of vegetative life.

To an ecologist, however, the importance of light as a factor determining the for­mation of plant communities is not so great as its physiological importance, on account of the fact that the action of light is comparatively uniform over wide areas. The gene­ral features of plant communities are not usually affected by light except in the case of relatively small plant communities, growing in deeply shaded situations, such as caves, etc.

However, individual species within a plant community, shows particular internal structures which are modified in relation to the operation of the light factor. The majo­rity of plants have definite requirements with regard to light.

Some thrive only in full intensity of sunlight others favour a moderate amount of shade while still others can tolerate deep and continued shade. Most plants can adapt themselves to moderate variations in the intensities of light. There are some plants, however, which exhibit a maximum range of adaptability: they are apparently equally at home in both strong light and in the deep shade. Plants may be classified ecologically according to their relative requirements of sunlight and shade; those growing best in full sunlight are called heliophytes and those preferring shade (low light intensities) are sciophytes.

Among heliophytes, some species can grow fairly well under shade (facultative sciophytes) com­pared to those sun plants which cannot do so (obligate heliophytes). Sciophytes can be similarly divided in two groups depending upon their ability to tolerate high light intensities. One of the very common and disagreeable weeds in India, Elephantopus (Compositae) seems to be a strict sciophyte growing luxuriantly only where there is deep and continued shade.

In comparison to plants grown in shade (sciophytes), species thriving best under full sunlight conditions (heliophytes) usually exhibit some characteristic features, e.g.; (a) thicker stems with well-developed xylem, (b) shorter internodes with more prolific branching, (c) usually smaller, but thicker leaf blades with stomata and veins smaller and closer together, (d) leaves generally hairy with more hairs per unit area, (e) thicker cuticle and cell walls, (f) better developed palisade tissue with weakly developed spongy cells, (g) intercellular spaces reduced in size and number, (h) leaf blades not flat and oriented at other than right angles, (i) roots longer and more numerous, resulting in higher root/shoot ratio, (j) greater fresh and dry weights of both roots and shoots, (k) lower photosynthetic (proportionately greater chlorophyll a/b ratio) rate but higher respiration rate, (l) more rapid transpiration, (m) higher salt content and consequently higher osmotic pressure of the cells, (n) cell sap more acidic, (o) higher carbohydrate/nitrogen ratio, (p) pronounced vigour in flowering and fruiting, (q) earlier blossoming and (r) a significantly greater resistance to injuries due to temperature, drought or parasitic infection (Fig. 763).

In moist climates, vegetation tends to be a complex series of tall trees, low trees, shrubs, herbs, mosses, etc., and a high per­centage of the flora, except perhaps the trees, must needs be facultative or obligate sciophytes. In dry climates, however, the reverse is true but even here there are many plants that thrive only in the shade of larger ones. Shade conditions are mostly brought about by plants themselves—forest trees produce the shade conditions under which the undergrowth community of vegetation exists. Different types of wood­lands cast different amount of shades, as for example, undergrowth in the very deep shades of pine woods is almost absent (deep shade) compared to the luxuriant ground flora of an oak wood (shade not so deep).

Underwater differences in light values are very important factors which deter­mine the depth to which submerged plants can live. Red algae as a group have lower light requirements than any other algae. This may be a case of chromatic adapta­tion, for it is well known that phycobilin pigments in Rhodophyta can make exception­ally efficient use of weak light of relatively shorter wave-lengths, e.g., green. Has the presence of non-green pigments then any bearing upon the ability of aquatic flora to grow in deep water?

Type # II. Edaphic Factors:

The soil:

In a seed plant though the root systems usually comprise not more than a quarter of the whole plant on a dry weight basis, they are so finely divided that they may occupy a mass of soil greater than the volume of the atmosphere surrounding the shoot. There is no doubt then of the tremendous amount of surface contact between the soil and the plant. The plants, more or less completely dependent upon the soil for anchorage, water and nutrients, cannot be over-emphasised.

