Effect of exotic tree plantation on floristic composition and phytodiversity of Rema-Kalenga wildlife sanctuary, Bangladesh

Effect of exotic tree plantation on floristic compo sition and phytodiversity status of Rema-Kalenga wildlife sanctuary of Bangladesh was studied. A tot al of 309 vascular plant species under 245 genera belonging to 83 families were found to const itute the vascular flora of the studied area. The maximum number of species (298) with the highest Sh annon-Weiner diversity index value (3.882±0.090) was recorded from natural forest, which was followed by 194 and 165 plant species with 3.441±0.205 and 3.398 ±0.103 diversity index values recorded from Tectona and Acacia plantation sites respectively. The minimum number o f plant species (142) with the lowest diversity index value (2.999 ±0.152) was recorded from Eucalyptus plantation site. The collected data on the selected forest sites of Rema-Kalenga wildlife sanct uary showed the trends of gradual decrease in floristic composition and phytodivesity status of t hree plantation sites ( Tectona to Acacia to Eucalyptus) in respect to natural forest, which indicated tha t exotic tree plantations might have negative impact on floristic composition and phytod iversity of this semi-tropical forest area and the fast-growing exotic tree plantation of Acacia and Eucalyptus should be avoided for sustainable development of Rema-Kalenga wildlife sanctuary.


INTRODUCTION
The global extent of plantation forests in 1990 is estimated to be around 135 million ha (FAO, 1993;Gauthier, 1991;Pandy, 1995).About 75% of these plantation forests are in temperate regions and about 25% in the tropics and subtropics; some 5% are found in Africa, a little more than 10% in each of the American continents, some 20% in the former USSR and around 25% in each of Asia-Pacific and Europe (Gauthier, 1991;Kanowski & Savill, 1992).Most plantation forests have been established as even-aged monoculture crops of tree species with the primary purpose of wood production (Evans, 1992).Around 90% of existing plantations have been established for the production of wood for industrial use and most of the remainder to produce wood for use as fuel or round wood.Some plantation forests are grown and managed, either primarily or jointly, for non-wood products such as betel leaf, medicinal plants or fodder.Fast-growing tree plantations, mainly Acacia and Eucalyptus are widespread in tropical countries (Bouillet & Bernhard-Reversat, 2001).In the 19th and 20th Centuries valuable timber species and fast growing tree species of Australian origin were introduced to facilitate the development of forestry in Bangladesh (Hossain & Pasha, 2001;Islam et al., 2003;Hossain, 2006).Last few decades some fast-growing exotic tree species like akashmoni (Acacia auriculiformis A. Cunn.ex.Benth.) and eucalyptus (Eucalyptus camaldulensis Hook.) have been planted in different areas of Bangladesh.Most of the original forest cover of Rema-Kalenga wildlife sanctuary has been removed or substantially altered through plantation programs or other anthropogenic activities.Plantation started in this region from 1922 and in early stage, the Forest Department started the plantation program by planting teak tree (Tectona grandis L.).Then they took several initiatives to extend natural forest in degraded and deforested areas.As a consequence they planted some fastgrowing exotic tree species like Acacia and Eucalyptus in some areas of the sanctuary.Environmental impact of fast-growing tree plantations in the tropics has been a controversial topic.Some authors argued that these species exhaust soil water and nutrient resources, and prevent undergrowth vegetation and in further soil erosion and loss of fertility (Poore & Fries, 1985;Abbasi & Vinithan, 1997;Bouvet, 1998).The fast-growing exotic tree plantations are not suitable for local undergrowth vegetation (Hossain et al., 2002) as well as for soil nutritional quality (Prasad, 1988) and microbial growth (Hossain and Hossain, 2009).The previous studies conducted on this sanctuary were on taxonomy, ethnobotany, soil ecology etc. (Uddin et al., 2001;Uddin & Hassan, 2004;Hossain & Hossain, 2009).Data on the effects of different plantations on the vegetation in this area is still lacking.Therefore, the present investigation in this sanctuary was conducted to know the impacts of plantations of exotic tree species on the floristic composition and phytodiversity status of this area and to provide data that might be helpful for proper management plan and sustainable development of the sanctuary.

MATERIALS AND METHODS
The Rema-Kalenga wildlife sanctuary is located at Chunarughat thana of Habigonj district under the forest administration unit of Habigonj-2 forest range of Sylhet forest division (Fig. 1).Geographically, the study area is situated between 24°06′ to 24°14′N latitude and 91°34′ to 91°41′E longitude with about 67 m elevation at the highest peak (Rizvi, 1970).In 1982, this area was declared as a wildlife sanctuary through a Gazette Notification No. 11/For-68/89/882, Date 22/12/1981 by the Government under the Bangladesh Wildlife (Preservation) Order, 1973(Amendment Act 1974) to provide for the preservation, conservation and management of wildlife in Bangladesh.The sanctuary falls under IUCN management category IV-habitat/species management area (Green, 1990).This area is under the bio-ecological zone-9b with broad zone 'Sylhet Hills' (Nishat et. al., 2002).According to Bangladesh agro-ecological zones, this area belongs to Region-29, 'Northern and Eastern Hills', sub-region-29c, 'Low hills and piedmont plains' (FAO, 1988).Current notified area of this sanctuary is 1795.54ha.(Canonizado & Rahman, 1998).This forest area covers an area of low-medium high hills formed primarily from soft sandstones and classified as a mixed tropical evergreen forest (Champion, 1936).Sub-tropical monsoon climate exists in the study area with three distinct seasons, viz.summer, monsoon and winter.The mean minimum and maximum temperatures varied between 11.8°C and 33.2°C, relative humidity ranged from 71.5% to 87.5% and mean annual rainfall varied from 7.3 to 456.8 mm (Uddin and Hassan, 2004).

