Introduction

Cut deep into the Middle Hills of eastern Nepal, a system of low-elevation riparian corridors extends tropical climatic conditions far into the high Himalaya. Remnant tropical and subtropical forest vegetation along these riparian corridors provides important biological habitat for numerous rare and endangered species (Cronin 1979; Shrestha 1989; Jackson et al 1990; Shrestha et al 1990) within an increasingly monotypic and cover-poor landscape. Remnant forest vegetation forms networks of habitat patches that are essential for numerous indigenous and migrating wildlife species. These forests contain a large number of rare and endangered plant species (Shrestha et al 1990). The occurrence of these forests provides essential habitat for a great variety of tropical birds and mammals, many of which are at considerable risk throughout the eastern Himalaya (Jackson et al 1990).

The vital role of forests within the mountain farming system and dependence on fertility transfer from forest to agricultural fields to maintain agricultural production are well described (Mahat 1987; Mahat et al 1987; Ives and Messerli 1989; Metz 1994, 1997). Within the agricultural zone of the Middle Hills, remnant patches of riparian vegetation constitute important resources for local subsistence agriculturalists, providing fodder, fuelwood, medicinal or other useful plant materials, and timber. Equally important to the mountain farming system, dense vegetation cover within the steep and highly erosive gullies and ravines provides important ecosystem and soil stabilizing functions (Young 1989). Local farmers generally recognize the important conservation value of protecting vulnerable riparian sites (Zomer and Menke 1992), although population growth, resource scarcity, and land tenure often put heavy demands on these marginal areas. Many of these sites are often highly utilized or, in some cases, semimanaged. Increased demands on these systems, with associated fragmentation and/or degradation of riparian forests, is facilitated through improved access, particularly in the case of extension of new roads along river corridors.

Tropical and subtropical monsoonal rain forests located along the upper reaches of the Arun River represent ecologically and economically important repositories of regionally endangered biological diversity (Cronin 1979; Shrestha 1989; Jackson et al 1990; Shrestha et al 1990). Approximately 7000 ha of these forest types occur within the boundaries of the Makalu Barun Conservation Area (MBCA) (LRMP 1986). The importance of riparian vegetation within the mountain landscape is generally recognized, particularly in relation to wildlife habitat, dispersal and migration (Harris 1984; Carlson et al 1991), refugia (Meave et al 1991; Dix et al 1997), and streamside management (Moyle et al 1996). However, relatively few hectares of these hill-variant tropical and subtropical forest types are found within protected areas of Nepal (Hunter and Yonzon 1993).

This paper reports on a detailed landscape-level analysis of land use and cover change in relation to the status of tropical and subtropical riparian forests within the MBCA. A comparison of classified satellite remote sensing datasets is used to describe both spatial and temporal dynamics of forest change. A time interval of approximately 20 years, leading up to the establishment of the MBCA in 1992, is evaluated for both impacts upon forests and changes in tree cover on agricultural lands. Areal extent, fragmentation, and recent historic changes in tropical riparian forest stands within the MBCA are discussed in the context of the dynamic nature of the mountain farming system.

Study site

The study area lies along the upper reaches of the Arun River of the eastern Nepal Himalaya, within the area designated as the Makalu Barun Conservation Area (MBCA) (Figure 1). The MBCA (27°30′N, 87°10′E), established in 1992 by the Department of National Parks and Wildlife Conservation, H.M.G. Nepal, lies on the edge of the vast and relatively undisturbed Makalu Barun National Park (MBNP). Envisioned as a development-oriented buffer zone for the adjacent 1500 km² of uninhabited wilderness, the MBCA (700 km²) supports nearly 40,000 subsistence agriculturalists. These farmers, with an average farm size of approximately 0.5 ha, depend heavily on local forest resources (Sherpa et al 1990; MBCP Task Force 1990).

Low-elevation riparian corridors within the eastern Himalaya are generally referred to as being within the tropical bioclimatic zone if below 1000 m (Shrestha 1989) or within the subtropical zone if below 2000 m. Although they are geographically outside of the tropics, the deeply cut river valleys are characterized by tropical climatic conditions due to orographic blocking of cold winter air from central Asia. This tropical/subtropical zone is frost free, with average monthly mean temperatures above 18°C throughout the year for elevations below 1000 m. Annual precipitation within this region of eastern Nepal is generally high (4000 mm), occurring mostly during the relatively long summer monsoon. Spatial distribution of precipitation is highly variable and strongly influenced by orographic effects associated with the complex mountainous terrain.

