Ecological Factors that Affect the Growth of Plants (With Diagrams)

The ecological factors that affect the growth of plants and determine the nature of plant communities are divided into three types.

The three types of ecological factors are: (1) Climatic factors which include rainfall, atmospheric humidity, wind, atmospheric gases, temperature and light (2) Physiographic factors which include altitude, effect of steepness and sunlight on vegetation and direction of slopes (3) Biotic factors which include interrelationship between different plants of a particular area, interrelationship between plants and animals occupying the same area and interrelationship between soil microorganisms and plants.

I. Climatic Factors:

The important climatic factors of a region are rainfall, atmospheric humidity, wind, temperature, and light. Of these climatic factors each one individually contributes to the general and overall effect of climate by influencing the life processes of plants which constitute the vegetation.

(A) Rainfall and Other Atmospheric Precipitations:

Rainfall affects indirectly through the medium of other ecological factors. As it directly affects the amount of available soil water, the annual rainfall is a major factor in determining the distribution of plants. Many plants like epiphytes and lithophytes have no source of water other than direct atmospheric precipitations. Such plants have special organs for the absorption of water from atmospheric precipitation. For instance, occurrence of aerial roots with special water absorbing spongy tissue called velamen.

The deposition of dew in areas with scanty rainfall is of great importance to maintain vegetation. In the sub-tropical tracts which receive only negligible amount of rains, strong deposition of dew takes place during dry season. Breazeale (1950) has quoted instances when the leaves of certain plants absorb water from saturated atmosphere, and this water exudes through the roots into the surrounding soil which, consequently, may attain field capacity.

Too much rainfall in a particular region determines the type of vegetation not only pertaining to that of humid climate but also types of plants with adaptation for soil percolated with water and against heavy showers. For instance, leaves of the plants growing in equatorial forests have drip-tip and furrows so that excess of water can immediately be removed. The moist climate increases the longevity of plants and their leaves, whereas the dry climate shortens the vegetative period, checks blooming, setting of fruits and maturation of seeds. Aridity also enhances the resting period.

Temperature is perhaps the most important environmental factor which determines the effectiveness of rainfall. Light rains in hot, dry weather will usually have no effect upon the soil moisture content, for the water does not get down to the roots, and quickly evaporates from the soil surface. Heavy rains of short duration may also have little effect upon soil moisture, for the run­off may be great.

(B) Atmospheric Humidity:

This is a very important climatic factor which directly affects the vegetation. It is so, chiefly because of its effect on the rate of transpiration in plants. The most important environmental factor that effects atmospheric humidity is temperature.

In the atmosphere, water is present in the form of water vapors. This is called atmospheric humidity. Evaporation of water from earth surface and transpiration from plants are the main cause of atmospheric humidity. Clouds and fog are the visible forms of humidity.

Humidity is described in three different terms:

(a) Specific Humidity:

It refers to the “amount of water vapours present per unit weight of air”.

(b) Absolute Humidity:

It refers to the “amount of water vapours present per unit volume of air”.

(c) Relative Humidity:

It refers to the “amount of water vapours actually present in the air, and is expressed as percentage of the amount which the air can hold at saturation at the existing temperature”. Absolute and relative humidity change with the changes in temperature.

Absolute humidity is maximum near the equator and it gradually declines as we proceed towards poles. Relative humidity is also maximum near the equator but decreases in the subtropical regions and increases once again in the temperate regions. Thus, the relative humidity is affected by temperature as well as latitude.

Humidity affects structure, form and transpiration in plants. In higher temperature the relative humidity is low and exposed water quickly evaporates and thus the rate of transpiration increases. Transpiration is one of the leading functions through which the habitat of a plant is determined. For instance, the highly humid air within a lowland forest or in a sheltered mountain gorge (narrow defile between mountains), is greatly responsible for the delicate and obviously moisture loving characters of the plants inhabiting that area. Plants like orchids, mosses and lichens depend on atmospheric humidity for their water requirement.

(C) Wind:

Wind is also an important ecological factor which affects both directly and indirectly. The direct effects of wind are to be seen in the regions which are quite often exposed to violent winds. Violent winds often break off twigs or branches of plants and sometimes even uproot the trees and shrubs. Such an effect of wind often prevents the growing of larger trees above a certain height. The vegetation of such areas is mostly composed of species which have a prostrate habit of growth and a tenacious underground root or rhizome system.

Larger plants which are often exposed to violent winds and are adapted to such condition have following important characters:

(i) Their trunk and branches are often bent,

(ii) The branching is irregular,

(iii) The crown presents a very peculiar shape, and

(iv) The leaves are smaller than usual. Sometimes fast, cold winds result in cushioned growth in plants—e.g., Androsace helvetica,

(v) some plants that grow in areas subjected to strong wind all the year round develop an overall shape that offers least resistance to wind.

Thus growth is restricted on the side on which wind effect is most. Its effect is quite common on sea cost and mountain plants as there the wind is more effective in killing buds and thereby checking branch development on windward side (Fig. 1.2).

Indirect effects of wind are more significant. Wind velocity has great effect on the rate of transpiration of plants. As the fast blowing air currents remove layers of humid air from the vicinity of leaf surface, rate of transpiration markedly increases. With the increasing altitude, wind velocity also increases thus promoting the rate of transpiration. Plants growing at higher altitudes show stunted growth because of the effects of wind.

