Light and Distribution
We have mentioned in the beginning that the variation in the amount of light generally affects the global and local distribution of plants and animals. Light plays a great role in species composition and development of vegetation. We have already discussed the global variation of light intensity. Let us study the causes of variation in light climate in terrestrial and aquatic ecosystem. In order to provide a comprehensive idea of light climate of any locality, information on the following three aspects needs to be provided:
i) Intensity or amount of light per unit area per unit time,
ii) The quality or wavelength composition, and
iii) Photoperiod or duration.
Significant local variation in the light in the terrestrial ecosystems results due to the interception of light by vegetation. In a forest, tall trees with fully expanded canopy receive maximum sunshine and absorb a major portion of the incident light especially in the red and blue regions. The undershrub and herb layers receive only light filtered through the tree canopy from above. In a thick forest the light interception by the multistoreyed vegetation is very efficient and on the forest floor light intensity may be only 1% of the incident solar radiation received at the top of the canopy.
Due to selective absorption, spectral light quality changes as it passes through the tree canopies. Yet, we find that some plant species are adapted to functioning in such low light intensities. On the basis of relative preference for natural growth in bright or diffused light the plants have been classified into two categories - sciophytes (shade loving) and heliophytes (bright-light loving). Some plants are more rigid in their preference for shade or bright light. These are termed as obligate sciophytes and obligate heliophytes respectively. There are some heliophytes that can also grow in shade but not so well. These are called facultative sciophytes. Similarly, the sciophytes that can also grow in bright light are called facultative heliophytes.
Plants can survive only when the total energy harnessed in photosynthesis exceeds that used in respiration. The intensity of light at which energy harnessed through photosynthesis is just sufficient to meet the energy requirement of respiration is called light compensation point. In deep shade, under trees the amount of light is not enough to carry on photosynthesis to satisfy the immediate need of the plants. Therefore, they lose leaves and usually branches. The leaves in a tree canopy are arranged in a way so as to function above light compensation point.