Thus the factors of the soil, the edaphic factors, are of supreme importance to the ecologist not only due to this intimacy of con­tact between the plant and the soil, which are strongly influenced by each other, but also due to the tremendous complexity of the structure of the soil. While climatic factors determine the general layout of vegetation, the edaphic factors are primarily responsible for local differences found within the same climatic regions.

With the very beginning of agriculture, the important part which soil plays in plant growth, was recognised and appreciated. Man soon discovered that in his attempt to control environment for the benefit of his good crops, little can be done in altering climatic factor but much can be accomplished with the soil.

In the first text-book on plant ecology written by Warming in 1895, plants were classified according to the characteristics of the soil, into oxylophytes (on acid soils), halophytes (on saline soils), psammophytes (on sandy soil), lithophytes (on rocks) and chasmophytes (on rock fissures and crevices, etc.).

Soil study now ranks as a highly technical independent branch of science and is known as Pedology. Soil is the surface material of the earth in which plants grows con­taining the underground parts of higher plants as well as algae, fungi and bacteria (soil flora) and also a soil fauna, ranging from protozoa, worms, insects to small-mam­mals, such as moles, mice, etc.

Soil may be defined as the weathered superficial skin covering of the earth’s crust with which are intimately mixed living oragnisms and the products of their decay (humus). The earth’s crust is created and continuously affected by surface agencies— primarily heat, frost, wind and above all water. These agencies act upon the rock from which the soil is derived and continually also on the soil itself.

The soil is also essentially affected, sometimes even newly created soil, e.g., peat, by the vegetation growing upon it. The spaces in the soil remaining between the solid particles are filled with water and gases. The continuing action of heat, wind, water, etc., naturally tends to the develop­ment of stratification—differentiation of soil into distinct layers from the surface down to the parent rock. This is the natural soil.

The basic framework of most soils consists of small fragments of mineral matters which have been derived from the parent rock by disintegration by weathering agencies, both physical and chemical. Besides these inorganic particles, an organic constituent (humus) is nearly always present.

According to the particular rock from which the soil is derived, the nature of the inorganic soil particle differs both chemically and physi­cally; but all soils contain basically three main inorganic chemical substances—alumino- silicates, silica and calcium carbonate.

With alumino-silicates are associated various ele­ments which form alkaline salts, e.g., calcium, magnesium, potassium and sodium —all except sodium, being elements on plant nutrition. Iron salts, mostly as ferric ions are also always present in all soils and which give many soils the typical brown or red colour. Besides these, phosphates and sulphates are always present. Nitrogen is partly derived from the organic humus and also from fixation of atmospheric nitrogen by certain soil micro-organisms. Traces of other elements in extremely minute quantities are found in most soils.

Almost all rocks liberate considerable quantities of soluble salts as they weather and the marine sediments also yield considerable quantities of sodium chloride by leaching. Salt accumulation on the surface of the soil is sometimes called ‘alkali’ though the deposits are not always basic. There are two major types of ‘alkali’ in saline soils. ‘White alkali’ consists chiefly of chlorides and sulphates of sodium, potassium, calcium and magnesium. During dry seasons, the concentration of these salts produces a thin, white incrustation on the surface of such soils.

The white alkali soils are fairly porous. The growth of most crop plants is adversely affected, except halophytes (can thrive even in soil with white alkali content of more than 5-8%), when the ‘white alkali’ content rises above 0’5%. ‘Black alkali’ soils are dominated by carbonate or bicarbonate of sodium and sometimes potassium—about 10-15%. These salts yield strong bases like caustic soda or caustic potash by hydrolysis, which, as is well known, has a strong corrosive action on plant tissue and humus.

As a result, large amounts of organic matters are dissolved into such soil which gives the soil surface a blackish colour. Black alkali soils are structure less and more or less impervious to water and gases. Owing to these characteristics of the black alkali soil most plants cannot tolerate as much of it as they can of the white alkali.