The plants growing at lower altitudes have less chances of being put to the effects of violent winds and undergoing excessive transpiration due to wind action. On account of the wind action, the height at which a plant can grow depends on its ability to absorb and transport water rapidly enough to replace that lost as a result of transpiration. A plant cannot survive under conditions where loss of water by transpiration is greater than gain in water by absorption from the soil.

Wind action will be most severe when soil temperature is extremely low. Very low temperature of soil almost or entirely arrests the absorption of water by the roots. While a majority of plants are very sensitive to wind actions, some show great degree of adaptability. For instance, Vaccinium myrtillus is a comparatively large bush two to three feet high under normal conditions, while growing on high mountains develops underground rhizome and root system and its aerial branches do not project more than an inch above the soil surface.

(D) Atmospheric Gases:

The Atmosphere or thick gaseous mass envelope is essential for all living beings. The air surrounding earth, within 15 kms affect weather and influence organisms.

All atmospheric gases are usually available in proper amount to living organisms because, concentration of gases does not vary in the environment, hence they are not considered as part of changing environment. Beside these, dust particles, smoke, microorganisms, pollen grains and various gases coming from industries and volcanoes like SO₂, NH₃, SO₃ etc., are also present in atmosphere.

Decomposition of dead plants and animals on muddy soils also releases some organic gases like methane into the atmosphere. In industrial area, sometimes plants die due to excess of smog. Plants on the road sides contain dust particles on then leaf surface. These particles influence photosynthesis and respiration, due to which growth is stopped and sometimes plants die.

(i) Oxygen (O₂):

Oxygen is used in respiration by all organisms all the time. Green plants during day time give out oxygen in the atmosphere during photosynthesis. This oxygen is released through stomata.

The oxygen is consumed by terrestrial and aquatic animals for energy production and they release CO₂ which is used by plants. This cycling of oxygen occurs in nature. Microbes also use oxygen. It is also required for burning.

A little quantity of atmospheric oxygen gets converted into Ozone (O₃) by photochemical reactions. Ozone layer covers the gaseous envelope around the earth and prevents the ultraviolet rays at that level. Ultraviolet rays are harmful to living beings. This conversion of oxygen into ozone does not cause any imbalance between oxygen and carbon dioxide.

(ii) Carbon dioxide (CO₂):

It constitutes about 0.03% of the air. It is more in water than air. It is one of the raw materials for photosynthesis of plants. Photosynthesis increases in CO₂ up to 15 to 20 times of what it is in normal air. Microorganisms living in soil release CO₂ by respiration and increase the percentage of soil-CO₂. Green plants take CO₂ from the air and are responsible for cycling of CO₂. CO₂ is also produced by burning of organic matter.

(iii) Nitrogen (N₂)

About 79% part of air is nitrogen. This is primary source of nutrients for plants and other biological systems. Proteins which are synthesized by amino acids and essential for protoplasm, and stored as food in the plants. Proteins also release energy by oxidation. Nitrogen is an essential constituent of chlorophyll also. All metabolic reactions are completed with the help of enzymes and enzymes are proteins.

It is also a part of DNA and RNA – the genetic material. However, nitrogen is not directly taken by plants and animals from the air. Atmospheric nitrogen is inert, and it is converted to available forms for plants by microbial activity, thunder and also by biological nitrogen fixation. Herbivores obtain their nitrogen requirements from plants and carnivores from herbivores. The nitrogen in the body of animals and plants is acted upon by microorganisms on their death. In this way nitrogen goes back to atmosphere to complete the cycle.

(E) Temperature:

Temperature is the “master factor” in the distribution of vegetation over the earth, though its action is always interwoven with those of light and water. Temperature influences almost all activities of plants—photosynthesis, respiration, absorption of water, transpiration, germination, growth and even reproduction.

Direct Effect of Temperature:

Temperature, as a habitat factor, may operate directly or indirectly.

Directly it operates in two important ways:

(i) It affects the rate of different physiological processes of the plants and consequently influences their germination, rate of growth and development.

(ii) Temperature range and fluctuations in an area go a long way in determining the kind of plants which will be able to grow and survive in that area. Plants greatly vary in their tolerance to withstand a particular range of temperature and its fluctuations.

Indirect Effect of Temperature:

Temperature operates indirectly through its influence on soil factors and other climatic factors. For instance, the rise in atmospheric temperature accelerates the rate of transpiration through its action on saturation-deficit of the air.

Another very important indirect effect of temperature which has significant effect on the vegetation of hilly areas is commonly known as cold air drainage. During spring and summer nights, the air in the close proximity of hill tops and upper hill slopes is cooled by radiation. This cool air flows down into the valley and lower hills where it brings about extreme cooling or even freezing.

Cold air drainage is an important factor in determining the distribution of particular plant communities in hilly regions. The ability to survive freezing temperatures varies greatly in different species and is one of the important factors which determine northward and altitudinal distribution.

A species also frequently includes geographic races which vary in their ability to withstand freezing. The ability to withstand freezing is commonly referred to as frost resistance or frost hardiness. Low temperature may affect plants both during the dormant stage and in the stage of active growth.

Winter injury may result in damage to the roots, bark and buds and even death of the plant. The death of plants when exposed to freezing temperature is not simply due to the direct effect of low temperature but is the result of formation of ice in plant tissues.

Adaptations against Changing Thermal Conditions:

Many plants have various means to guard against changes in thermal conditions. For instance, plants growing on the eastern side of a forest often suffer from night frosts where the sun rays strike them early in the morning. As an adaptation against frost the starch changes into fat during autumn. The fatty oil in the form of emulsion depresses the freezing point and increases the power of resistance to frost.

