Abstract

Anthropogenic factors and pressures on protected areas often have negative consequences on the ecosystem function which leads to loss of flora and fauna. The goal of this research aims to investigate the alteration of forest vegetation by utilizing remote sensing techniques profiled from 2006 - 2018 in Pualreng Wildlife Sanctuary with an area of 50 km². The spatial distribution of forest cover was mapped using the IRS-P6 LISS III satellite images. The satellite-based survey revealed that 5.23 km² of forest cover was lost during 2006 - 2018.The periodical observations showed that the forest cover was 48.82 km² (97.64 % coverage of PWS) in 2006, which moderately declined to 48.17 km² (96.33 %) losing 0.65 km² of forest cover in 2012. However, a steep decline was observed from 2012 to 2018 where forest cover was reduced to 43.59 km² (87.19 %) losing 4.58 km² of forest area. The main cause of this loss is due to the construction of Tuirial Hydro Electric Power Station based on the rivers alongside the sanctuary. The reservoir of this dam has submerged large quantities of trees and forest areas creating many small fragmented and inhabitable lands, thus, disturbing the ecosystem function of this sanctuary. The study also feature the potential of periodic monitoring using multi-temporal satellite observations for the monitoring and management plans of these protected areas.

1. Introduction

Forests are the primary source of natural wealth that contributes to human well-being and environmental sustainability; however, they undergo rapid alteration due to the combination of climate change and anthropogenic influences ¹. Tropical forests account for 7% of the earth’s total land surface and more than 50% of the world’s species are found in these forests ². This type of forest are declining at a much-accelerated rate than other forest types due to anthropogenic activities primarily from slash-and-burn agricultural practices, rapid urbanization, and increasing population ³. Expansion of agricultural lands, deforestation for construction of houses, and firewood to accommodate the rising human population have led to unconstrained destruction of natural resources which will likely lead to extinction crisis in the future ⁴. Therefore, management and preservation of any forest are imperative. Environmental monitoring and forest cover change are paramount components in natural resource management so it is crucial to produce relevant information for natural asset management ⁵.

Forest fringe villages are directly or indirectly dependent on forest products for fuel-wood, traditional medicines, and food for personal and commercial purposes ^{6, 7}. Buffer zones of protected areas often experience high pressure from fringe villages as these areas are cleared for agricultural purposes and pasturelands which adversely disrupt the ecosystem function and services in the territory ^{8, 9}.

Construction of dams to meet growing urban and industrial energy demands is ever increasing. More than 45,000 dams higher than 15 m have been built worldwide, altering more than 50% of all major rivers whose reservoirs store more than 3,600 km³ of water ¹⁰. These reservoirs have immersed millions of hectares of forests especially in the tropical regions disrupting the ecosystem by causing fragmentation and deforestation of pristine forests; destroying habitats of flora and fauna; interfering with the route of migratory fish and animals; submerging millions of flora; transportation of sediments to lowlands and as flooded forests decompose they emit methane and carbon dioxide into the atmosphere which are greenhouse gases thus contributing to climate change. The damages done to the ecosystem by dams are believed to be permanent in most cases ^{11, 12, 13}. Over the next 20 years, the capacity of hydro electric plants are projected to increase by 73% globally. The north-eastern states of India including Mizoram have an immense capacity for hydroelectricity accounting for almost 40 % of the total hydropower potential of the country ¹⁴. India has 197 hydroelectric power plants throughout the country out of which 3 are present in Mizoram. Tuirial hydroelectric plant was constructed in 1998 having a length of 700 m which is based upon the rivers in and around Pualreng Wildlife Sanctuary, hence submerging portions of its forest areas disturbing the ecosystem function.

According to the Forest Survey of India 2019, forest cover of Mizoram is the second-highest among the Indian States/UTs covering 18,006 km² of the geographic area. However, forest cover have decreased from 19,240 km² (91.27%) in 2009 to 18,006 km² (85.41%) in 2019 ^{15, 16}. Reduction of forest cover in Mizoram occurs mainly as a result of shifting cultivation practiced by the rural populations in which almost every year, new areas are cleared for cash crops, developmental projects, and expansion of district capitals due to urbanization.

The advancement of remote sensing techniques, availability of high-resolution images in the optical and non-optical regions, and the high temporal availability in conjunction with geographic information system (GIS) have resulted in an efficient tool for the continuous detection, quantification, and mapping of forest areas and their changes over time. Temporal satellite data provide accurate information concerning deforestation and forest alterations. On account of rapid progress in satellite technology and digital image processing, remote sensing has improved the scientific comprehension of Earth’s land and waters ^{17, 18}. Since remote sensing technology has the ability for system observations at various scales, it can extend possible data archives from present time to several decades back which can delineate vegetation cover at a large scale efficiently ¹⁹. Studies and research aimed toward monitoring the land and forest cover change of protected areas, before and after they are established is very limited. This study will yield insight into the influence of the management of these protected areas by the administration over the period.