An interesting type of soil is found in the regions of high rainfall, especially on sandy ground. Rainfall here exceeds evaporation and iron and other minerals tend to be washed down from the surface layers. The minerals are deposited lower down as a hard pan which is naturally dark in colour, in contrast to the pale colour of the leached surface layer. The leached surface is acidic, and as the hard pan is impervious to water, bog vegetation often develops there.

Calcium is the dominant basic ion in the soil and when present in quantity, imparts the physical and chemical stability to the formation of the soil. These ions sometimes aggregate into fine colloidal particles and sometimes also into compound articles, thus giving a structure to the soil.

The basic reaction of the soil is usually due to the presence of free calcium carbo­nate. The extremely acid soils with pH values between 3 and 4 can support only spe­cialised natural vegetation (oxylophytes). These soils are sometimes referred to as ‘sour’ and they need the addition of lime to render them fertile.

The terms calcicoles or calciphiles for plants growing exclusively on calcareous soils and calcifuges or calciphobes (lime-hating soils) for those found only on acid soils, have long been recognised by ecologists. Calcifuge plants are typical of sandy soils. Some plants may occur indiscriminately on either types of soil but the reality of the above ecological classification may not be overlooked. Species of one genus may differ and even physiological types adapted to either type of soil, may occur within one species, e.g., ^‘edaphic ecotypes’ of Trifolium repens (Snaydon, 1962).

The species of moss genera, Tortella and Neckera can thrive only in the soil or rock surface which is rich in calcium or lime. These soils are apparently alkaline in nature and the species of such plants are calcicoles (acidophobic); other moss species such as Polytrichum and all Sphagnum grow exclusively in acid soils and these calcifuges conse­quently strictly avoid lime-rich soil or rock (acidophilous).

The growth of different moss species may thus reveal something of the potentialities and nature of the land on which they grow. Consequently in recent years the importance of moss as ‘indicator plants’ (indicating whether the soil is basic or acidic in nature) is coming to be appreciated in increasing extent.

Sometimes growth of natural flora of a particular habitat may act as an index for judging the quality of land from the point of view of fertility and deterioration. The following species of plants serve as such indicator plants in some dry or semi-dry regions of the Punjab: Salvadora oleoides, Qzyphus nummularia, etc., are indicators of good pro­ductive land; Calotropis procera, Saccharum munja, etc., indicate sandy soils; Tamarix articulata, Butea monosperma and Suaeda fruticosa indicate saline soil in the process of deterioration.

The timber yielding Shorea robusta communities in India, the conifer, Pinus roxburghii seem invariably to avoid calcium-rich soils (alkaline soils) and they can thus only be classed as calcifuge or calciphobe, i.e., acidophilous (best growth between pH 6.0-6.5).

Use of indicator plants in geobotanical prospecting is being extensively employed for indicating the presence of heavy metal ore deposits. Indicator plants, designed ‘universal’ are those species which always indicate the presence of a definite element and are confined to soils over certain ore deposits—mapping the distribution of a parti­cular indicator species, most affected by the minerals sought, has been economically very profitable. Examples are afforded by Astragalus sp. for uranium prospecting in Colorado (U. S. A.), Viscaria alpina for copper in Norway, Silene cobalticola for cobalt in Katanga (Africa), Equisetum arvense for gold in Czechoslovakia, etc.

The four essential constituents of the soil are:

(a) Mineral particles constituting the inorganic framework of the soil,

(b) Organic matter or humus arising from the death and decay of plants, and animals such as protoza, insects, etc., living on the surface of the soil,

(c) Soil water containing dissolved substances and

(d) Air in the soil.

(a) Mineral particles in the soil:

The nature and size of the inorganic particles resulting from the disintegration of rocks not only influence the chemical processes but also directly determine the physical nature of the soil and its effects upon plants. The texture of a soil depends primarily on the sizes of its mineral particles and this feature is of considerable importance to vege­tation because the texture of the soil controls aeration and water-holding capacity of a particular soil.