Again during winters the insoluble organic reserves may transform into dissolved substances like sugars which depress the freezing point of plant tissues and thus prevent any lethal injury. Presence of such substances like sugars, oil and resinous bodies in the protoplasm greatly enhances the ability of cells to withstand extremes of temperature.

Excess of water in plant tissues adversely affects the ability of plant to withstand extremes of temperature. For this reason the young shoots of temperate trees which contain plenty of water suffer from late frosts but the older shoots are not damaged. Similarly the dry seeds can withstand a temperature as high as 100°C but if they are soaked in water they cannot endure temperature higher than 70^oC.

In certain cases plants develop special structures to protect their delicate organs against extremes of temperature. For example, a number of arctic and alpine plants have a grey cotton-like hairy outgrowth. Buds are protected by bud scales and hairy envelop.

Effects of High Temperature:

The effects of high temperature are altogether different. Besides increasing the rate of transpiration and affecting the soil water contents, high temperature has certain other injurious effects which may consequently result in the death of the plant. If temperature rises above maximum limits, the plant becomes inactive, and may develop choruses.

High temperature tremendously increases the rate of respiration and plant may die of starvation. Plants are variously adapted to withstand high temperature. Plants growing in extremely warmer regions are usually succulent, their leaves very much reduced and stomata are sunken and covered with hairy outgrowths.

In general, the effects of temperature are more obvious when considered from the wider point of view of plant geography rather than only plant ecology. Temperature normally does not affect the types of plant communities to be found in a particular area so much as it determines the species present.

Limits of Atmospheric Temperature:

A temperature that is most supporting to reproduction, seed germination, growth and development is called optimum temperature. Life activities of other organisms are at their best during optimum temperature range.

However, organism can still survive, though with low efficiency, at a temperature below (minimum) or above (maximum) the optimum temperature. Most organisms tolerate temperatures 0°C to 50°C. Generally, 25°C (±5) is optimum for a large number of organisms.

Depending upon the response of plants to temperature of environment, the entire vegetation of the earth can be divided into following four classes:

1. Megatherms:

Plants which require more or less constant high temperature throughout the year for their optimal growth and development, e.g., dominant vegetation of tropical rain forests.

2. Mesotherms:

These plants are capable of enduring considerably lower temperature during some period of the year such as winter months, followed by high temperature, such as during summers. Many plants of tropical, subtropical regions of the world can be included in this class, e.g., vegetation of tropical deciduous forests.

3. Microtherms:

Plants of temperate regions of the earth need much lower temperatures for their growth and development. These plants are incapable of enduring high temperatures even for a few months of the year. All high altitude plants (upto about 3600 metres) of the tropical and subtropical regions can also be included in this group, e.g., mixed coniferous forest.

4. Hekistotherms:

These plants are restricted only to arctic and alpine regions above 4800 metres in tropics and above 3600 metres in the temperate zones of the world. The plants have the lowest thermal requirement and they are also adapted to short summer which prevails in the extreme temperate regions of the world. They endure long and extremely cold winter months without any permanent injury, e.g., alpine vegetation.

(F) Light:

Light is the primary factor in photosynthesis and flowering. It is evident, therefore, that light has a very profound ecological importance. The study of light as an ecological factor is complicated by the fact that the sun emits not only the light rays used for assimilation, but in addition heat rays and ultraviolet rays, both of which influence many other processes in the plant. The table given below gives some idea of the action of different parts of the spectrum upon the living plant.

1. Effects of the Quality of Light:

Generally speaking, there exists no sharply defined distinction between the action of the different rays. All rays that the plant absorbs exert a definite heat effect. Yellow and red rays are photo-tropically active, though to a much smaller degree than the ‘blue-violet rays. Growth, differentiation and tropic movements are influenced chiefly by the blue-violet and ultra-violet rays. This sensitiveness to the blue-violet part of the spectrum has only been proved for phototropic reactions.

Light has a two-fold influence upon growth. One part of the spectrum provides the energy for carbon assimilation, and hence for the production of raw material, while another part acts as a stimulus, and influences directly the rate of growth and differentiation. Even the form of the plant is determined to a certain degree by light.

2. Effects of Light Duration:

Initiation of flower buds in plants is another factor affected by light period. Plants are grouped according to their response to day length into what are called short-day plants, long-day plants and day-neutral. The short-day plants in general develop flowers when the days are less than 13 to 14 hours long (e.g., Chrysanthemum, Xanthium, Salvia, Maryland Mammoth tobacco). The long-day plants develop flower when the days are longer than 13 to 14 hours (e.g., Hordeum, Petunia, clover, radish, lettuce etc.). The day neutral plants are insensitive to day length (e.g., Tomato, tobacco, cotton, roses carnation, sunflower etc.).

3. Effects of Intensity of Light:

Light is subject to cyclic fluctuations in intensity, depending upon the altitude, latitude, season, and climatic conditions. One of the most striking effects of light intensity is poor growth of grasses when shaded by trees with dense foliage. The undergrowth of flora may be almost entirely absent in extremely dense forests, where trees grow very closely together.

Many species of plants are shade tolerant (Heliophobous plants or sciophytes), they are capable to survive and grow in low light intensity, while others are shade intolerant (photophilous plants or heliophytes) which require high light intensity for growth and survival. In certain cases 50 per cent of full sunlight is needed.

