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.