2. Materials and Methods

The research was carried out in Pualreng Wildlife Sanctuary which was established in 2004. Mizoram is located in the north – eastern part of India which shares boundaries with Myanmar and Bangladesh. This sanctuary is located in Kolasib district of Mizoram, situated between 24^o 6’35” - 24^o 14’16’21”’ North Latitude and 92^o50’17.6” - 92°54’2.64” East longitude covering an area of 50 km². Pualreng Wildlife Sanctuary conserves, protect, and provides shelter to 16 endemic species of flora and fauna including Ficus religiosa L., Mesua ferrea L., Michelia champaca L., Acrocarpus fraxinifolius Arn., Bombax ceiba L., Clouded leopard, Hoolock gibbon, Sun bear, Slow lorries, Phayr’s leaf monkey, Sambar, Serow, Khaleej pheasant, Peacock pheasant, Common hill partridge, and Pied hornbill. Doria’s foam-nesting tree frog was recently rediscovered in this sanctuary after 108 years where the last report of this species was in Arunachal Pradesh’s Tenga Valley in 1912 ²⁰.

The forest type of Pualreng Wildlife Sanctuary is a semi-evergreen tropical forest with sandy soil and sedimentary rocks. The altitude ranges from 260 m to 750 m and the fringing villages include North Hlimen, Thingthelh, Bukpui, Ratu, and North Khawdungsei (Figure 1). The climatic condition is very mild and pleasant, during summer the temperature ranges between 20°C – 30°C and in winter it ranges between 10°C – 20°C having a mean annual rainfall of 2000 mm – 3900 mm. The main economic activity of the surrounding villages is agriculture through jhum cultivation (slash-and-burn). As a result of the expanding agricultural lands caused by the increasing population, much vegetation in the buffer zones and some core zone has been degraded and disturbed by the local communities.

In this study, the spatial distribution of forest cover was mapped using the IRS-P6 LISS III (Linear Imaging Self Scanning Sensor) which dispense multispectral data in four bands. LISS III is a remote sensing satellite camera from ISRO, India. The spatial resolution for visible (two bands) and a near infrared (one band) is 23.5 meters with a ground swath of 141 kms which follows a 24 day repeat cycle (Table 1). The satellite imagery from LISS III was taken in three cycles i.e. 27^th January 2006, 25^th February 2012, and 17^th February 2018. The images were used to depict forest and non-forest (scrubland, water-bodies) regions during the year 2006 – 2018 using visual interpretation techniques and analyzing forest cover change. The ArcGis 10.5 version software was used for data preparation and geospatial analysis.

3. Results

The multi-temporal satellite images during 2006 – 2018 of Pualreng Wildlife Sanctuary (PWS) were utilized to delineate forest and non-forest cover of the region. The satellite-based survey revealed a reduction of forest cover with an overall change of 5.23 km² (- 10.45 % change) during 2006 – 2018. The periodical observations revealed that the forest cover was 48.82 km² (97.64 % coverage of PWS) in 2006, which moderately declined to 48.17 km² (96.33 % coverage; - 1.31% change) losing 0.65 km² of forest cover in 2012. Steep decline was observed from 2012 to 2018 where forest cover was reduced to 43.59 km² (87.19 % coverage; - 9.14 % change) losing 4.58 km² of forest area. On the contrary, the satellite-based survey exhibited an increase in non-forest cover with an overall change of 5.23 km² (10. 45 % change) from 2006 – 2018. The non-forest cover was 1.18 km² (2.36 % coverage) in 2006 which slightly increased to 1.83 km² (3.67 % coverage) in 2012 to 6.41 km² (12.81 % coverage) in 2018 (Figure 2; Table 2).

As shown in Table 3 & Figure 2, gradual decline of Open Forest Area was observed from 2006 (16.92 km²) to 2012 (16.56 km²) to 2018 (15.59 km²) likewise, Medium Dense Forest Area decreased from 2.98 km² in 2006 to 2.84 km² in 2012 further declining to 1.99 km² in 2018. Dense Forests account for the largest area in the study site and although moderate reduction of dense forests was observed in 2006 (28.92 km²) to 2012 (28.77 km²), a sharp decline was observed in 2018 (25.66 km²). Initial increase of Dense Scrubland Area occurs during 2006 – 2012 i.e. 0.15 km² to 1.02 km² but later declined to 0.37 km² in 2018. The areas occupied by Open Scrubland declined from 0.38 km² to 0.16 km² during 2006 to 2012 which was no longer present in 2018. Area covered by water bodies was constant during 2006 to 2012 i.e. 0.65 km², however, it increased drastically to 6.04 km² in 2018 immersing many areas of the sanctuary.

