Tuesday, March 11, 2014

Forest of Bhutan I. The biodiversity of the forest in Bhutan

Forest of Bhutan I. The biodiversity of the forest in Bhutan

By András Darabant forester in Bhutan

If the readers of this blog have heard of Bhutan, it was likely in the context of the country’s preeminent role in environmental conservation and the judicious use of its natural resources.  First acquaintances with Bhutan are frequently made also through images depicting pristine forests and picturesque settings of human settlements in perfect harmony with nature. 

I. The biodiversity of the forest in Bhutan
How does a slightly more nuanced picture of this simplistic view look like? 

II. Traditional uses and nature conservation of the forest in Bhutan
What are the forests like and how are they conserved and used?

I. The biodiversity of the forest in Bhutan
How does a slightly more nuanced picture of this simplistic view look like?

Bhutan’s forests cover an incredible 70% of the country’s area. 
Given the extreme differences in altitude, ranging from 130 to 7500 m above mean sea level within a horizontal distance of just 175 km, along with the rugged topography and high precipitation due to monsoon rains, the country harbors a truly impressive biodiversity.  Not surprisingly, Bhutan is part of the Himalayan biodiversity hotspot described by IUCN.  On an area comparable to Switzerland, 5500 species of vascular plants have been described, and many more are expected to be added to the list.  For comparison, Kazakhstan harbors comparable numbers of vascular plants as Bhutan, on an area more than seventy times in size.

At closer look, vegetation zones in Bhutan are primarily defined by altitude.   Common dominant species in subtropical hill forests, covering lowest elevation zones, are Shorea robusta, Bombax ceiba, Pterospermum acerifolium, Acrocarpus fraxinifolius, Chukrasia tabularis, Ailanthus grandis, Duabanga grandiflora, and Aquilaria agallocha.  This belt occupies a narrow band along Bhutan’s southern border with India and harbors large commercial centers, serving the exchange of goods with Bhutan’s main commercial partner.  At the same time, relatively gentle slopes in many locations make the subtropical belt well suited for agricultural production, accommodating a relatively dense rural population.  The country’s oldest national park, RoyalManas, a prominent refuge of the Bengal Tiger, is partially located within this zone.

Rising in altitude, warm-temperate broadleaf forests soon follow up to an elevation of 1000 meters.  Common species include Fagaceae (Castanopsis sp., Lithocarpus sp.), Engelhardia spicata, Evodia fraxinifolia, and Schima wallichii, with Betulaceae (Betula alnoides, Alnus nepalensis), and Macaranga pustulata growing on disturbed sites.  The warm-temperate broadleaf zone is comparatively sparsely populated and only Royal Manas National Park protects notable stretches of this forest type.

Cool-temperate broadleaf forests represent the next forest type between 2000 and 2900 meters, frequently dominated by various Lauraceae (Beilschmiedia sikkimensis, Persea sp., Lindera sp.) and to a lesser extent by Magnoliaceae (Michelia sp.).  Other common taxa are Acer sp., Betula alnoides, Brassaiopsis sp., Exbucklandia populnea, and Ilex fragilis.  Daphniphyllum himalense is common on disturbed sites, and Symplocos sp. frequently dominates the sub-canopies.

The altitudinal zonation of forest types is locally modified by topography, leading to distinct local climatic conditions.  These modifications include rain shadows on the leeward (northern) side of east-west oriented mountain ranges, as well as local climatic conditions in inner dry valleys, commonly created in deep Himalayan valleys by strong mountain-valley wind systems.  These drier local climates lead to the formation of distinct forest types.  Evergreen oak forests can be found between 1800 and 2300 meters and are dominated by Fagaceae (Castanopsis sp, Quercus sp., Lithocarpus sp.) in the overstory, while Acer and Symplocos spp. are common dominants of the subcanopy. 
Blue pine forest. Photo from http://www.hotelmee.com/blog/hotel-olathang-bhutan/    

Stronger climatic modifications in inner dry valleys shelter two distinct – mostly mono-specific – conifer forest types: the Chir Pine (Pinus roxburghii) forests between 900 and 1800 meters and the Blue Pine (Pinus wallichiana) forests between 2200 and 2700 meters.  While Chir Pine forests are major throughfares providing connectivity along deep V-shaped valleys, Blue Pine forests, located in glacially formed U-shaped valleys are prominent settlement areas, harboring the majority of urban centers in the country, including the capital Thimphu.  A large proportion of rural population settled in higher altitudes also resides in the Blue Pine forest zone.  Blue Pine is the preferred timber used for the construction of the iconic Bhutanese farm houses.

Above 2500 meters, a mixed broadleaf-conifer zone of variable range follows on zonal sites, gradually giving way to conifer forests above 2600-2800 m.  The lowest zonal conifer forest type are mixed conifer forests, dominated either by Himalayan Hemlock (Tsuga dumosa) or by East Himalayan Spruce (Picea spinulosa) along a gradient of decreasing moisture availability.  This forest type has immense importance for the national economy, as most commercial forest operations are concentrated here.

The uppermost belt of closed forests follows above 3200 m, comprised of mono-specific stands of Himalayan Silver Fir (Abies densa).  The understory of this forest type is frequently dominated by bamboos and large rhododendrons.  The timberline is between at 4200 m in mountains closer to the plains of Assam and at 3900 m in the rain shadows of northern Bhutan.

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