Shade tolerance plays a significantly important role in forest plants where their seedlings must become established in very poor light conditions on the floor of the forest. Although shade tolerance is an important factor in determining the establishment of a species in a particular area, availability of soil water and essential elements are equally important.

Shade conditions are always invariably associated with increased atmospheric humidity, as compared with conditions of full light. Plants grown under shade show certain peculiar anatomical features which distinguish them from plants grown in full light.

II. Physiographic Factors:

Physiographic factors of the habitat include form, behavior and structure of the earth’s surface which consists of erosion of land, silting up of river, lakes, accumulation of sand and shingle along the sea coast etc., and also topography and elevation of land from sea level. Strong topographical relief, such as steep hills and deep valleys, has a profound effect on vegetation, chiefly, because it produces characteristic “local climates” (also called microclimate).

Some of the important physiographic factors are discussed below:

(i) Altitude:

Effect of altitude is best visible in the mountains. With the increase of attitude (viz., height from sea level), the temperature and atmospheric pressure decreases, but wind velocity, relative humidity and light intensity increase.

These changing climatic conditions also bring about changes in the pattern of vegetation of the region. Starting from the base of the hills, the vegetation pattern changes from tropical to temperate, taiga, tundra and polar (in the region totally covered with snow) (Fig. 1.3).

(ii) Effect of Steepness and Sunlight on Vegetation:

In addition to determining the character of the soil, slope also brings about variations in soil water contents. The steepness of slope determines the rapidity with which water flows away from surface, the degree of wetness of the soil surface, the intensity with which the sun rays can heat the soil surface and the denseness and height up to which vegetation can occur. Steepness of the slope also affects the amount of humus and other degenerating organic matter in the soil. On very steep slopes most of the humus is carried away along with rain water (Fig. 1.4).

(iii) Direction of Slopes:

This factor plays an important role in determining the rainfall, and hence on the characteristics of vegetation of the area. The mountain chains steer wind into definite directions, capture moisture from the wind on certain sides and condense aqueous vapours in the form of clouds and rains in higher region; thus, on certain sides rich vegetation is found, while the other side of the mountain with lesser height, have only xerophytic vegetation (Fig. 1.5).

Edaphic Factors-The Soil:

Edaphic or soil factors play significantly important role in determining the nature of vegetation. These factors are mainly responsible for the more local differences between plant communities to be found in great climatic regions. As the plants depend upon soil for anchorage, water and mineral nutrients, edaphic factors go a long way to affect the vegetation.

According to W.B. Turrill and Marsden Jones (1945), the edaphic factors have manifold effects on the vegetation—such as germination of seeds, size and vigour of the plant, size of the root system, formation of wood in the stem, susceptibility to various plant pathogens, flower formation and setting of fruits etc.

Here the term “soil” has been used in a wider sense. From the ecological point of view, the soil might be defined as that part of the earth’s crust which bears plant life. According to this definition, the solid rock covered with lichens is as much soil as forest humus, or mud in still pools. Soil profile, soil texture, tenacity of the soil, air in the soil, soil water, soil temperature, organic and inorganic nutrients are the conditions which determine the kind of vegetation in an area.

(1) Soil Profile:

If we dig a trench and examine its cut end we find that soil is made up of distinct layers which often differ in colour and which are known as “soil horizons”. The sequence of horizons from the surface downward is called “soil profile”.

A soil profile consists of the following major horizons (Fig. 1.6):

1. Horizon A:

This is the topmost soil and is composed of the bodies of plants and animals undergoing humification. In a mature soil this horizon is subdivided into distinct layers in progressive stages of humification.

From the surface downward the following layers can be conveniently made out in this horizon:

(A) A₀₁, (B) A₀₂ (C) A₀₃, (D) A₁, (E) A₂

This horizon is usually sandy and the roots of plants are embedded in this zone.

2. Horizon B:

This is known as subsoil and is formed with clayey soil. Roots develop poorly in this zone. This is composed of mineral soil. In this horizon the organic compounds have been converted into inorganic compounds by the process of mineralization and thoroughly mixed with finely divided parent material. The upper part of the horizon B (B₁) appears as a dark band. The soluble materials which are formed in horizon A are leached by downward flow of water and deposited in horizon B.

3. Horizon C:

It is at the bottom of the soil profile and represents the more or less unmodified parent material which is in the form of weathered rock. Below this zone hard rocks are found. The soil profile and the relative thickness of the horizons differ with climate and topography. For example, in grassland soils (as compared to forest soils) humification is rapid but mineralization is slow.

4. Horizon-R:

This is the lowermost layer or horizon of the soil. It is made of bed rocks which are still un-weathered.

(2) Classification of Soil:

On the basis of its origin or formation, there are two major types of soil:

(i) Residual Soil:

At some places weathering and development (viz., pedogenesis) takes place simultaneously. Hence, the soil occupies the same area where the parent rocks were present earlier.

(ii) Transported Soils:

The weathering takes place at one place. The soil is then transported to another place where it is fully developed.

These soils are classified into the following three categories depending upon the mode of transport:

(a) Alluvial Soil:

The soil is transported by running water.

(b) Colluvial Soil:

The soil is transported due to force of gravity.

(c) Eolian:

The soil is transported by wind.

(d) Glacial:

The soil is transported by melting snow etc.