Within the MBCA, the main zone of cereal cultivation overlies the tropical and subtropical bioclimatic zones. Many of the native forests in these zones have been cleared for rice terraces and/or upland maize, particularly in the less rugged areas. Although the tropical zone is generally not as intensively cultivated as the subtropical zone, it is common for subtropical zone farmers to also have fields at lower elevations. The resulting vertical zonation associated with the agricultural calendar is partly due to a fragmentation of landholdings but is also regarded as an adaptation of the mountain agricultural system to climatic differentiation associated with the altitudinal gradient. Livestock are generally rotated to graze stubble on lower fields during the winter and are grazed in, or fed fodder from, nearby forest and riparian patches.

Lower slopes along the deeply cut rivers and streams tend to be steep and soils rocky due to active steam cutting processes and the unstable and erodable geologic substrate associated with the lower Middle Hills of the eastern Himalaya (Selby 1988). Geomorphologic processes are accentuated by sustained periods of intense precipitation, with frequent disturbance within riparian corridors due to seasonally swollen rivers and monsoonal downpours. River- and stream-side zones are often left uncultivated with dense scrub and/or forest vegetation in a semiwild state. These patches of remnant forest provide access to fuelwood, fodder, and other valuable resources for local subsistence agriculturalists. Ives and Messerli (1989) outline the important interrelationship of agriculture and forests in the Nepal Himalaya. A transfer of fertility from forest to farm, primarily mediated by livestock and the collection of fodder, maintains on-farm agricultural productivity. Recently, farmers have responded to diminishing forests by planting more trees on their farms (Gilmore and Nurse 1991).

The forest vegetation of eastern Nepal is known for high levels of both species and community diversity (Hooker 1852; Singh and Singh 1987, Shrestha 1989). Low elevation riparian corridors within the MBCA are described as containing significant and important biodiversity and forest habitat (Jackson et al 1990; Shrestha 1989; Shrestha et al 1990). This point was highlighted at the time of establishment of the MBCA (MBCP Task Force 1990), as the study site lies adjacent to the proposed (but now dormant) Arun III Hydroelectric Project (Kattelmann 1990). Descriptions of forests of the Arun Basin include, among others, Hara (1966), Numata (1966), Stainton (1972), Dobremez (1976), Oshawa (1983), Numata (1983), and LRMP (1986). Lower elevational forests, specifically of the MBCA, have been described by Oshawa (1983), Shrestha (1989), Shrestha et al (1990), and Carpenter and Zomer (1996). Three major formations were identified within the study area (Zomer et al 2001a):

Methods

Results

Visual analysis of the Landsat TM scene from 1992, based on an overlay of the forest classes on the enhanced false color imagery (Figure 2), reveals a landscape pattern typical of the relatively heavily populated Middle Hills of the eastern Himalaya. Forest fragments are clustered along the steep riparian corridors of tributaries and along the banks of the Arun, interspersed by agricultural land uses, including terraced and nonterraced cropping areas, grazing land, and agricultural scrub. Tree cover appears relatively contiguous along the major tributaries and forms a patchy corridor finely articulated along the riparian network. Several conspicuous breaks in cover (ie, agricultural zones) are evident on the less steep slopes along the west side of the Arun corridor. Two main lower elevation forest formation types were distinguished: tropical semideciduous monsoonal forest (defined to include chir pine and sal community types) and subtropical evergreen hill forest. Forest represents approximately 26% of all area below 2000 m, with agriculture making up 32%. Forest is estimated to occur on approximately 7132 ha classified into 2 broad forest classes (Table 1). Tropical forest comprised 29% of the total areas classified as forest within the TM scene (% TM forest), with the subtropical forest class comprising 71%. Tropical forest (2080 ha) occupied more than 40% of all area below 500 m, with the majority (1178 ha) found between 500 and 1000 m. The tropical forest formation showed a distinct northern latitudinal limit at approximately 27°34′, consistent with reports by Shrestha et al (1990). Subtropical evergreen forest (5052 ha) was found primarily above 1000 m, with a substantial presence in the upper tropical zone between 500 and 1000 m (575 ha; 16% of zone). There is very little agricultural land use in the lower tropical zone, extending from 345 to 500 m (3 ha, 3% of zone), with more than 90% of the total study area classified as agricultural (8656 ha) occurring above 1000 m. Agriculture occupied more than 40% of the lower subtropical zone (1000–1500 m) and almost 30% of the upper subtropical zone (1500–2000 m). Less than 600 ha of agricultural land use types occurred below 1000 m.