The mechanical composition of the soil, i.e., the relative proportions of the different sized particles determines to a considerable extent the availability of water for absorption by plants. By mechanical analysis in the laboratory the physical compo­sition of a soil may be determined.

The mineral part of the soil is separated into a number of generally accepted by arbitrary fractions each consisting of particles whose size lies between definite limits, which are as follows:

All soils contain, in fact, particles belonging to more than one of these categories and a soil is conveniently named after the fraction which is preponderant. A loam is a soil which consists of a thorough mixture of particles of widely different sizes with an adequate humus fraction (Fig. 764).

Gravels are soils with a preponderance of large mineral particles above 2 mm in diameter and always mixed with coarse sand as well as some fine sand and silt. Aeration and percolation of water in such soils are extremely free. Thus rain falling on this coarse- textured soil penetrates almost immediately so that very little water is lost. Gravel soils are not very suitable for plant life in general because of their dryness (they lose their water too quickly) and dearth of nutrients.

Sandy soils have a preponderance of particles between 2 mm (coarse sand) and 0^.02 mm (fine sand) in diameter. The mineral particles are mainly silicates. As in gravels, percolation of water and aeration are unrestricted and as a result sandy soils are generally dry, warm soils, warming up quickly in spring.

As in gravels, they are poor in nutrients because of the deficiency in the finer particles of silk and clay with which the bases are associated. Because they are always liable to thorough leaching, the nutrients are usually washed away. The sandy soils are referred to as light soils.

Sandy soils with sufficient proportions of the finer particles of silt and clay and with a little humus readily support deciduous summer forests. Sandy soils make good agricultural soils for certain food crops.

Silt soils are intermediate between sand and clay in the size of their particles. They are the most favourable soils for all types of vegetation as they have considerable water- holding capacity accompanying easy percolation and aeration. The name is derived from the texture of alluvial soil (silt) laid down in the plains by flooding of rivers. Fine silts when wet form heavy stiff soils but unlike clay, they are easily crumbled, when dry.

Clay soils differ very strikingly both chemically and physically from the three types of soil constituents mentioned before. The mineral particles, below 0-002 mm in dia­meter of hydrated aluminum silicates are numerous in such soils and when any soil has more than 30-40% of these particles, it may be called a clay soil. Clay soils may in some measure be secondary in origin as they are not entirely composed of particles of rock-forming minerals but in addition also contain precipitations of dissolved CO₂ and organic acids, derived from the weather­ing of rocks.

Clay, owing to smallness of its particles, is essentially colloidal in nature and is thus able to take up and retain considerable quan­tities of water, swelling up and increasing markedly in volume during the process. Clay soils have the qualities exactly opposite to those of sand; extre­mely slow percolation of water, imperfect aeration and high water-holding capacity. Clay soils are wet, heavy (compared to sandy soils which are light and warm) and cold as they warm up only very slowly.

The very fine silt and clay offer considerable resistance to penetration by roots and hence clay soils are unsuitable for many plants. In plants adapted to these heavy soils (fine silt and clay), the root systems tend to be shallow because of the difficulty of aeration at greater depths. The extent and the degree of branching of the roots are decreased. This becomes very important in those regions where relatively long dry periods intervene between rains.

This effect can be seen on clay grass land where the grasses are mostly shallow-rooted species. Clay soils are sometimes much improved as regards their ferti­lity by an abundance of humus, which make them lighter and secures better aeration and freer water movement.

Loams are soils which have a good mixture of particles of all the different sizes and are the most suitable soils for the great majority of plants as they combine the good qualities of all the types of soils—clay and humus fractions give consistency and water- holding capacity and also supply nutrients essential for plant growth; silts permit capil­lary rise of water and sand particles facilitate good aeration. Loams can be classified as heavy (dominant particle, clay), medium and light (dominant particle, sand) (Fig. 764).

