In the environment, organic matter is synthesized in its biotic component i.e. biosphere. Autotrophic organisms are the only organisms that can synthesize organic matter using solar radiation and mineral matter taken from atmosphere, hydrosphere and edaphosphere. Autotrophs synthesize organic matter either by photosynthesis or by chemosynthesis. While chemosynthesis is important for cycling of nitrogen and certain other processes in the environment, photosynthesis is the major process responsible for formation of organic matter in the environment. Autotrophic green plants, particularly land plants are most important from the point of view of photosynthetic production of organic matter. In the photosynthesis, carbon dioxide and water are used and a certain portion of short-wave solar radiation is absorbed and expended within the plant cover. In considering the role of plant cover of Earth in the global energy and water balance, it is necessary to consider the amount of solar radiation and water utilised by plants in production of biomass i.e. in photosynthesis. For this, the quantities that are calculated and studied are efficiency of photosynthesis and productivity of transpiration.
Efficiency of photosynthesis: It is the ratio of energy expenditure on the synthesis of biomass to the total quantity of solar energy absorbed by plant cover in an area. Many experimental studies have shown that this efficiency of photosynthesis is very modest and under normal conditions, usually does not exceed 0.1 to 1.0 percent. However, under very favorable conditions, it may increase to several percent.
Productivity of transpiration: It is the ratio of the amount of biomass produced to the quantity of water transpired by photosynthesizing plant cover. This productivity of transpiration usually ranges from 0.5 to 0.1 percent which indicates that photosynthesizing plants use very little water and abundant transpiration in them merely circulates the water in the environment.
Thus general low values of both the above quantities indicate that under natural conditions, plant cover assimilates only a negligible part of available energy and water resources i.e. there is substantial limitation on the use of natural resources in production of biomass in the environment. It is important to establish the causes of this limitation for the study of the relationship of productivity of plant cover to climatic factors. Experimental studies of maximum possible efficiency of photosynthesis in controlled environmental conditions when carbon dioxide of the atmosphere is fully utilized indicate that under such conditions, plants can assimilate 5% or more of the solar energy received and the productivity of transpiration also increases manifold. However, in natural conditions maximum possible photosynthesis and, therefore, the production of biomass is greatly limited by various factors other than the availability of resources.
PHOTOSYNTHESIS WITHIN TERRESTRIAL PLANT COVER
In nature, most of the photosynthesis takes place within the terrestrial plant cover in which different meteorological conditions exist at different levels. The efficiencies of photosynthesis at various levels within the plant cover are not same and are determined by particular microclimatic (meteorological) conditions prevailing at different levels. The microclimatic effects of a forest cover are explained in terms of:
Plant coverage characteristics: These characteristics depend upon:
Density of dominant forms in the forest covers.
Distribution of different forms in the forest covers.
Stratification characteristics of plant cover: These characteristics depend upon:
Total vertical height of plant cover.
Number of vertical strata in the plant cover.
Morphological characteristics of each strata in the plant cover which are determined by branching pattern of plants, evergreen or deciduous nature of foliage, size, density, texture and orientation of leaves.
Importance of the above features can be judged from comparison of tropical and temperate forest plant covers. In tropical forests, average height of tall trees is 46-55 metres, species diversity is 40-100 species per hectare, stratification is strong with 4-5 strata, undergrowth is dense commonly with two upper foliage strata and lower strata being denser. In temperate forests, average height of tall trees is about 30 metres, species diversity is less than 20 species per hectare, stratification is poor with usually 2-3 strata which are almost continuous from low shrubs to top of trees.
In the study of photosynthesis at various levels within plant cover, averaging of the values of meteorological elements at one level along the horizontal line is appropriate and it makes it possible to exclude the influence of individual plants on the meteorological regime. By applying such averaging techniques, following conclusions have been established:
Microclimate within the plant cover may be represented by a series of vertically varying profiles of meteorological elements, particularly of solar radiation, water vapor pressure, air temperature, carbon dioxide concentration and wind speed.
The profiles of meteorological elements show diurnal and seasonal variations.