(3) Soil Texture or Physical Structure of the Soil:

Upon the physical nature of the soil depend a number of fundamental properties—the power of absorption, the water holding capacity and aeration. The determination of the physical structure, therefore, is of importance if one is to assess the value of the soil in ecology or in agriculture. Soil consists of a mixture of different substances. This mixture has been divided into two large groups: the humus, and the mineral constituents.

The humus material are gels of very variable dispersion. Humus always occurs in macroscopic “flocks” or aggregates. It is clear that the size and distribution of these aggregates, even when the total amount present is constant, will influence differently the degree of aeration and water holding capacity of the soil. A very fine divided humus imparts to the soil an impervious structure, and a coarser humus promotes drainage.

Finally, it should be borne in mind that it is in the humus layer of the soil that the fungal and bacterial population is to be found, so that both the chemical nature of the humus and its mechanical condition will exert an influence upon the soil flora.

With regard to mineral constituents of the soil, some of the mineral particles occur as gels, as for instance, the very fine particles which compose clay. The fine particles form a suspension with the soil water, and the degree of dispersion varies, according to the ions present and the salt content, from particles of ultramicroscopic size to particles visible under the microscope.

Now the concentration of ions in the soil, upon which the state of dispersion of the clay depends, varies continuously throughout the year according to the concentration of soil water and the temperature. A mechanical analysis of the soil, therefore, provides only some idea of the physical “capabilities” of the soil.

The diameter 0.002 mm., taken as the upper limit of the clay fraction, makes an important limit in the colloidal properties of the material. For when clays reach a degree of dispersion of 0.002 mm. they behave as suspension colloids. They are very sensitive towards small quantities of electrolytes and hydrogen ions; they cause flocculation, which process is reversed on the removal of the ions. Hydroxyl ions, on the other hand, stabilize the dispersed state of the clay. The clay, in fact, behaves like a negatively charged colloid.

A remarkable property of clay is its plasticity when damp. The particles cohere together, and when present in the soil in small quantities, they increase its water-holding capacity, though when the clay is present in excess, the soil becomes very heavy.

It is well known that a layer of clay is almost impervious to water. When the clay is dry it becomes very hard, and shrinks, with the formation of cracks in the soil. A high percentage of particles as small as 0.002 mm., diameter in the soil is disadvantageous for agriculture, in fact heavy clay soils can be quite unproductive.

Particles of silt in the soil, of diameter 0.002 to 0.02 mm., also retain colloidal properties, and are precipitated by electrolytes. Particles of diameter greater than 0.02 mm., do not exhibit any of the peculiarities of clays. Particles of this size begin to be visible to the naked eye, and the root hairs are able to penetrate between them. Clay, on the other hand, is so thick in consistency that root hairs are unable to penetrate into it.

Coarse silt, of diameter 0.006 to 0.02 mm, prevents the clay particles from becoming too closely compacted together, and is for this reason a most valuable constituent of clay soils, in fact, the fertility of a soil is to a great extent dependent upon the presence of suitable proportions of clay and silt. The fine and coarse sand in the soil also promotes the drainage and aeration of the soil.

If sand is present in the soil in too great a proportion, however, the water holding capacity and the ascent of soil water by capillarity are reduced. As the clay content of a soil diminishes, the humus in the soil becomes more and more important for its fertility, on account of the resemblance between the physiochemical properties of humus and those of clay.

(4) The Aeration of the Soil:

Bacteria, fungi and the plants in the soil use up oxygen in respiration, and produce a corresponding amount of carbon dioxide. Since the provision of oxygen and the removal of carbon dioxide depend entirely upon diffusion to and from the atmosphere, the concentration of these two gases in the soil will depend upon the intensity of respiration and the resistance to diffusion in the soil If the soil is highly impervious to diffusion, the aeration is bad; the oxygen concentration decreases, and an accumulation of carbon dioxide follows.

The intensity of respiration depends upon the partial pressure of oxygen, and upon the respiration depends the growth and vigour of the whole plant. An excess of carbon dioxide, moreover, is poisonous to the plant.

It is clear, then that aeration is a factor of the first importance in ecology. In a cultivated field, the concentration of carbon dioxide in the soil atmosphere is markedly increased by manuring. Too intensive manuring of a good field can depress the aeration of the soil below the normal, and bring great harm to the crops growing in it.

The oxygen content of the water table is also of great importance, and it depends to some extent on the aeration of the soil above. According to Hesselmann, damp moorland soils are almost free of oxygen; and the water which percolates through the soil of a wood loses a great proportion of its oxygen. Stagnant water most frequently suffers from lack of oxygen, and foul products formed in it are due to this oxygen shortage.

In dry climate, plants very sensitive to aeration will grow in a soil which is highly impervious; the soil of steppes might be cited as an example. In very damp climates, therefore, ordinary plants prefer soils with a loose structure, such as the sand soils and mould soils of the temperate climate. In spite of their high water capacity, mould soils are very loose in structure, so that although the production of carbon dioxide is high it does not accumulate in the soil.

This is only another illustration of the importance of the state of aggregation of a soil; in these mould soils the aeration and the drainage are good, yet the water storage capacity is not impaired. It is for these reasons that a good mould soil represents the optimum of edaphic conditions.

(5) Water in the Soil:

The water factor is peculiar in that it is a climatic as well as a soil factor. Effects of water on growth and development, determination of vegetation and water storage capacity of the soil will be discussed here.

Hygroscopicity:

The amount of water available in the soil depends upon the size of the soil particles, their volume and closeness. In the sandy soil, pore space between large-sized sand particles is more and, therefore, water gets drained off very fast. Since water retention capacity of sandy soil is very poor, such soil is called physically dry soil.

