Fire induced invasion of an endemic plant species alters forest Structure and diversity

[dropcap]I[/dropcap]ntensive alien species are regarded as principal threats to global biodiversity.
In this paper, we describe a situation where and endemic plant species, the dwarf bamboo ‘Sinarandinaria rollaona (Gamble) Chou and Renv’, invades the surrounding following a fire and becomes the dominant component suppressing other native species. The dwarf bamboo is endemic to eastern Himalayas and also grows in the Dzukou Valley and surrounding hills of north-east India (Figures 1 a and b). The scenic beauty of the area attracts thousands of tourists to the valley throughout the year. The valley can be called the valley of flowers during the flowering season, i.e. from April to September. It may also be called the “valley of bamboo” as the open valley and the surrounding hillocks devoid of forest trees are fully covered by the dwarf bamboo which appears like an open grassland from a distance (Figure 1 a).Over the past several years we have observed that the pristine forests of Dzukou valley and the surrounding hills are being destroyed by frequent annual forest fires due to anthropogenic activities, causing a serious threat to biodiversity. The main cause of the fires is the thick growth of native dwarf bamboo in the valley and open spaces in the hills that get burnt easily during the dry season. The dwarf bamboo grows more vigorously after the fire resulting in rapid colonization and invasion of the open spaces caused by the forest fire. The fire-dwarf bamboo cycle has become an annual affair due to the heavy influx of tourists in the valley. The magnitude of destruction cab imagined from the devastating fire in January 2006 which lasted for over a week. The State Forest Department, Government of Nagaland with the help of Indian Air Force conducted aerial surveys to gauge the extent of damage using GPS to map the area of devastation, which spread over 70 sq. km of Dzukou hills and valley and the adjoining Japfu hill ranges (Figure c and d).
We undertook a study of 2006-08on three sites in Dzukou valley and surrounding hills with the objective to determine the the status of regeneration of dwarf bamboo in relation to oither native species following the forest fire.
The Dzukou valley is situated at the border between Nagaland and Manipur states (Figure 2). The valley and the hills surrounding it have an area of about 27 sq. km. The elevation ranges from 2400 m in the valley to 2994 m above mean sea level in the surrounding hills. Geologically the area is of recent origin. The soil is black in colour indicating that it is rich in humus. The natural drainage is maintained by a number of streams which have their origin in the hills and later join in the valley to form the main Dzukou river. The river flows down the valley criss cross towards the west to Peren district of Nagaland and down to Assam plains. The climate is warm temperate monsoon with an annual rainfall of more or less 2000-3500 mm. The vegetation of the area can be briefly described as temperate to sub-alpine (2400-2994). The hills surrounding Dzukou valley have temperate forest vegetation on the hill slopes. The dominant tree species found in this forest are Betula utilis, Rhododendron spp., Gaultheria spp., Peiris Formosa, Lyonia, ovalifolia, Lithocarpus pschyphylla, Quercus lamelosa, Hex spp., Taxus wallichiana, Juniperus recirva, Symplocos spp.,etc. The main valley (c.2400m) is devoid of big trees but rich in herbaceous plant species with few small shrubs her and there. The dominant species are Lilliium macklini, Euphorbia sikkimensis, Rhodiolaheterodonta, Polygonatum, Satyrium nepalensis, Primula spp., Caltha palustris, grasses, Sinarundanaria rolloana, Anemone spp., A. Elswii, Thalictrum spp., Oenanthe spp., Ligularia spp., Carex spp., Juncus spp., Osmunda claytonia subsp, vestitia, Potentilla spp., etc.
The sites which differed from each other with respect to fire incidence were selected for the study. Site-1, the valley (c. 2400 m) gets burnt by mild fire almost every year during the dry season, i.e. November to March. Site-2, (c. 2550 m) is situated in a forest area on the hills surrounding the valley. The phyto-sociological analysis of dwarf bamboo and associated plant species was carried out by using five, 3 x 3 m randomly distributed quadrates on each site according to Misra. In each quadrate, each species were individually counted. Frequency, density, relative frequency, abundance, frequency ratio and importance value index (IVI) of each species were calculated.