The average vertical flow of short wave and long-wave radiation, heat and water vapor within the layer of plant covers and the momentum of the system depend substantially on height.
Microclimatic profiles within plant cover
Solar radiation: Plant cover significantly changes the pattern of incoming and outgoing radiation. Short-wave reflectivity of area depends somewhat on the density and characteristics of the plant cover. The albedo of areas having coniferous forests is about 8-14 while that having deciduous forests is about 12-18. Albedo of semiarid savannas and woodlands is much higher.
Large amount of solar radiation is trapped within the foliage canopy e.g. Fagus sylvetica forest traps about 80% of incoming radiation in the top strata of canopy and less than 5% reaches the ground. Such trapping is more pronounced on sunny days.
The foliage canopy absorbs more short-wave radiation than long-wave infrared radiation e.g. in tropical forests of Nigeria, only 7.6% radiation of <0.5 m reaches the ground while 45.3% of radiation of >0.6 m reaches the forest floor.
Effect of the age of plant cover on the penetration of light into the plant cover can be judged by the observation that in Pinus sylvestris forest in Germany, percentage of light reaching the ground floor is about 50% at 1.3 year, only 7.0% at 20 years and again 35% at 130 years.
Penetration of solar radiation within the plant cover generally obeys Bougner-Lambert Law:
I = Io e-KL
Where, I = radiation intensity on a horizontal plane within the plant cover; Io = radiation intensity on a horizontal plane above the plant cover; L = leaf area index; K = Extinction coefficient.
Extinction coefficient (K) is constant for a given species and is related to:
- Amount and type of leaf chlorophyll.
- Canopy architecture and
- Reflectivity of leaves. Its value lies between 0.3 and 0.5 for grass-type plant cover and approaches 1.0 for nearly horizontal leaves. Value of K shows inverse relationship to chlorophyll content and reflectivity of leaves.
In general, light penetration into plant cover depends upon the type of plants (particularly trees), spacing of plants, age of plants, crown density, height of plants (particularly trees) and time of year. Percent light reaching the forest floor in some types of forests is given below:
- Birch-beech forest 50-75%
- Pine forest 20-40%
- Spruce-fir forest 10-25%
- Tropical forest 0.1-0.01%
In deciduous forests, light penetration increases during leafless conditions.
Thus the intensity of solar radiation decreases exponentially from top of plant cover towards Earth’s surface due to absorption and radiation scattering by the surface of plants. The resulting radiation balance, therefore, also decreases in the same direction due to screening effect of plants.
Air temperature: During day time, heating of foliage canopy causes a convectional transfer of sensible heat and so air temperature within upper canopy may be higher than above the canopy or below. At night, the relationship is reversed as upper canopy layer of air is cooled by contact which are both losing heat by radiation and also transpiring slowly.
Modification of thermal environment is due to shelter from sun, blanketing at night, heat loss by evotranspiration, reduction in wind speed and obstruction to vertical airflow.
Blanketing causes lower maximum and higher minimum temperatures and causes lower mean monthly temperatures in tropical and temperate forests.
At sea level, mean monthly differences in air temperature in temperate forest may reach 2.2OC in summer but only 0.1OC in winters. In hot summers, this difference can be more than 2.8OC.
In forests, which do not transpire greatly in summers e.g. forteto oak maquis of Mediterranean area, day temperatures in woods may cause mean monthly temperatures to be higher than in open.
Altitude in the same climatic zone may affect the degree of temperature decrease in temperate forests. At 1000 meters altitude, lowering of temperatures may be twice that at sea level.
Vertical stratification in plant cover modifies the thermal profile within it in complex ways. In tropical forests, dense foliage canopy heats up greatly during daytime and cools rapidly during night. It shows a much greater diurnal temperature range in denser canopy than in the lower strata. Whereas daily temperatures of second story are intermediate between those of the tree tops and undergrowth, the nocturnal minima are higher than either tree tops or undergrowth because the second story is insulated by trapped air both below and above.