Sample collection and analysis
Shannon-Wiener diversity index of the recorded undergrowth plants of different selected sites was calculated through the following formula described as Kent and Coker (1992).Shannon-Wiener Diversity Index (H′) = -Σ pi ln pi where, pi = The proportion of individuals or the abundance of the ith species expressed as a proportion of total cover, ln = Log base n

RESULTS AND DISCUSSION
The collected data on the floristic composition and Shannon-Wiener diversity index of the undergrowth plant species of selected forest sites of Rema-Kalenga wildlife sanctuary have been presented in Tables 1 to 2 and Figs. 2 to 5. In this study, a total of 309 vascular plant species under 245 genera belonging to 83 families were identified as undergrowth species (Table 1).The maximum number of species (250; 80.91%) belonged to dicotyledons and it was followed by 47 (15.21%) and 12 (3.88%)species, respectively of monocotyledons and pteridophytes (Fig. 3).The highest number of species 100 (32.36%) were recorded as herb, which was followed by 93 (30.10%), 71 (22.98%) and 45 (14.56%) species, respectively of tree, shrub and climber respectively (Fig. 4).
In natural forest, a total of 304 plant species under 240 genera and 81 families were found (Fig. 2), of which 298 species under 237 genera of 79 families were recorded during monsoon season, 287 species under 234 genera of 75 families during winter season, whereas 245 species under 221 genera belonging to 69 families in summer season (Table 2).The values of Shannon-Wiener diversity index of the recorded undergrowth species of natural forest site varied between 3.960 and 3.784.The highest index value 3.960 was calculated during monsoon season which was followed by 3.901 and 3.784 recorded during winter and summer seasons, respectively (Fig. 5).2).In Tectona plantation forest, the highest Shannon-Wiener index value 3.614 was calculated during monsoon season which was followed by 3.494 and 3.215 found during winter and summer seasons, respectively (Table 2; Fig. 5).
In Acacia plantation forest, the highest index value 3.496 was recorded during monsoon season which was followed by 3.406 and 3.291 recorded during winter and summer seasons, respectively.Similarly, In Eucalyptus plantation forest, the highest index value 3.171 was recorded during monsoon season which was followed by 2.944 and 2.882 were recorded during winter and summer seasons, respectively (Table 2; Fig. 5).The average number of taxa and their diversity index values recorded from four different types of forest sites during three different seasons were found to decrease by different sites of plantation forests in the sequence from Natural forest to Tectona plantation to Acacia plantation to Eucalyptus plantation forest.Similarly, the number of plant taxa and Shannon-Wiener diversity index values varied among the studied seasons and found to show the trends of gradual decrease from monsoon to winter to summer seasons.
The number of undergrowth plant taxa as well as the phytodiversity index values was found to be the highest in natural forest site which was followed by Tectona and Acacia plantation sites and the lowest in Eucalyptus plantation site (Fig. 2 and Fig. 5).The finding of lower number of undergrowth species in the plantation of fast-growing exotic tree species is supported by that of Hossain et al. (2002) The occurrence of lower number of undergrowth species in the plantation forest of fast-growing exotic tree species (Hossain et al., 2002) that due to thick, leathery, flattened and expanded leaves of exotic plants were degrade slowly and ultimately the soil of their habitat become hard and unfertile.The finding of natural forests harboring the maximum species diversity index in their comparatively stable ecosystem in respect to any other plantation forest ecosystem is supported by Narayan et al. (1994) because they stated that higher values of diversity showed greater stability of the community.Eucalyptus plantation depressed the forest species more than other plantation either fast-growing Acacia or slow-growing Tectona plantation.Due to allelopathy Eucalyptus plantation is often regarded as damaging for the environment (Poore & Fries, 1985) which was also shown by Lisanework & Michelsen (1993), Souto et al. (1995), Bernhard-Reversat (1999), Reigosa et al. (1999).Moreover, the floristic composition and phytodiversity of fast-growing plantation forests in tropical regions have been reduced through the losing of fertility of top soil.The finding of Acacia and Eucalyptus plantations sites of the study area harboring fewer indigenous plant species and less species diversity is supported by the findings of Hossain & Hossain (2009), Bouvet (1998), Abbasi & Vinithan (1997).
The present findings concluded that any kind of plantations, either of fast-growing Acacia or Eucalyptus or slow-growing Tectona species in this semi-tropical region might affect the species composition and diversity of local forests.Fast-growing exotic tree plantations of Acacia and Eucalyptus affected the floristic composition and phytodiversity as well as on the environment more than that of Tectona plantation.Therefore, it might be suggested that monoculture plantations especially for the fast-growing exotic species like Acacia, Eucalyptus are not suitable for local forest plant species in the semi-tropical region of Rema-Kalenga wildlife sanctuary.These types of fast-growing species may be selected for plantation in unfertile barren lands or less fertile grasslands, because they grow well in those types of habitats without any care and supplementary nourishment (Hossain et al., 2002).So, the fast-growing exotic tree plantation of Acacia and Eucalyptus should be avoided for future sustainable management and development of Rema-Kalenga wildlife sanctuary.

. Shannon-Weiner diversity index values recorded from different selected forest sites of Rema-Kalenga wildlife sanctuary
In plantation forest, the highest number of 194 plant species under 178 genera and 67 families were recorded from Tectona plantation site during monsoon season, which was followed by 164 plant species under 131 genera and 63 families were recorded from Acacia plantation site during the same season, whereas the lowest number of 105 plant species belonging to 86 genera and 45 families were recorded from Eucalyptus plantation site during summer season (Table