4. Discussion

The satellite-based mapping of vegetation has utilized visual interpretation techniques to assess forest cover change revealing the declining forest cover area of Pualreng Wildlife Sanctuary from 2006 to 2018 which was established in the year 2004 with an area of 50 km². The total forest cover lost is 5.23 km² disclosing that 10.45 % of its forest cover was lost during a span of 12 years (Table 2; Figure 2). Non-forest areas have increased over the period as a result of anthropogenic influences leading to habitat destruction, vegetation loss, and vulnerability to the fauna and flora. Scrublands appeared near the fringe villages such as N. Khawdungsei and Thingthelh during 2006 and 2012 (Figure 2). This is the result of illegal clearing of forests within the sanctuary areas by the communities of the aforementioned villages for agricultural lands. Illegal cutting of trees for commercial purposes has also contributed to the decrease of dense and medium-dense forests transforming it to open forests and scrublands.

Forest cover areas exhibit a decline from 2006 to 2012, even before the dam starts storing water in the reservoir, revealing that human activities of the fringing communities have negative consequences since the core areas of the sanctuary are a major source of bamboo and timber extraction. Forests on the buffer zones are cleared for agricultural lands and grazing of cattle in which they often venture into the sanctuary core areas (Figure 3). Forest fire which often breaks out in the sanctuary caused by natural as well as man-made had caused further deterioration of the forest. The combined impact of anthropogenic and natural factors with varied intensity may be attributed to the gradual decrease of forest cover in Pualreng wildlife sanctuary. Human activities taken to accommodate its ever-growing population have negatively impacted the ecosystem causing alteration of species composition and modify forest structures to such length that enormous territory of forests had been replaced by scrubland.

As a result of the construction of Turial hydroelectric power plant in 1998, based on the rivers alongside Pualreng Wildlife Sanctuary, many trees and animal habitats have been submerged leading to formation of many small fragmented and inhabitable lands thus decreasing the forest cover. About 5.39 km² of forest have become submerged due to the reservoirs of Tuirial power plant from 2012 to 2018 (Table 3). Migratory birds, animals, and fishes have been adversely affected and the transportation of sediments to the low lands has also been disrupted by the construction of this dam. As a result of the increased water level, untouched forest areas are now easily accessible by boats on which hunters and fishermen from nearby villages would often chop trees to make temporary camps and smoke their catches illegally. Dense forest regions of the sanctuary have lost the most area (6.24 % coverage) than other regions due to the increasing reservoir of the power plant. Since trees are not chopped down and left to rot in the reservoir, they release carbon dioxide and methane through decomposition consequently contributing to climate change (Figure 3). However, decrease in scrubland areas can be seen from 2006 – 2018 which shows the effectiveness of environmental monitoring plans for this sanctuary. Regular estimation of forest cover is among the crucial data for the management and preservation of protected areas.

5. Conclusion

This study utilizes remote sensing technologies to evaluate forest cover change caused by anthropogenic influences in Pualreng Wildlife Sanctuary during 2006 – 2018. The study indicated that the forest cover in 2006 was 48.82 km² (97.64 %) of the total geographical area of the sanctuary. A gradual decline of forest cover was observed in 2012 (48.17 km²; 96.33 % coverage). In 2018, forest cover decreased to 43.59 km² (87.19 %) revealing a total loss of 5.23 km² of forest area from 2006 – 2018. The illegal activities of fringing communities alongwith the submergence of forest areas by Tuirial Hydroelectric Power Plant have negatively impacted the forest cover causing disruption to the ecosystem function. Due to the ever rising water level of the reservoir, more forest areas are under the threat of being submerged. Untouched forest areas are now easily accessible by boats because of the rise in water level on which illegal hunting of animals are practiced by the nearby communities. Periodic monitoring using multi-temporal satellite observations assisted in the successful estimation of change in forest and non-forest areas and can supply essential information for the management and monitoring plan of Pualreng Wildlife Sanctuary.

Acknowledgements

The authors would like to thank Mizoram Remote Sensing Application Centre (MIRSAC) for preparing the satellite images and datasets.

Statement of Competing Interest

The authors have no competing interests.

Abbreviations

PWS: Pualreng Wildlife Sanctuary

FSI: Forest Survey of India

IRS-P6: Indian Remote-Sensing Satellite-P6

LISS III: Linear Imaging Self Scanning III

References