On the other hand, in the clay type of soil, the soil particles are smaller and are also associated with organic matter. They collectively form a colloidal system and, hence, water retention capacity of the clay is much more. The power of solid particles (here soil particles) to adsorb water upon their surface is called hygroscopicity; the larger the relative surface, the larger is hygroscopicity.

Types of Soil Water:

Rain water is the main source of water present in the soil. Major part of the rain water runs off and pass down into lakes and rivers etc. This water is generally called runoff water. However, a small part of rain water percolates down deep into the soil and stored there.

The water stored in the soil may be classified into four groups:

(i) Hygroscopic water,

(ii) Capillary water,

(iii) Gravitational water and

(iv) Chemically bound water.

(i) Hygroscopic water:

It refers to the water that is present as a thin film around the soil particles. It adheres very firmly to the soil particles and cannot be removed from there easily. This water is not of much use to plants, and, hence, is generally called unavailable water.

(ii) Capillary water:

It refers to the water present in narrow capillaries in between the soil particles. The water held by capillaries is of great importance for vegetation, since it provides a reservoir from which the plant can draw it in times of low rainfall.

(iii) Gravitational water:

It is that surplus of water which, after rain, gradually sinks downward, under the influence of gravity. This forms ground water or water table. When the rain showers follow one another sufficiently often, this water, too, forms an integral part of soil moisture, and can be of importance in soils like sand, which have a low water capacity. In heavy soils, on the other hand, this water can be definitely harmful, since it inhibits aeration.

(iv) Chemically-bound water:

This water occurs in the form of hydrated oxides of iron, aluminum, silicon, etc. This type of water in the soil is also not available to the plants. Holard, Chresard and Echard Water. Holard refers to the total amount of water present in the soil Chresard or available water refers to the amount of water that can be used by plants. The amount of water that cannot be absorbed by plants is called echard or non-available water.

Field capacity is the total amount of water in the soil, except the gravitational water. It is sum of the total amount of capillary water and hygroscopic water present in the soil. Water holding capacity or storage capacity of soil is the amount of available water in the soil and is equal to capillary water.

(i) Water holding capacity or storage capacity = Field capacity – Non-available water

(ii) Field capacity = Capillary water + Hygroscopic water

(iii) Non-available water = Hygroscopic water

(iv) Water Holding Capacity or Storage Capacity = (Capillary + Hygroscopic) water.

(6) Soil Temperature:

Temperature of the soil has significant effect on the growth and development of plants mainly through its action on the absorption of water and minerals. Low temperature decreases the rate of respiration in the embryonic cells of the root and thereby checking its elongation, resulting in a slower rate of penetration into new areas of soil where water is available.

A plant growing in soil saturated with water may wilt if the temperature of the soil falls below a certain degree, because at a very low temperature roots cannot absorb water from the soil. This effect is primarily due to increased viscosity of both water and protoplasm at low temperature.

Uptake of minerals is also greatly affected at low temperature because the reduced respiration results in less available energy during the absorption process and probably also because of great viscosity of the protoplasm. A certain degree of heat is necessary for seed germination, root growth and microbiological activity in the soil.

All these activities almost cease at or near the freezing point of water. The temperature needed for seed germination and root growth vary with species to species. Microbiological activity is retarded by low soil temperature. As a result, the nitrification processes in the soil are slowed down and plant nutrition and growth are affected adversely. Plants growing on cold soils mostly show prostrate growing habit, whereas plants of the warm soils are usually slender and tall.

Direct radiation from the sun, the heat generated by the decomposition of organic matter in the soil, and the heat from the earth’s interior are the chief sources of soil heat. The temperature of the soil is affected by its colour, texture, slope and water content. Dark coloured soils absorb more heat than those of lighter hue. Sandy soils absorb heat during the day and lose it at night quicker than the finer grained silt and clay.

(7) Mineral Nutrients in the Soil:

Soil is the natural source for the supply of nutritive substances to plants. Compounds of silicon, calcium, magnesium, iron, potassium, sodium and aluminum form the principal chemical constituents of soil. Besides these, the soil also contains small quantities of other mineral elements like copper, zinc, cobalt, molybdenum, manganese, boron, iodine and fluorine: these e elements are commonly known as minor or trace elements, and are required in extremely small quantities tor the well being of plants.

The total amount of mineral elements present in soils depends partly on the nature of the rocks from which they are formed and partly on their age and the extent to which soluble products have been leached away. The chemical composition of different horizons of a soil also shows a good deal of variation.

Acidity and Alkalinity of the Soil:

Soil water is generally a weak solution which contains almost all the mineral elements in sufficient quantity. Hence, soil solution is a major source of nutrition for plants. The elements are usually absorbed in the form of cations. Absorption of mineral elements, therefore, depends upon cation exchange capacity of the soil. This capacity is maximum in clay soils. Hydrogen (H⁺) and hydroxyl (OH^–) ions determine acidity and alkalinity of soils. It is expressed in terms of pH. Fertile soil is slightly acidic, its pH varies from 6 to 7. pH plays important role in determining the type of vegetation.

(8) Organic Matter in the Soil:

The amount of organic matter present in the soil has very significant effect upon the vegetative growth of the plants. Both plants and animals are equally responsible for contributing to the organic matter of the soil.

Dead plant organs and animal bodies are acted upon by microorganisms and become incorporated into mineral substances. The amount of organic matter in the soils ranges from less than one per cent in arid sandy soils to as much as 90 per cent in peaty soil.

