Human Disturbances Impact On Vegetation Structure Of Forests Biology Essay


Abstract: Assessments of human impacts on forests within protected areas will provide information on the causes and processes of deforestation. We conducted a study on the dependence of the local communities on forest products and the impact on vegetation structure and diversity in the dry deciduous Sal dominated forests of Similipal Tiger Reserve (STR) in Eastern India. We assessed human disturbance in 8 villages using two plots per village: one plot was in the forest close to the villages where the local people collected forest products such as fuel-wood and fodder. All woody species ≥3.18 cm dbh (diameter at breast height) were permanently marked and tagged. The control plot was further away from each village in a site where harvesting was not carried. We compared plant densities, basal area, mean height of trees, species richness and diversity in the study plot and control plots. We assessed extraction pressure in the forest in the study plots at 3-month intervals. The species richness, diversity, basal area and stand density and mean height were lower in plots with ongoing disturbance compared with the undisturbed sites. Mean extraction pressure ranged from 1.77-11% and was positively correlated with village size. This indicates that harvesting affects forest structure and leads to loss of diversity.


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Forests have sustained human needs in the Indian subcontinent for centuries. Local communities have harvested the forest biomass in the form of fuel-wood, fodder, various plant material, hunted wild animals for meat and for livestock grazing (McNeely 1995; Ghimire & Pimbert 1997; Hegde & Enters 2000; Harris & Mohammed 2003; Arjunan et al. 2005; Meheta et al. 2008). It has been estimated that about 350 million poor people and 84% of tribal in India live in forested areas (Mehta & Shah 2003), and significantly depend on the forest resources for sustenance and cash income ( Poffenberger et al. 1996; Hegde & Enters 2000; Kutty & Kothari 2001; Harris & Mohammed 2003). However, due to the increase in human density near and within protected areas, the pressure on plant and animal populations has increased leading to forest degradation, loss of biodiversity and forest cover (Kothari et al. 1989; Gadgil & Guha 1992; Murali et al. 1996; Somanathan & Borges 2000; Rai & Chakrabarti 2001; Silori & Mishra 2001; Puyravaud & Garrigues 2002; Anitha et al. 2003; Rahmani 2003; Arjunan 2005; Arjunan et al. 2005; Madhusudan 2005; Roessingh 2006; Karanth et al. 2006; G. Shahabuddin & Kumar 2007; Davidar et al. 2007). The intensity and causes of forest loss and degradation need to be identified and causes understood to formulate better management and policy decisions.

Similipal Tiger Reserve (STR) is a large protected area in the Northern Eastern Ghats (Fig. 1), in the Mayurbhanj district of Orissa. It was declared a Tiger Reserve in 1972, but still more than 12,500 people live in the 65 villages located within the Reserve and own about 10,000 livestock. The human impact has led to a progressive degradation of the forest, reduction in regeneration potential of the vegetation and loss of soil (Mishra et al.2008).The non sustainable extraction of biomass, together with grazing pressure from livestock and burning are the chronic disturbances to STR (Parida 1997; Raut & Behera 1997). In this study, we selected 8 villages, 4 within the core zone and 4 in the buffer zone of STR and investigated the impact of extraction of forest products; grazing and other disturbance on the sal (Shorea robusta) dominated dry deciduous forests. We compared vegetation diversity, structure and regeneration in one hectare plots situated near villages with high levels of extraction and grazing pressure with control plots in the same area but further away from the village in relatively undisturbed forests. We then assessed extraction pressure in the plots close to the villages at three month intervals over a period of 18 months. We tested the hypothesis that plots with higher levels of human impact will have lower diversity, structure and levels of regeneration than the control plots, and extraction pressure will increase in village size.

