Factors Structuring Treeline Dynamics of the Nepal Himalaya
Date
2017-05-22
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Abstract
The alpine treeline ecotone is an important component of mountain ecosystems of the Nepal Himalayas; it plays a vital role in the livelihood of indigenous people and provides ecosystem services. However, the region faces a problem of paucity of data on treeline characteristics at the regional, landscape, and local scales. Therefore, I applied remote sensing and geographic information science approaches to investigate the treeline ecotone at the regional (entire Nepal) and landscape (Barun and Manang Valleys) scales. Treeline elevation ranges from 3300–4300 m. Abies spectabilis, Betula utilis, and Pinus wallichiana are the main treeline-forming species in the Nepal Himalayas. There is an east to west treeline elevation gradient at the regional scale. No slope exposure is observed at the regional scale; however, at the landscape scale, slope exposure is present only in a disturbed area. From the landscape scale study, I found that topography and human disturbance are the main treeline-controlling factors in Barun and Manang, respectively. Diverse treeline-forming species and treeline nature observed in the landscape and regional scale study suggested more investigation was needed at the local scale. Therefore, I established two transects of 20 m width and 120 m length (100 m above and 20 m below the forestline) in the Betula utilis sub-alpine forest of the Dhorpatan Hunting Reserve in western Nepal to understand the local scale treeline dynamics. Poor regeneration was observed above the forestline in both transects compared to below the forestline. Low regeneration at the treeline ecotone suggested site-specific biotic and abiotic controlling factors. Seedling and sapling establishment above the forestline is limited by a lack of moisture, an absence of suitable microsites, and the presence of herbivores. I found the treeline stable at the local scale. I used the Maxent species distribution modeling approach to predict the likelihood of treeline advance in the Nepal Himalayas by modeling the habitat suitability of three dominant treeline species—A. spectabilis, B. utilis, and P. wallichiana—under present and alternative future climates. Temperature-related climatic variables and elevation explained the greatest amount of variance in the distribution of the study species. Under future climate models, I found a regional increase in habitat suitability of all three treeline species that predicted a potential for northward and upslope advance.
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Keywords
Treeline, Climate Change, Nepal Himalaya, Remote Sensing, Geographic Information Science, Dendroecology