The organic matter may occur in following three forms:

(i) Litter:

This is the fresh, dead organic matter, recently fallen to the ground,

(ii) Duff:

This refers to partially decomposed organic matter derived from last year’s or season’s litter.

(iii) Humus:

This is dark-coloured, amorphous, completely decomposed organic matter produced as a result of microbial activity.

Plants obtain mineral elements from the soil and return them to the soil in the form of organic matter. This organic matter is decomposed by microorganisms and these nutrients are once again made available as inorganic elements. This process is called mineralization.

Importance of Humus:

(i) Humus has very high water holding capacity.

(ii) Soil with high quantity of humus shows higher capacity of ion absorption.

(iii) Inorganic nutrients are made available to plants by humus in the soil.

(iv) Clay particles in association with humus form soil aggregates.

(v) Humus is important to enhance aeration and porosity of the soil.

(vi) Percolation of water into soil depends upon the quantity of humus in the soil.

(9) Soil Organisms:

A large number of organisms are generally present in the soil. The soil organisms include bacteria, fungi, algae, protozoa, rotifers, nematodes, earthworms, molluscs, arthropods etc. Bacteria and blue green algae are very important flora of the soil as they fix atmospheric nitrogen and improve soil fertility. The activity of other organisms also improve soil structure for the benefit of vegetation.

The structure and nature of soil is greatly influenced by the organisms present in the soil. These organisms indulge in various activities, such as nitrogen fixation, antibiosis, decomposition of organic matter and formation of soil.

III. Biotic Factors:

Biotic factors have their origin in the activities of living organisms, such as green and non-green plants, and all animals, including man. The activities of these living organisms have profound direct and indirect effects upon growth, structure, reproduction and distribution of plants on the earth. These effects result from the biotic relationships between the plants themselves comprising a plant community, between these plants and animals living in close proximity, and between the micro-fauna and flora of the soil.

Biotic factors may be classified in three groups for the sake of study.

I. Inter-Relationship between Different Plants of a Particular Area:

The plants of a community affect the morphology, reproduction and other activities of other plants of the same community. These plants of a community show intensive competition for food, light, water, essential minerals and organic matter. This kind of competition is usually called intraspecific competition.

Some group of plants exists widely in a particular environment and other group of plants in another environment, and they influence the environment. For instance, big trees reduce the intensity of the sun light reaching the underground small plants. However, big trees protect small plants from strong wind and high temperature.

Following are some examples of plants to show mutual relationship between individual plants growing in the same area:

(i) Lianas:

They are woody plants rooted on ground but climb up with the support of other trees and reach almost on the top of the plants canopy. They are autotrophs and commonly found in tropical or dense forests. Some of the common examples of lianas are Bauhinia vahlii, Entada gigas, Tinospora etc.

(iii) Epiphytes:

They grow on other plants and are not attached to the soil. They are also autotrophs and do not obtain food from the supporting plant, e.g.. members of the family Orchidaceous like Vanda, many mosses and ferns. They obtain water and minerals through their absorbing roots from the soil present in the crevices and cracks on the surface of the supporting trees. Orchids absorb water from saturated atmosphere and also absorb rain and dew drops with the help of special hanging roots. These roots have a special, water-absorbing tissue, called velaman, present outside the exodermis.

(iii) Parasites:

These plants exist on other autotrophic plants, called host, from where they (parasites) obtain their food. The parasites have no contact with the soil. They have special sucking roots, called haustoria, to obtain food from the host plants. The parasites may be either total parasites, obtaining their total requirement of food, water and minerals from the host plant, or partial parasites which draw water and minerals from their host. They grow either on stem or root of the host plant.

Accordingly, the parasites may be of the following types:

(a) Total stem parasites (e.g., Cuscuta);

(b) Partial stem parasites (e.g. Cassytha, Loranthus, Viscum);

(c) Total root parasites (e.g.. Balanophora. Orobanche. Rafflesia etc.) and

(d) Partial root parasites (e.g., Santalum album. Striga) etc.

(iv) Symbiotic Plants:

Lichens are the example of perfect symbiotic relationship between two plants In lichens, the algae (called phycobionts) and fungi (called mycobiont) live together and mutually provide benefits to each other. Here, the alga synthesize organic food and provides the same to the fungal partner, whereas the fungus in return provides moisture and mineral elements to alga.

II. Interrelationship between Plants and Animals Occupying the Same Area:

Animals and plants interact in different ways, such as given below. These interactions may be beneficial or harmful:

1. Pollination:

Bright coloured flowers or highly scented or honey producing flowers attract insects for pollination which is an essential act for the formation of seed and fruit, e.g, Salvia. Calotropis, Ficus, flowers of family Compositae etc. Animals grazing on the ground or eating fruits also sometimes help in pollination.

2. Dispersal of Seeds and Fruits:

Animals of all groups are helpful in the dispersal and distribution of seed and fruits from one place to another. Some seeds fail to germinate unless the same have passed through the gut of birds, e.g., Reus. Grazing animals greatly help in dispersal of seeds and fruits of many plants (e.g., Xanthium).

3. Insectivorous Plants:

Insectivorous plants, such as Drosera (Sundew), Dionea (Venus fly-trap), Utricidaria (Bladder wort). Nepenthes (Pitcher Plant), grow where nitrogen is deficient. They make up the deficiency of nitrogen by eating the insects. Leaves of these plants are usually modified to capture insects, and they also secrete proteolytic enzymes to digest the insects and make nitrogen available to the plant.