The study revealed that dwarf bamboo showed the highest IVI on site-2 followed by site-1. On site-3 Gaultheria hookers showed the highest IVI. The dwarf bamboo thus dominated the burnt sites and exhibited excellent power of regeneration following the fire. Other species with low IVI are of little significance to the community and are not able to compete with dwarf bamboo in the prevailing environment. If such a condition (repeated fire) prevails for long time, dwarf bamboo will eliminate other species. The relative density of dwarf bamboo in the three study sites especially in site-2 indicates that the species grows more vigorously after forest fire contributing to its success in invasion. On site-3 apart from dwarf bamboo, all the other species found were trees and shrubs. The population of dwarf bamboo in the valley is so thick now that it grows like the grasses with an average height of about 30 cm. However, in the forested area not affected by fire in the recent past, the understorey population of dwarf bamboo is sparse and height of the culms 2 to 3 m. If such a forested area is burnt, the dwarf bamboo assumes dominance through rapid regeneration and suppresses the regeneration of other species.
The dwarf bamboo embodies several of the advantageous characteristics of the adapted invasive species, such as (1) it resprouts readily on being burnt. In fact, the present study has shown that it grows more densely in response to burning as reported in Lantana. This bamboo forms dense impenetrable thickets and it severely restricts the regeneration of other species. Like in Lantana, dwarf bamboo once established, fuels further fires, setting up self-feeding fire-dwarf bamboo cycle. This dwarf bamboo flowers sporadically and not gregariously (usually the bamboo species dies after flowering) which is another characteristic of dwarf bamboo. Thus, forests once colonized by the dwarf bamboo will fall victim to the fire-dwarf bamboo cycle. (2) A second mechanism underlying the success of dwarf bamboo is its ability to compete for scarce nutrients. In can grow even on rock crevices where it is devoid of soil and nutrients which is an indication that it is exceedingly efficient at nutrient uptake and use, enabling it to grow on highly impoverished soils as does Lantana, such an ability to extract and use nutrients efficiently would give it a competitive advantage over other species. (3) Finally, a third mechanism underlying the success of dwarf bamboo is its colonizing ability. The rhizomes grow deep in the soil, solid and strong, so physical removal is very difficult. Thus, a complete eradication of dwarf bamboo requires repeated uprooting of sprouts for several consecutive years.
The phenomenon of the invasive grass-fire cycle has been especially well-documented for the Americas, and on the Hawaiin Island. Historical record on fire regimes in the Dzukou valley and surrounding hills is not available. However, there is good reason to infer that forest fires have been more frequent in recent years than before. Burn stumps of tree trunks standing on the hill slopes speak of the ferocity of recent forest fires (Figure 1 c and d). The alteration fire regimes is thus causing widespread wide-ranging consequences for the forest structure, composition and functioning. The altered fire regimes have probably made the eco-system susceptible to invasion by the fire-tolerant or fire-resistant dwarf bamboo.
According to Ewel, the ecological traits of invasive species alone do not necessarily determine invasions. The presence of invasive species may only be a “symptom,” the underlying cause being eco-system invasibility. There are reports of increasing ecosystem invasions due to disturbance: more disturbed ecosystems are more vulnerable to invasion than the less disturbed ones. In the present case also, the area is greatly disturbed by the influx of thousands of tourists throughout the year and their careless behaviour that results in forest fires.
In our view, the biggest threat to the biodiversity of the area is the fire induced by the dwarf bamboo which is rapidly colonizing and altering the forest structure and diversity of the Dzukou valley and the surrounding hills. There are still patches of pristine forests in the surrounding hills with its unique flora and fauna. The populations of several species are facing survival threats to their natural habitats, whereas a few are already on the verge of extinction. If the present trend of annual forest fires and consequent dwarf bamboo invasion is not controlled, the entire valley and the surrounding hills will be covered by the dwarf bamboo.
There is an urgent need to put in place large-scale monitoring of fires and spread o f dwarf bamboo to better gauge the spatial extent of the other. The task requires active participation of stakeholders, namely, the local communities, wildlife conservationists, forest managers, government agencies and the civil society at large. Also, well-planned ecotourism will certainly be a solution for protection of these hill ranges which will conserve the unique biodiversity of the area in general and endemic ones particular.
Courtesy: Current Science