Lime soils are those which are derived directly from limestones containing 90-100% of calcium carbonate. The lime soils are shallow and contain surface-humus saturated with calcium (calcareous soil). These soils support a herbaceous vegetation, e.g., Clematis (calcicole) which tolerates drought exceedingly well.

(b) Organic matter or humus in the soil:

All soils contain organic material derived from the dead remains of plants or parts of plants such as dead leaves, roots, rhizomes, etc., with a small addition of animal ex­creta or dead bodies. When the dead remains are returned to the soil, the complex organic compounds are broken down by the activity of micro-organisms into humus and ultimately mineralised (the process, an oxidative decomposition, takes a long period for completion) into molecular or ionic forms (the chief of which are CO₂ and water) which then become available to the future generation of plants.

The term humus is usually applied to the whole complex of continually disintegrating organic material in the soil. Sometimes the term humus is restricted to the intermediate conversion of humus into brown substance, soluble in acidulated water (humification). Humification takes place only when oxygen supply is deficient.

The soils of deserts in which vegetation is scanty and new soils freshly formed from rocks, apparently contain very little humus, while mature soils with vegetation, the most.

Peat is the term given to a soil which is really a deposit of partially decomposed humified organic matter which accumulates as a layer on a soil. Peat formed on wet soil or under water is sometimes referred to the type of vegetation from which it is derived. If the peat is strongly acid in reaction, due to oxygen deficiency and shortage of calcium, it may be called acid peat or acid humus.

In shallow lakes, where temperature and calcium supply are suitable and oxygen supply plentiful, organic matter is decom­posed into what is known as neutral peat or neutral humus, which is usually neutral but may also be slightly basic in reaction.

In general humus is an important source of mineral nutrients, and the fertility of a soil is usually correlated with its humus content. Peat soil is, however, notoriously low in phosphorus and potassium. Humus, like clay and perhaps, even more efficiently than clay, helps to make aggregate structure of the soil possible.

Humus is the source of food for most soil organisms. Green plants certainly add much more to the soil than what they extract from it. While they absorb only small amounts of soluble minerals they return these (the minerals) plus even greater quan­tities of synthesised organic materials such as starch, protein, cellulose, etc., when they die. The vast and complex groups of saprophytic soil organisms and organisms which grow as parasites on these organic substances thrive only in humus.

(c) Water content of the soil:

The amount of water contained in a soil at any particular moment apparently depends upon various factors. Water-holding capacities of soils of different textures are very different; the amount of water held in the soil depends on the size of soil particles and the amount of humus present. The coarser the particles are, the larger are the air-spaces between them.

A clay, because of its constituent fine particles, takes longer to dry and holds water so very strongly that plant roots can scarcely extract from it nearly all the water present (plants can use nearly all water in sand which is relatively loosely held)—a small percentage of water remains unavailable to the roots all the time (hygroscopic water).

The water content of the soil is more important than any other single ecological factor in causing differences between plant communities. Soil water is not only impor­tant in connection with direct water requirements of plants, it is, as far as it is known, also the medium by which mineral salts essential for plant nutrition enter the plants. Water is also responsible, as we have seen before, for most of the actual soil-forming processes.

A heavy carpet of vegetation sometimes dries out a soil although a close vegetation decreases direct evaporation from the soil surface. The reason for this anomaly is clearly understood if we consider that a soil may lose water more rapidly by absorption through the roots and transpiration from the extensive leaf surface during hot weather than by direct evaporation from the soil surface.

(d) Air in the soil:

The presence of a continuous air system which ramifies all but waterlogged soils is essential to the respiratory activity of underground plant organs and of the soil micro­organisms. Decay of roots, leaves the soil full of minute channels and borrowings also by worms and insects create minute passages through which oxygen can enter from atmosphere and the respiratory CO₂ passes out readily.

Composition of soil air differs only very slightly from that of the atmosphere proving an active gaseous exchange bet­ween the two—the soil and the atmosphere. Good aeration supplies oxygen for root respiration and the energy derived seems to be essential for active absorption of minerals from soil by the root.