4. Myrmecophily:

This term refers to the association of ants with trees. In this kind of association, both are mutually benefitted. The ants get comfortable habitat in the tress, while the trees get protection with the help of ants, (e.g., Acacia, litchi guava, mango etc.).

5. Grazing and Browzing:

Grazing by domestic and wild animals is an intense factor which has its effects on the character of the grazed plant community which we commonly call as pasture. Only when the grazing is intense, striking changes in the character of the vegetation are brought about by this factor. Intense grazing results in almost disappearance of the vegetation and the soil is no longer protected from erosive influences of wind and water.

Arid zones, if subjected to intense grazing, assume desert-like appearance. The characteristic vegetation of the Mediterranean region is the result of excessive grazing. Grazing causes introduction of the nitrophilous communities. Rodents like rats, squirrels, rabbits and birds etc., feed upon standing crops and cause tremendous losses.

III. Interrelationship between Soil Microorganisms and Plants:

A large number of plants and animals, as listed below, live under the soil surface and as a result of their activities vegetation on the surface is greatly affected. Among plants, several algae, fungi, bacteria and roots, rhizoids and rhizomes of some plants are important from our point-of-view. Among animals, protozoa, nematodes, mites, insects, earthworms and burrowing vertebrates have significant effect on the flora.

Some of the important activities of these soil flora and fauna are as under:

1. Decay of Dead Organic Matter:

One of the most important roles played by soil micro-flora is action on dead organic matter present in the soil and its conversion into simple forms which can be used by higher plants as nutrients. This activity results into increased fertility of the soil and, therefore, has significant effect on the growth of the plants growing on that soil.

2. Nitrogen Fixation:

Productivity of both terrestrial and aquatic plants is greatly dependent on nitrogen than any other element. The atmospheric nitrogen is in the elemental form; it is inert and cannot be directly utilized by most plants. The nitrogen which is taken up by most plants exists in the combined form such as nitrates, ammonium salts, or possibly in organic compounds.

It is strange to observe that certain micro-organisms inhabiting soil are capable of fixing free nitrogen of air. Among bacteria, the three genera Clostridium, Azotobacter and Beijerinckia contain species that are nitrogen fixing. Certain blue-green algae, such as Nostoc and Anabaena which inhabit soil are also capable of fixing atmospheric nitrogen. Such organisms greatly add to the fertility of the soil.

3. Production of Growth-Promoting Substances:

Some of the heterotrophic soil organisms, that may include both bacteria and higher fungi, produce growth stimulating substances, thus affecting the growth of the plants present in the near vicinity.

4. Mucilaginous Secretions:

A number of bacteria and blue-green algae secrete certain mucilaginous substances in the soil. The mucilage converts soil micro-particles into large aggregates which adversely affect the growth of higher plants growing on that soil.

5. Plant Diseases:

Soil borne diseases of plants that cause tremendous losses every year are initiated by a number of fungi, bacteria and nematodes living in the soil.

6. Aeration of the Soil:

Burrowing vertebrates, earthworms and decaying roots improve aeration and water holding capacity of the soil.

7. Mycorrhiza:

This is a symbiotic association between fungi and roots of higher plants. The fungi may be present either outside the roots or inside the roots. If the fungus occurs on the outer surface of the root, the mycorrhiza is called ectotrophic mycorrhiza (e.g., Pinus). If the fungus is present inside the root, then the mycorrhiza in known as endophytic mycorrhiza.

8. Antibiosis (Biological Antagonism):

This refers to the phenomenon of complete or partial inhibition (i.e. destruction) of one organism by another either by secreting some substance or modifying its immediate environment. The most common example is the production of antibiotic substances which inhibit important metabolic activities of other organisms.

9. Influence of Man on Vegetation:

In more settled regions of the earth the biotic factors which have caused greatest impact on the vegetation are those which have resulted from man’s manifold activities Farming is one of the major activities that results in changing vegetation in a definite direction. On well-managed farms the impact of human activities is most striking, whereas on badly- managed farms human interference are less intense and, therefore, on the latter the vegetation possesses many natural features.

Human interests in introducing plants from other parts of the globe pose another problem to the native ones, and competition for existence among them becomes more intense. Sometimes introduction of new animals by men brings about significant effect on the existing vegetation. For an example, introduction of rabbits into Australia has become a major economic problem for the natives. These rabbits have caused devastation of large areas of that country.

Modifications of vegetation by intermittent partial destruction by fire is another indirect effect due to man’s activities. When vegetation is burnt, almost the whole of environment is changed. When the fire is extremely severe, the vegetation is almost destroyed and upper part of the humus is also burnt down. This reduces the fertility of the soil.

Compounds of calcium, potassium and phosphorus are converted into soluble forms which easily leach away from the soil thus making it deficient in these minerals. Nitrogen compounds are converted into their gaseous forms and hence disappear. When the fire is over, such areas are invaded by low nitrogen requiring plants such as Marchantia and Funaria.

Fire of low intensity may sometime result in increased soil fertility. When the temperature is below 100°C but well above the normal, bases are released thus raising the pH of the acidic soils. This temperature also promotes the growth and activity of the nitrogen fixing bacteria that are present in the soil. The effects of fire are not always destructive on every plant. Epilobium augustifolium may be quoted as an interesting example whose growth and flowering is stimulated by fire in close proximity.