The supervised classification of the 1972 Landsat MSS data was not able to distinguish between the tropical and subtropical forest classes, and consequently all forest (ie, tree cover or high biomass) classes were aggregated into 1 forest class. The MSS data had significantly more unclassified area. Forest classes represent approximately 26% of all area below 2000 m, with agriculture occupying approximately 20%. Total forest cover in the classified 1992 data was approximately 12% less than in the classified 1972 data. Change detection analysis of forest cover (Figure 3) reveals substantial forest loss at higher elevations and the upper margins of the study area, while gains in tree cover appear at relatively lower elevations. Within areas designated as forest types in the LRMP Land Use Map (Figure 4), 628 ha of forest have been converted to agriculture, comprising more than 8% of existing forest in 1972 (Table 2). Within this same area, almost 300 ha are shown as having gained substantial tree cover, representing an increase of slightly less than 4%. The resulting net loss of forest within the LRMP-designated forest zone was 4%. Within areas designated as agricultural in the LRMP Land Use Map, 1282 ha have lost high biomass cover in agricultural areas of almost 17% of all forest in 1972 (Table 3). Within this same area, 680 ha are shown as having gained tree cover, representing an increase of more than 9%. The resulting net loss of forest within the LRMP-designated agricultural areas was more than 7%. Net loss of low-elevation forest for the study area was 11%.

Discussion

Extent and spatial distribution of current forest cover below 2000 m in the MBCA indicates that closed canopy tree cover is now mostly restricted to riparian corridors and the slopes of associated gullies and ravines. However, within the highly articulated and complex topography of the study area, this network of remnant patches continues to represent an essentially contiguous corridor. Upland subtropical evergreen (nonriparian) forest community types appear to have been the most impacted by recent conversion to intensive agricultural uses. Although intensity of cultivation within the MBCA has increased, most riparian zones along the tributaries of the Arun still remain heavily vegetated.

The highly articulated landscape and dissection of the narrow riparian corridors by their streams are major factors influencing the evaluation of forest and landscape. Because the linear nature of the deeply cut corridors limits dispersion and migration of both plant and animal species, even relatively limited fragmentation within corridors can be significant, although impact is species specific. Within the study area, low elevation riparian forests form a fairly contiguous network and intermingle at their upper margins with the densely covered and extensive areas of oak, hemlock, and fir forests of the upland National Park area. This spatial continuity enhances both their value to seasonally migrating species and their resilience to periodic disturbance.

Conclusions

Analyses of land use and cover change processes within the MBCA suggest that remaining riparian stands are under pressure and are subject to disturbance and degradation. Evidence that overall extent of low elevation forests within the MBCA has decreased in the period from 1972 to 1992 indicates substantial conversion to agricultural land uses. Complete loss of forest stands occurred primarily within the subtropical nonriparian forests of the MBCA. Loss of spatially limited but ecologically significant remnant stands within the monotypic agricultural landscape further increases the significance of remaining riparian zones.

The ecological and economic significance of remnant riparian forest within the landscape and the mountain farming system create a unique conservation challenge for managers, policymakers, and researchers interested in regional or landscape-level approaches to biodiversity management. Increasing population and livestock numbers threaten riparian forest with degradation and local extinction throughout the Middle Hills of east Nepal. On this regional level, conservation and biodiversity management are faced with several challenges. Spatially discrete patches within roughly contiguous corridors, interspersed within a primarily agricultural landscape, do not easily lend themselves to protection within the national park or protected area conservation model. Community forestry (Gilmore and Fisher 1991) on a local level has proven an effective model for other similarly restricted forests in the Middle Hills but has not specifically addressed the role of biodiversity conservation or wildlife habitat. Further research on biodiversity and wildlife habitat relationships within these patches and a specific recognition of the value of remnant riparian forests within the landscape and rural economy are required if conservation goals for the eastern Nepal Himalaya are to be met.