In marshy waterlogged soils, the regular aerating process of other soils seems to be inoperative, for water fills up all the places between the soil particles thereby excluding the air. Under such circumstances oxygen deficiency and an excess of CO₂ (from ana­erobic root respiration) may be accompanied by production of other gases in the soil, such as marsh gas or methane. The composition of soil air may easily be an important factor in determining the characteristics of a community, such as a marshland vegeta­tion.

(e) Soil Temperature:

The porosity of coarse-textured, light sandy soils and even of heavy soils such as clay, which are well aggregated, tends to favour a condition of equilibrium between the soil and the atmospheric temperature. But due to lower moisture-holding capacity of light sandy soils they become warm much sooner in spring than the clay. Root injury due to low temperature in winter is more pronounced in sandy soil than in clay.

The rate at which a soil surface is warmed up, is considerably more rapid than the rate of cooling—on an average the temperature of the soil is higher than that of the air immediately above at all seasons.

Temperature of the soil is one of the most important factors which influence the rate at which absorption of water and nutrients from the soil can take place and as such, as we have seen before, plays a conspicuous role in determining the geographical distri­bution of plants on earth.

Type # III. Physiographic Factors:

Strong topographical relief (steep hills and deep valleys) has a profound effect on vegetation largely because it may produce characteristic local climates or even micro­climates and may also modify the edaphic factors of the habitat. These physiographic factors originate from behaviour and the structure of earth’s surface.

Mountains, hills and valleys result from irregularities in the form of earth’s surface. This behaviour of the surface of the earth definitely also includes such processes as erosion, silting up of lakes and river mouths and the deposition of sand along the sea coast by wind.

Steepness of slope especially in comparatively high latitudes increases the exposure to the sun. A steep southern slope receives rays of the midday sun almost perpendicularly while the northern slope receives only oblique rays during morning and evening and sometimes none at all, except perhaps for a short period at summer. This difference in light and heat sometimes affects the vegetation profoundly.

Sometimes striking and almost incredible effects on vegetation are seen due to this difference in as much as the steep northern slope protected from the sun (where fog may often form) may bear virgin forest with hygrophilous ground vegetation, whereas the southern face constantly heated by the sun, can support only a xerophytic vegetation of shrubs and herbs, strongly protected against excessive transpiration.

In fact, there may not be a single species in common in the two slopes separated only, say by 90-120 metres of rock face. This complete contrast in vegetation types may entirely be due to difference in the clima­tes, produced locally in the two slopes.

Slopes may play a very important part in determining the character of the soil. Due to gravity, ground water always moves downwards and in a slope of a mountain, this downward movement of water removes soil from a slope and carries it down and may deposit in the valley.

Sometimes, if the slope is steep, this result in a soil surface of unstable material and the vegetation may be wholly unable to establish there or is just able to maintain a precarious foothold. Water content of a sloping soil surface is less compared to a horizontal one and consequently the soil is drier, for rain water runs off before it can be retained by the sloping soil. This may result in almost complete absence of higher green plants from a sloping hill side.

In general, northern slopes usually bear a vegetation adjusted to wetter conditions than southern slopes in the Northern Hemisphere (opposite is true in case of Southern Hemisphere) and often the same communities of vegetation establish in the southern slopes at higher altitudes, as in general, greater the altitude, wetter is the climate.

The vegetation of a deep sheltered slope sometimes differs considerably from that of an exposed slope, even in the immediate neighbourhood. This effect of shelter on windy sea coasts and mountains is sometimes very pronounced.

Local climates may vary considerably within a few metres or even centimetres on an uneven ground (microclimate). The shelter from wind and heat provided by a hillock or even by a small rock may enable a plant community to establish itself which could not exist if it were not so sheltered.