Saturation water vapour pressure: This profile within plant cover shows close correspondence with temperature profile both during day and night. Forest temperatures differ strikingly from those in open and the forest water vapour pressures were found to be higher within an oak stand than outside it for every month except December.
At night, actual water vapor pressure almost reaches saturation as the air and canopies are cooled by radiation and convection. Some water vapor is transferred through transpiration from the canopy. During day, upper canopy is air heated by convection and water vapor pressure curve shifts much from saturation curve. Deficit between the two increases the downward and at quite lower level, actual water vapor curve inflects. Towards the bottom of canopy, it reapproaches saturation curve due to transpiration coupled with low air movement and low temperature towards base of plant cover.
The flow of water vapor within the plant cover increases with height because of the influence of transpiration by plants and the momentum of the system declines downwards from the plant cover’s upper boundary as a result of the inhibiting effect of plants on the movement of air. This effect is associated with the reduction in turbulent exchange within the layer of plant cover compared with higher air layers. The coefficient of turbulent exchange within the layer also declines towards Earth’s surface.
Humidity conditions within the plant cover are very much different from those outside it due to evotranspiration characteristics of the cover. It generally depends upon the type of plant cover, density of plant cover, structure of vertical stratification and temperature effects. Time of the day and season also affect evotranspiration and, therefore, humidity within the plant cover.
Evotranspiration generally increases with density of vegetation and within the plant cover, relative humidity may be 3-10% higher than outside. This effect is more pronounced in summers.
Rainforests have high transpiration and so have high humidity inside their plant cover. Mean annual relative humidity excess is reported to be 9.4% in beech, 8.6% in Pinus abies forest, 7.9% in larch forest and 3.9% in Pinus sylvestris forests.
In tropical forests, night exhibits complete saturation while in daytime, the humidity decreases with height.
Carbon dioxide: The profile of carbon dioxide concentration within plant cover shows much diurnal variation due to photosynthetic uptake of carbon dioxide during daytime and respiratory addition of this gas during night. Carbon dioxide concentration in soil is very low and its use by plants is spatially and temporally very inhomogeneous.
During daytime, CO2 concentration decreases from upper canopy towards ground. It reaches a minimum point near middle of canopy. Below this point, CO2 concentration rapidly increases towards ground and becomes equal to CO2 concentration outside the canopy at a point that roughly corresponds to compensation light intensity. It reaches fairly high level at soil surface. This profile is due to photosynthetic depletion of carbon dioxide in upper canopy, equilibrium corresponding to compensation point lower in the canopy and respiratory addition of carbon dioxide from lowest shaded leaves and soil microorganisms.
In the night, concentration of CO2 gradually increases towards ground level due to its respiratory addition.
Wind velocity: The profile of wind speed shows no strong change in day and night but overall wind speed is higher during daytime due to convectional effects. Wind profile within the canopy develops due to steady state boundary layer flow. The profile is logarithmic above the canopy and becomes exponential within the canopy. The zero plane displacement (D) depends on the height of plants. The roughness height (zo) is a measure of community roughness and it is effectively the thickness of a laminar sublayer through which individual elements project. Value of zo is related to height variation and spacing of individual elements which in the plant cover are plants. In extrapolation of logarithmic curve downwards, the zero velocity intercept is found to lie at the height D+zo. If the canopy were rigid, it would have a constant value but variation of surface roughness depends on leaf flutter, movement of branches and leaf streamlining. These variations cause variations in value of zo with wind speed. Surface frictional characteristics are entirely specified by D and zo. The wind profile is major factor in establishment of profiles of saturation water pressure and carbon dioxide within the plant cover.