Study area

Similipal Biosphere Reserve (86° 03' to 86° 37' E and 21° 28' to 22° 08' N) is a protected area spreads over 2750 sq. km. across the district of Mayurbhanj in the state of Orissa in Eastern Ghats. The reserve is divided into two distinct zones: the inner core zone of 845 km2., also designated as Similipal National Park, and encircling this, a outer buffer zone of 1904 km2 (Fig. 1) (Patro 1985; Das & Das 1997; Setty & Siddiqi 2007). Similipal Tiger Reserve primarily originated as a hunting ground for royalty prior to India's independence, and was declared as a 'Project Tiger' reserve in May 1972 to protect declining populations of the Royal Bengal Tiger. Vegetation comprises of semi evergreen forests, moist to dry deciduous forests and grass land and is dominated by Sal (Shorea robusta) (Saxena & Brahmam 1989; Misra 2004). About 1076 species of plants belonging to 102 families, 42 species of mammals, 242 species of birds and 30 species of reptiles have been recorded in STR (Rout 2008). There are 61 villages in buffer zone and four in the core zone. The Santals, a Scheduled Tribe community, constitute more than half the population, and other groups are the Bhumijas, Bhuiyans, Hos and Gonds, who practiced agriculture, and the Kharias, Mankidias and the Sabharas, who are nomadic hunter-gatherers. They depend completely on the forest and collect NTFPs like honey, wax, resin, arrowroot, siali fiber, herbal plants, fruits, leaves and flowers etc. and sell these to earn a livelihood ( Das & Das 2008 ).

Materials and Methods

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Estimates of extraction pressure

To evaluate the extraction pressure on the forests, a total of 16 one-hectar square plots were laid. The location of these plots covered a representative area of Similipal Tiger Reserve. Eight of these plots were in forests where the villagers collected fuel wood and other forest products. Eight of plots were controls and located in the same village as the study plots but further away in the forest where resource extraction was minimal. Therefore each of the 8 villages had one extraction plot and one control plot in its vicinity. In each of the 16 plots, all woody species >10cm gbh were identified and tagged and their height recorded. These plants were observed every three months and removal of whole plants and plant parts and the number of new cuts and notches were recorded. The study was carried out over a period of one and half years from November 2006 to June 2008 and a total of six assessments were made. An index of extraction pressure was formulated for each of the three month intervals. Extraction pressure was given as the [(total number of cuts*100/total number of plants in the plot prior to assessment)/ 3] to give monthly percentages for each village every month (Arjunan et al. 2005). All the cuts and removals were summed for the study period and the annual and monthly averages were estimated for each village. The control plots were not assessed except at the initial period.

Socio-economic correlates of extraction pressure

Mean extraction pressure for each village was correlated with the total population of that village as an indicator of village size. The village statistics were obtained from the office of the Village Administration Officer. Comparison of extraction pressure by villages with the highest and lowest mean extraction over the 3 month period was made using a non parametric Wilcoxon signed rank test.


Extraction pressure of biomass

The mean monthly extraction pressure was 6%, and ranged from 1% in Jenabil to 11% in Gurguria (table-1).This shows that on average 6% of all trees had new cuttings every month and most of the cases the whole plant were removed. The buffer zone villages like Gurguria, Makabadi, Naawna and Balarampur had very high level of extraction pressure than core areas villages, There were significantly difference in the mean monthly extraction pressure between the core zone and buffer zone villages (Wilcoxon signed rank test: p< 0.02). The village size was negatively correlated with stem densities of each plot (r= -0.23, p <0.05). It indicating that larger villages had fewer plants ha-1, indicating increasing pressure on forests with increase in village size. The mean monthly extraction pressure is positively correlated with village size (r=0.43, p< 0.05). This finding suggests that the extraction pressure increases with as the size of the village increases thereby leading to deforestation over time due to increasing pressure from villages.

