According to some workers, the tropical vegetation types are largely determined by the duration of the dry season rather than by the amount of rainfall.
For many groups of organisms, the neotropical region is the most species- rich area in the world, being much more species diverse than the palaeotropics but no more diverse at the family and only slightly so at the generic level (Prance, 1994).
Only 6.4% of neotropical species belong to plant families that are confined to the region and there is a greater generic diversity in Africa than in the neotropics indicating that species may be much better indicators of total diversity. Indeed, there may exist about 90,000 spp of higher plants in the neotropical region (Gentry, 1982) as compared to about 45000 for Africa (Beentje et al., 1994) and about 50,000 for the area covered “by Flora Malesiana (John, 1992; Prance, 1994). According to Prance, accurate assessment of the taxon component of biodiversity of neotropics should be based at the species level rather than at that of higher taxa, and that species data make a valuable tool for conservation planning.
5 Major types of Tropical Vegetation are described below:
1. Rain Forest:
About one-half of the world’s forest area is located collectively in tropical Africa, tropical America and tropical Asia.
Tropical forests play a vital role in global ecology and are extremely important not only in maintaining environmental quality within the tropics but also beyond the tropics. Mature tropical forests characteristically have large amounts of nutrients and materials trapped and bound in the vegetation biomass. They have closed nutrient cycles with relatively little loss through runoff into streams, etc.
In highly humid and warm regions (annual rainfall approx. upto 500 cm, temperature 25r28°C) luxuriant rainforest type of vegetation occurs and a most interesting fact is that such luxuriant vegetation occurs on very poor soils which contain only traces of nitrogen, phosphorus and calcium; such nutrients are rather stored in the plant biomass. It has been generally observed that the more humid the climate, the more luxuriant the vegetation and the poorer are the soils.
High-rising tropical mountains characterized by rainforest vegetation exhibit a regular vegetation zonation, with successive zones of tall trees, low subalpine forest or scrubland, and alpine vegetation. The plant complex also includes various climbers, lianes, epiphytes, parasites, herbs, shrubs and others. The entire vegetation is quite dense and compact.
As an ecosystem, the tropical rainforest is in dynamic equilibrium with environmental and biotic factors. Cutting or burning of such as forest leads to sudden mineralization of the stored nutrients and their washing off by rain, causing eutrophication in streams. Only the poor soils are left behind in this way. Recent studies in Mexico have made it seem very likely that the tropical rainforest is a nonrenewable resource when it is disturbed over sufficiently large areas.
Some virgin tracts of rain forest occur around the Silent Valley area (the Nilgiri plateau) in India. These forests mostly have tall woody plants with “flying buttresses”. The trees are rather weak, having thin baric, and are usually loaded with abundant epiphytes and climbing lianes. The vegetation tends to change in species composition in space and time.
The terrain provides much habitat heterogeneity for the establishment of diverse species. Some more important plants in these forests include Elaeocarpus tuberculatus, Palaquium ellipticum, and species of Eugenia, Alseodaphne, and Ficus (Singh etal., 1984).
2. Monsoon Forest:
This type of vegetation occurs in regions where annual rainfall ranges between 100-200 cm, during an approximately 4-month period of June to September, with the remaining months being mostly dry. In contrast to the rainforest type of vegetation, in Monsoon forest the trees are more widely spaced. Fewer epiphytes and parasites are found in this vegetation.
Some complex interactions occurring among physicochemical and biotic factors in tropical moist evergreen forest are illustrated.
3. Savanna Woodland:
The savanna is grassland (a rather dense stratum of grasses and herbs) with scattered shrubs or trees. It occurs in areas where rainfall (70-100 cm) is even lesser than in Monsoon climate. Here, the vegetation which consists of tall shrubs or small trees, both fairly xerophytic, is even less dense and much more open and widely spaced than in Monsoon forest.
Very few lianes and epiphytes are found but xerophytic shrubs and grasses are more abundant than in Monsoon forest. The important point is that
in a savanna, grasses and woody plants grow together in an ecological equilibrium.