Lateral air movement is generally lesser within the plant cover than outside it. Even large variations in outside wind velocities do not affect airflow inside forest cover. Vertical stratification structure, leaf canopy architecture, density of stand and season have marked influence on wind velocity profile within a plant cover. For this reason, reduction in wind velocity within the forest cover is different in temperate and tropical forests. Reduction in wind speed from outer edge towards deep inside a forest is greater in tropical rainforests. In temperate European forests, wind velocity at outer edge of forest is reduced to 60-80%, 50% and 7% at points 30 m, 60 m and 120 m respectively deep inside the forest. In Brazilian evergreen forest, wind velocity of 2.2 m/second at the outer edge of forest is reduced to 0.5 m/second at 100 m deep inside forest while at 1000 m inside forest the wind velocity becomes negligible. In this forest, outside storm velocity of 28 m/second was reduced to 2 m/second at 11 km deep inside the forest.
Flow of water vapor and momentum of system: The flow of water vapor within the layer of plant cover increases with height because of the influence of transpiration by plants. The momentum of the system decreases downwards from upper boundary of plant cover towards ground level as a result of the inhibiting effect of plants on the movement of air. This effect is associated with the reduction in the turbulent exchange within the layer of plant cover compared with higher air layers. The coefficient of turbulent exchange within the layer also decreases towards ground level.
The theory of photosynthesis within plant cover and numerical models of this process developed in recent years are based on the general idea of a transition from photosynthesis within a single leaf to photosynthesis within a layer that is homogeneous horizontally but possesses different physical conditions at various heights. Application of the theory of photosynthesis within a layer of plant cover indicates following general conclusions:
When assimilation process is not very sensitive to different meteorological elements, total assimilation within the plant cover strongly depends on the radiation flux for low levels of radiation. For large values of radiation, the total assimilation is independent of radiation flux and becomes dependent on other factors particularly temperature.
Within he plant cover, increase in total assimilation with increase in inflow of CO2 from soil is much slower than would occur if all the inflow of CO2 from soil were to be expended on assimilation. This is because the inflow of CO2 from soil first encounters the leaves located in shade, which are not able to photosynthesize intensively due to insufficient radiation. The general increase in CO2 concentration produced by its inflow from below is compensated by a reduced contribution of CO2 from above. Thus assimilation within plant cover is influenced very little by the upward inflow of CO2 from soil and is largely influenced by flow of CO2 coming downwards from the atmosphere.
PRODUCTIVITY OF PLANT COVER
The productivity of plant cover () is the difference between total assimilation and the expenditure of organic matter on respiration within a plant cover. Thus the productivity of a particular plant cover depends on photosynthesis and respiration in it. The leaves of plants are the major organs association with both photosynthesis and respiration. Therefore, the productivity of plant cover substantially depends on the value of the index of leaf surface (leaf index) and decreases for both very small and very large values of this index. In view of this, the value of productivity of a plant cover is calculated for an optimal value of leaf index i.e. the value of this index that corresponds to the highest value of productivity. From various studies, it has been established that the parameters and factors that affect the photosynthesis within a plant cover also influence the productivity of plant cover. Thus the productivity of plant cover is mainly determined by parameters characterizing the properties of plant cover itself and the climate.
In general, following important points can be observed in relation to productivity of plant cover in nature:
The structure of plants in the plant cover continuously changes throughout their life cycles and photosynthetic activity of leaves is never optimal throughout the entire vegetative period of any plant.
Availability of mineral nutrients in nature is always less than optimally required for maximum possible photosynthesis.
Under natural conditions, water regime of soil is also not constantly maintained at optimally required level.
Thus the productivity of plant cover in real natural conditions is always less than theoretically possible maximum level due to complex interactions between a variety of biological, climatic and soil factors.
Climatic factors and productivity of plant cover
In conditions of sufficient moisture, two climatic factors i.e. photosynthetically active radiation and temperature are particularly important in relation to productivity of plant cover.
The influence of radiation and temperature on productivity of plant cover is quite complex. In real natural situations, radiation is always a factor whose value is a ‘minimum’ because radiation available to leaves in lower layers of canopy is always insufficient. Therefore, increase in radiation flux always results in increased productivity of plant cover.
With increase in temperature, the productivity of plant cover increases initially. After attaining a certain maximum value that depends on the value of radiation flux, productivity begins to decrease with further increase in temperature. Thus productivity of plant cover substantially decreases above a certain threshold value of temperature which is determined by the radiation flux.