Species richness, Species diversity and basal area

The study resulted in documentation of a total of 93 species of trees, shrubs and lianas belonging to 41 different families were recorded in study sites, 71 species recorded in disturbed sites and 88 species in undisturbed sites (Table 3). About 55 species were common to both sites. There were 16 more plant species in undisturbed compared with disturbed sites (74 species versus 58 in disturbed). Only 21 species recorded in undisturbed sites were not found in disturbed sites but five species were recorded in disturbed sites were not found in undisturbed sites (Table 6). Species diversity was lower in disturbed sites as compared to undisturbed sites (Table 4). The population density of different species varied in different sites and in every site Shorea robusta was dominant. Tree density at different sites ranged between 528 in Balarampur to 1380 in disturbed sites and in undisturbed sites 760 in Makabadi to 2165 in Yamunagard (Table 3). Stem density significantly differed between two sites there were higher density in undisturbed sites than disturbed sites as well as the tree density were more in core areas than buffer areas. In disturbed sites basal area were ranging from 20.25 in Gurguria to 108. 47 in Jenabil and in undisturbed sites the basal areas differ in 33.52 to Jenabil 123.34 (Table 3). Basal area was significantly lower in disturbed sites (55.28) as compared with the undisturbed sites (71.18) in (Table 3). This finding suggests that degradation is more in more populated areas.

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Uses of plant part for different purpose

Out of 93 species recorded in study regions 90 species were used for fuel wood as well as other purposes.


Forest degradation in Similipal Tiger Reserve

This study clearly shows that there is intensive and ongoing degradation of forests in Similipal Tiger Reserve due to anthropogenic pressure. The levels of degradation were higher in the buffer zone as compared with the core zone. The plots where people extracted forest products showed a significant decrease in plant species richness, diversity, basal area and mean height, which are indicative of degradation (Arjunan et al. 2005; Davidar et al. 2007; Verma et al. 1997; Kakati 1999; Ramesh 2003). Extraction pressure was variable between sites, and was related to size of the village. Larger villages had a greater impact on the forest. This level of human pressure is not sustainable within a protected area. The deciduous sal forests which cover most part of the central and eastern India (Champion and Seth 1968) have been subject to high anthropogenic pressure. Sal is a commercially important tree and human intervention, such as controlled burning have favored the dominance of sal and reduced the density of other plant species (Uma Shanker, 2001). Sal wood has a good market in urban centres such as Kolkata, and therefore there is also an ongoing and illegal harvesting of sal trees, which are transported out of the forests by foot (personal observation). In buffer zone, large trees were ringed and branches of all sizes were notched to kill the tree. After removing the branches the main trunk was cut and removed for sale in the urban areas (Sahoo own observation). Even small trees and saplings were thus destroyed, but for local use. The intensive exploitation of tendu leaves could also cause damage to the trees, but was not explored in this study. Apart from the commercial extraction of sal trees, there is heavy dependence on the forests for fuel-wood (Davidar et al. 2010). In Similipal tiger reserve, the major source of energy is fuel wood, and all the households used fuel-wood for their domestic requirements (Davidar et al. 2010). This is because there was no locally available substitute for fuel-wood for the local tribal communities. Fuel-wood is harvested in a daily basis and is therefore an important cause of forest degradation. We documented that 90 out of the 93 species were harvested by the local people to supply their fuel-wood needs. This situation is prevalent throughout India and it has been estimated that about 85% of the rural population in India depends on locally available fuel wood and agriculture residues (Natarajan 1997). Therefore the local energy needs of the population need to be considered if forests are going to be conserved for posterity. Shifting cultivation and livestock grazing are other causes of forest degradation (Sahoo unpublished). The dependence of the local communities on the forests for fuel wood, fodder, livestock grazing and other impacts such as burning the undergrowth has been responsible for extensive degradation of the forests.

Impact on vegetation

Basal area, stem density, species diversity, species richness and average height of tree declined the extraction of fuel wood and has reduced species richness and diversity in Similipal Tiger Reserve, as reported elsewhere (Verma et al. 1997; Kakati 1999; Ramesh 2003) , (Rao et al. 1990; Vetaas 1993; Murali et al. 1996; Ramirez-Marcial et al. 2001; Kumar & Shahabuddin 2005 ) showed that decrease the density and basal area, average height of tree along the disturbance intensity, (Smiet 1992) basal area values may be related to the stand disturbance index in different sites and different types of forests. So, in the heavily disturbed forest stand the basal area is lower than in the undisturbed stands, practice of livestock grazing and trampling affect on reduction of wild herbivores population, which indirectly affects flagship species such as tiger in protected areas (Silori & Mishra 2001; Madhusuddan 2004; Arjunan et al. 2005).