INDIA: Invasion of Exotic Weeds in the Natural Forests of Tropical India due to Forest Fire- A Threat to Biodiversity
(IFFN No. 27 – July 2002, p. 90-92)
Ancient human race metamorphosed into a modern race. In course of time his greedy needs started changing the landscape by way of cutting, burning and apriority the natural resources. Forest cover started shrinking with limited flora and fauna. Introduction of exotics reflected positive and negative impact. But if we take in term of local biodiversity it actually played a negative role. Slowly and slowly it started replacing the natives. Certain plants, which were introduced as ornamental plants, have now become a weed. For example Lantana camara, Ulex europaeus, Acacia mearnsii, Eupatorium glandulosum and Cytisus scoparius have become a menace in Western Ghats and have replaced the valued flora at places. Fire is one of the major factors for such species, which is not only depleting undergrowth but also facilitating the germination of above-mentioned weeds. Present paper reports impact of fire on frequency, abundance, density and basal cover of these five species.
Vegetational composition of various places differs with each other. Interaction between abiotic and biotic things brings out positive as well as negative changes. Undesirable changes force system to work in negative direction. In, India, as the civilization started ascending towards the evaluation of modern man, the landscape started getting changed and also effected the genetic pool, whether we take the Himalaya as in the North or Western and Eastern Ghats in the peninsular India. Now, when the whole world is concerned about the conservation of endemic flora and fauna it is the right time to think about certain exotics, which have become a real menace and threat to the local natives. (Srivastava 1994, Methew 1965).
A characteristic feature of the Nilgiri biosphere is the occurrence of sholas above 1500 m. They are found in patches in hollows and sheltered folds surrounded by rolling downs in Anamalais, Nilgiri, and Palni hills and the high ranges of Kerala and Karanataka. (Methew 1959, 1965; 1949-74; 1987) Evergreen forests with thick undergrowth consists of short boled trees mostly belonging to the families Myrtaceae, Stryaceae and Lauraceae with a low height of trees up to 20m. The undergrowth consists largely of Rabaceous plants and Strobilanthes (Acanthaceae). The ground flora consists of ferns and mosses. The biodiversity of this region is adversely effected by increasing biotic pressure, specially over grazing (Fvtenally 1992). However, fire is also very important factor for the degradation of the forests, which not only convert vegetation into ash but also facilitates in the spread of weeds and the escaped exotics, now weeds, to come up more vigorously. The spread of such weeds has taken away much of the fodder resources as such obnoxious weeds cannot be grazed. Measures to control such factors have been suggested by Gadgil (1984).
Not only this, the forest fires also effects on climate changes vigorously. In the past decade researcher have realized the important contribution of biomass burning and it is recognized now as a significant global source of emission contributing as much as 40% of gross Carbon dioxide and 30% of troposphere ozone (Anderson 1969).
The study area is located at Udhagamandalam-Mysore road in Wenlock Downs Forest Reserves in the Nilgiris district of Tamil Nadu. The study area consists of two small adjoining watersheds (each about 32 ha) having nearly identical topography, slope, vegetation and soil characteristics. The vegetation of the watershed is typical of the Nilgiris with “Shola” forests, rolling grasslands and swamps. The “Shola” forests are largely confined to the valleys or folds, while the adjoining hill slopes are covered with grasses.
Observations were taken in ecotone of shola grassland in current year and after one year of fire incidence along with the area without fire. The other associated species were mostly undergrowth consists of Berberis tinctoria, Plectronia neilgherrensis, Hypercum mysorense, Rubus spp. Mahonia leschenaultii, Michelia nilagirica, Viburnum species, Hydrnocarpus alpina, Pittosporum nilghirense, Eugenia arnottiana, Casoeria esculenta, Garcinia gambogia and Schefflera spp. etc.
Two plots of 50x50m were selected for the study after natural fire in the first plot. Second associated plot was unaffected by fire. During the study period of two years, fire incidence occurred only in first year and observations were taken in subsequent years after fire.
Observations revealed that frequency, abundance, density and basal cover of all the exotics seems to be decreased immediately after fire incidence in the first year and in some species it became almost zero because due to fire vegetative shoots were burned. Maximum frequency and basal cover were recorded in the species Lantana camara followed by Acacia mearnsii. After fire incidences the basal cover of Lantana camara has increased tremendously in second year.
It has also been observed that generally most of the species present in the study area shows increasing trend in terms of frequency, abundance, density and basal cover in subsequent year (second year) after fire incidences. Fire plays a vital role in invasion of exotics in existing natural vegetation in due course of time.
Exotics namely Eupatorium glandulosum, Ulex europeus (Gorse), Lantana camara, Acacia meansii and Cytisus scoparius (Yellow broom) have made considerable change into the grasslands in recent years. Eupatorium glandulosum is a Mexicon species and is an escape from gardens into which it was introduced early in the past century. It readily takes possession of areas where the soil is even temporarily exposed. It comes up on the cuttings made for hunt rides in the Wenlock Downs and in places where the hooves of Toda buffaloes have wounded the soil. It occupies areas gregariously in unsightly colonies, is apparently not easy to eradicate and bids fair in course of time to impair the beauty of the downs. Ulex europeus was introduced on the plateau many years ago as an ornamental plant. It has since spread over considerable areas, especially on the slopes falling to Parson’s valley and in the neighbourhood of Nanjanad. Its yellow flowers make a show all the year round, but there is little otherwise to recommend it and it is certainly not to be preferred to the grass, which it is steadily displacing. Cytissus scoparius is much less invasive than the other two species mentioned above. The Forest Department apparently introduced it on the downs in an effort to afforest the grasslands. It has established itself in small areas at the head of the bridle path to Bangi Tapal. It is also to be found on the slopes near Nanjanad along the road to Avalanche.
During the course of the study deletion of local biodiversity by the way of fire, grazing and invasion of exotics and weeds were also recorded. In the process of repeated burning, the species composition is gently changed. Other factor for survival of exotics due to their morphological characteristics like presence of modified leaf into spine protect them from frost and drought and also does not allow water to escape and more or less fertile soil is sufficient for their survival and this is adverse in case of very succulent species. A vicious circle with repeated pressure on the ecosystems paving way to the species, which welcome the activity of fire and grazing. The result is quite visible in most of the places, species like Acacia mearnsii, Ulex europaeus, Eupatorium grandulosum and Cistrum nigrum have already found their place comfortably and further slowly and slowly they are changing the landscape with replacing the natives.
Here it is worth mentioning that the Shola is very sensitive type of vegetation. Once it vanishes from its original habitat, it is very difficult to make it to reappear in view of a drastic change in microclimate (Meher-Homji 1967, Vishnu Mittre and Gupta 1968), which does not allow shoal seedlings to grow in open grassland. The exotic plantations such as Wattle, Acaica melonoxylon and Eucalyptus, which took entry in shola-grassland ecosystem, are now giving shelter to the shola seedlings (specially the pioneer species) under their canopy. (Srivastava 1994). But these exotics are increasing vigorously in due course of time they may replace the native vegetation.
Due to advent of civilization and population pressure on one side and another side invasion of exotics create havoc to natural treasure. Physical cutting and burning can be check by strict policing but clandestine entry of some of the exotics into a vast landscape, which goes unnoticed, is creating havoc by replacing the native species. If it is not checked at this juncture it will affect not only the biodiversity alone but whole ecosystem.
IFFN/GFMC contribution submitted by:
Rajeev K. Srivastava
Assistant Director General, Monitoring & Evaluation Wing
Indian Council of Forestry Research and Education
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