4. Thorn Woodland:
Here the average annual rainfall is about 40-80 cm, most plants are highly xerophytic with reduced foliage and have thorns, prickles or spines. Grasses are usually less conspicuous though not in India.
Here the rainfall is extremely low (less than 10 cm annually) and the vegetation extremely sparse or even lacking. Mostly two broad categories of plants occur: (a) the drought evaders, which avoid drought by confining their life cycle and growth period to the brief period of rainfall, and (b) certain micro-organisms such as bacteria and blue-green algae, which can often withstand drought and other adverse conditions. Such microbes occur in suitable microhabitats beneath certain diaphanous (translucent) pebbles, stones, quartz or sand particles, etc.
Most of the world’s deserts are at present either completely devoid of any higher vegetation or harbour only restricted vegetation during the favourable growth seasons. However, at least in some cases, it is possible to reclaim the deserts and make them suitable for plant growth and for farming.
Walter (1964) mentions an interesting practice in Iraq, where the desert has a water table at the depth of 15 metres, an annual rainfall of about 10-12 cm, and summer temperatures often exceeding 50°C; the farmers have dug wells to draw water for repeated irrigation of vegetables grown on the dry desert. Because of high evaporation and frequent irrigation (even up to five irrigations daily), salts are deposited in the soil thereby making it unfit for further cultivation.
To overcome this problem, the salt-resistant plant Tamarix articulata is planted in between the vegetables. By the time the latter are harvested and irrigation stopped, the roots of Tamarix would have penetrated deep and reached the water table. These plants can then grow into big trees and desert area become converted, in due course, into a forest.
UNESCO and MAB (1973) launched a research project to study the impact of human activities on mountain ecosystems through the application of systems analysis and mathematical modelling techniques. The mountains characteristically exhibit gradients in environmental conditions and such gradients result in corresponding zonations of soils, vegetation and animal communities.
Such human activities as technological operations, road building, tourism, recreation, etc., can have considerable impacts not only on the mountainous areas but also on the adjacent non-mountainous areas.
Mountain soils and vegetations play a significant role in the proper maintenance of water quality as well as rates of flow in the plains.
Continued overgrazing, deforestation and poor land use practices in the mountainous regions can cause serious problems of soil erosion and flooding. The preservation of forests is also necessary for the maintenance and perpetuation of the reservoirs of biological diversity and germplasm and for the conservation of wildlife.
Professor T. Kira has studied the productivity aspects of lowland tropical forests in Japan (see Mueller-Dambois, 1971). Quantitative studies of carbon cycling in different forests have indicated that the total biomass accumulation does not differ significantly between tropical and temperate forests but significant differences exist in the accumulation of soil organic matter, which may be as high as 200 tonnes/ha in temperate, and less than 50 tonnes/ha in tropical forests.
On the other hand, litter fall and net primary production increase from temperate to tropical forests. Annual decomposition rates were much low in temperate than in tropical forests. On the basis of his observations, Kira has emphasized carbon cycling as an important index of measure of the overall rate of turnover of organic materials and this seems reasonable in view of the fact that carbon constitutes almost one-half of a plant’s biomass.
A similar research project to study the impact of human activities on grazing lands and grasslands has been initiated by UNESCO (1972) under the MAB Programme. As in forest ecosystems, the intention is to study the complex and interacting effects of human activities and land use practices on grazing lands by means of systems analysis and mathematical modelling techniques. Both tropical and temperate grazing lands are intended to be covered within this project.
Pastures and grazing lands constitute an important means whereby agriculturally unsuitable land can be used to raise and support cattle and other herbivores. A moderate amount of grazing activity is necessary for the proper equilibrium of these lands as grazing lands; if grazing is prevented, the sub climax vegetation will, in due course, become converted to the more woody type consistent with the climate. Thus, moderate grazing is an important factor necessary for the maintenance of a dynamic stability in which biota may be preserved.
Excessive grazing can be harmful and may even cause serious damage to the quality of soil, sometime leading to desert formation. The post-grazing recovery process also requires some time and over-grazing may prevent such recovery.