Eroded rock faces, hills, steep slopes and river banks produce a special class of habitat in which only a certain species of plants can precariously maintain themselves. If the erosion is rapid enough, the habitat may be absolutely devoid of any plant com­munity. Gravel, sand or clay, eroded by water and wind from rock surfaces, may be brought down by rivers and streams and deposited as silt at their mouths, providing new habitats (like salt marshes) for colonisation by plants of different types.

Wind-blown sand along the sea coast accumulates near the coast on which some­times grass vegetation with a creeping underground stem and root system develops and establishes itself on the loose sand. These grasses eventually bind the loose sand together and ultimately render it partially suitable for the other species of plants which ordinarily require a heavier stable soil.

Type # IV. Biotic Factors:

The biotic factors are those involving animals and plants other than the organism being considered. The biotic factors are often overlooked, perhaps because they are the most complex and we know least about them.

The interaction of organisms, the reciprocal effects of plants and animals are sometimes termed Coaction.

The average green plant is commonly referred to and thought of as an autotrophic or independent organism in contrast to fungi, bacteria and animals which cannot synthesise their own food. In reality, however, not even green plants are independent in the strict sense of the word for they are also considerably influenced by other living organisms.

For example, a flowering plant may depend upon insects for pollination, upon birds for seed dispersal and upon fungi and bacteria associated with its roots for absorption of nutrients from soil. Even the CO₂ used in green plant photosynthesis has been released in respiration by other organisms and respiratory oxygen has accumulated in the air by generations of green plants.

The amounts of heat, light, moisture and nutrients available to one plant are all conditioned and determined by the proximity and competition of other living organisms. Environment of any organism is in reality mostly biologic and only partly physical.

Thus the biotic factors of the habitat (so-called because they have origin in the activities of plants and animals) depend directly on the action of living organisms on the vegetation. Plants in any community have a profound effect upon one another just as mutual effects of the individuals of a human society. In the theoretical sense, a parti­cular plant community, adapted to a habitat must also include the soil bacteria, either free living or in symbiotic association with other organisms, earthworm and other soil fauna, parasitic fungi, the snails or the insects which live on the plants or in the soil and the birds that live in the trees which may be destroying as well as distributing fruits and seeds.

The name biome, a major climax community, has been suggested for such a complex of organisms, both plants and animals, naturally living together and forming a sociological unit. It is said to be equivalent to climax or formation. Biome should also include the sheep or cattle which graze on the semi-natural pasture communities and help to keep the plant community in equilibrium.

Out of man’s diverse activities have arisen to a large group of biotic factors which operate on vegetation—farming largely consists of modifying vegetation in certain definite directions. Actually cultivation of the soil and growing of food crops are biotic factors due to intense human interference.

Grazing by domestic animals such as sheep may result in particular vegetation being dominated by certain species of plants, whereas in its absence, perhaps some other species would have dominated the field. Modification of vegetation by destruction by fire (an indirect biotic effect due to man’s activity) may cause a temporary replacement of one dominant species by another—in extreme case it might even lead to the permanent alteration of the whole character of the vegetation.

Animals including man himself react on and determine the nature of vegetation in many other ways. In areas where the soil is loose and sandy, grazing factors may cause serious results, leading to total destruction of the soil surface. Trampling by man and animals may often prevent any vegetation except a few mosses to grow in such soils.

Relics of an ancient era, survivors of a former prevailing flora, are referred to as Epibiotic. When regions harbour such relic species and have not been drastically altered, these areas are termed Refuginum.

Interactions occurring between various plants which grow together in a commu­nity do not seem to affect the community as a whole but perhaps only individual species. In any plant community, the dominant and also the largest species seem to make the greatest demands on the resources of the habitat. As such, the life form of the dominant species is chiefly the outcome of climatic conditions, physiographic and biotic conditions seem to be only of minor importance.

It is evident that in many cases biotic factors cannot be looked upon as definite factors in the same sense as climatic or edaphic factors—biotic factors only modify or limit the expression of other primary important factors of the habitat.