To a causal observer the pattern of vegetation in Chitwan probably seems stable. On the low lying flat land near the rivers, including the large islands in the Narayani river, there is a lush growth of short and long grass interspersed with patches of mixed forest. On the hills the forest is more uniform, consisting mainly of stately, straight-trunked sal (Shorea robusta). Everything, it seems, has been like this for some time.
Yet the apparent stability is an illusion. Nature is constantly in a state of flux, particularly in a monsoon area of this kind, and it is a process – a kind of continuous, creeping takeover – whereby some species of plants and trees gradually gain supremacy over others.
Two contrasting elements – water and fire – affect this environment, altering the course of plant succession and creating constant changes in vegetation patterns.
Every summer during the monsoon floods the rivers change their routes to a greater or lesser extent, altering the configuration of the floodplains. The floods destroy whole tracts of vegetation at various stages of growth, and the islands and sandbanks which emerge as the waters recede become sites for primary succession. Thus, every year, water wipes part of the slate clean and allows a new start to be made.
The freshly-exposed sandbanks are soon colonized by various species of grass. One of the first to arrive is usually Saccharum spontaneum, which can eventually grow to become 20 feet tall. Short, fast- growing grasses, and some creeping types, also invade, together with Herb’s and shrubs. Among the trees the sishoo or Indian rosewood Dalbergia sissoo and the Khar or cutch Acacia catechu, colonizes the newly-created silt-beds almost as fast as fast as grass. Both these species stabilize the soil and create conditions favorable to other trees such as kapok Bombax ceiba, and thus the foundations of a new forest are laid.
Shade provided by the first trees creates a more suitable environment for smaller Herb’s and shrubs and eventually a riverine type of forest dominates the grasslands. Patches of stable soil with exceptionally good drainage may even be taken over by sal.
Yet the speed of succession is strongly influenced by the second great controlling factor: fire. This strikes no less regularly than the monsoons.
Since time immemorial the aboriginal inhabitants of the valley have been burning the grasslands in winter and early spring, partly to ensure themselves a good, fresh growth of Imperata, the grass they use for thatching, and partly to harden the taller, cane-like grass reeds which they need for the walls for their houses. In the old days local people harvested grass and reeds whenever they wanted; now there is a limited season, usually in the first two or three weeks of January, in which the park authorities issue entry-permits to villagers at the nominal cost of 10 Rupees – less than 25 US cent – a head.
So important is the occasion in the lives of the local Tharus that they hold special festivals to mark the beginning and the end of the grass-cutting season. During this period more than 10,000 entry permits are issued, and thousands more illegal entrants no doubt poured into the park as part of the mass invasion.
To prevent poaching and illegal cutting of firewood, there is a rule that nobody may spend the night in the park. Thus hundreds of small temporary settlements suddenly spring up just outside the boundaries, so that the villagers, especially those who live some distance away, can hoard as much grass and reeds as possible during the period allocated. The Rapti and Narayani rivers become densely crowded with dug-out canoes and boats, which provide continual ferry services from the misty mornings until dusk.
Having collected what they need, the villagers set fire to the grasslands at random, without much supervision. Because, early in the year, many of the grass stands are still green, the first fires are relatively cool: they spread slowly, and are generally put out by the dewfall of winter nights. The numerous water- courses, open banks and artificially prepared clearings which act as fire breaks all help contain them.
By March and April, however, the grass is much drier, and now the fires spread much more quickly, fanned by the afternoon winds to such an extent that some areas are burned two or three times over. The flames spread into the riverine forests, and many young trees are destroyed; but they do not damage the mature trees. The effect of fire is not as devastating to vegetation as might be imagined; and on the plains, where the water-table is high, the grasses produce new shoots within 2 weeks. Although the rate of growth is not high early in the year, it is greatly accelerated by the occasional rains of April and May. By the time the monsoon has set in around mid-June, the new grasses are already 10 feet tall.
Fire appears to be integral to the ecology of Chitwan; if the grasslands were left unburned, the thick, matted stalks would inhibit new growth and create conditions suitable for trees to establish themselves. Burning is a traditional practice used to perpetuate grasslands and discourage trees from moving in. In the perpetuate grasslands and discourage trees from moving in. In the park, the natural plant succession is from grassland to forest, and burning retards this process. It has been established that grassland and riverine forest produce a greater animal biomass than the monotypic sal forest. Without fire to retard woody invasion, large grassland areas would very likely be taken over by forests, except on the low lying floodplains; wildlife populations, especially of ungulates and therefore of predators, would be likely to decline not only in numbers but also in quality.
The tall, coarse grasses have little food value once they have grown past the young, palatable stage. By the time they have flowered and are dying, most of their food has been transferred to their roots for storage. From the animals point of view, the main importance of dead or dying grass appears to be that it affords cover and shelter; but regrowth is so fast that this factor is regained in a few months after burning. Moreover, not all grass is burnt simultaneously, and animals can and do seek refuge in the sal forest and other areas.
All these factors indicate that, as far as the large mammals are concerned, the grassland-burning is an ecologically-sound exercise. It not only renders the grass edible for more months of the year, but also provides a period of maximum protein/fibre ratio. The herbivores readily move into recently-burned patches to feed on the succulent and nutritious new shoots. The existing mosaic of vegetation is, in part, a result of the fires, and it offers a variety of vegetation types that meets the food requirements of most ungulates.