Search Results (4)

The Himalayan region is likely particularly exposed to climate change indicated by the high regional rate of change. The number of high-resolution, well-calibrated, and long-term paleoclimate reconstructions are however regrettably few, to set this change in a longer-term context. The dendroclimatic reconstructions over Himalaya that do exist have only reconstructed summer season temperatures, and rarely or never attempted to reconstruct mean annual temperatures. The paucity of long meteorological records is a matter of concern when developing chronologies of climate sensitive tree-ring data in Bhutan, but the chronologies would theoretically be of high potential for extending short meteorological records back in time using trees in high-elevation ecotones. The objectives of this study were to explore dendroclimatic signals in tree-ring width chronologies of Abies densa growing in these extreme ecotones and to reconstruct, if possible, annual temperatures over Northern Bhutan. A point-by-point regression analysis revealed that the regional composite chronology was significantly and positively correlated with temperatures of all months of the current year, i.e., January to December. The chronology was highly correlated with annual temperatures (calibration period R = 0.67 and validation period R = 0.50; p < 0.001) allowing a reconstruction of temperature over Northern Bhutan (NB-TEMR). The NB-TEMR reveals some common variations with summer temperature reconstructions of the Northern Hemisphere as well as the Himalayan region, particularly w.r.t to the recent warming trend. The reconstruction covers the period of 1765 to 2017. This reconstruction reveals a warming trend since 1850 with higher rates of warming 1935 to 2017, but with a pause around 1940–1970. The warming is consistent with reduced volcanic activity and increase of greenhouse gases. We anticipate that our new reconstruction of annual mean temperature could be an important contribution for future climate change studies and assessments of climate models. Full article

The alpine treeline ecotones are an early indicator of vegetation’s response to changes in climate, and the advancement of diffuse treeline ecotones has been associated with mean annual warming temperatures. However, the knowledge of how tree demographic size, age and population distribution, and regeneration decrease with increasing elevation and mean annual temperature remain fragmentary in Bhutan. There was no explanation of how treelines migrate in response to the climate. Therefore, the objectives of this study were to investigate tree demographic size and age and population distribution, as well as the regeneration expansion of treeline ecotones of Abies densa trees in response to climate change. Demographic data from thirty transect bands from treeline ecotones and reconstructed mean annual temperatures from tree-rings were used. Regression analysis was used to establish a relationship between elevation/temperature and demographic tree size and age, as well as to determine recruitment frequency distributions and whether these could be driven by climate change. The tree demography indicated that the treeline ecotone in our sampling site is temperature limited. Hence, cooler temperatures at higher elevations should drive decreases in basal diameter, age and recruitment frequencies. From the dendroecological analysis, the diffuse treeline ecotones appear to be climbing on average 1.00 m per year in Northern Bhutan. We also found that the recruitment frequency has increased over recent years (1850–2017), as temperatures continue to rise. The thermal treeline ecotones will be likely to serve as a line of bioclimatic reference against which other zones of bioclimate can be defined. With documented responses of treeline ecotones toward mean annual temperatures, the expectation is that additional warming will continue to influence regeneration expansion in the future. This dynamic response of treeline ecotones towards the climate acts as an indicator of climate change. Information about climbing treelines and altered ecotones should be a vital part of the material for decision makers to consider, to assess impacts and threats to Himalayan alpine biota. Full article

The Sikkim region of the Himalayas (NE India) may form an important microplate between Nepal and Bhutan. Here we report high-resolution pressure-temperature (P-T) paths taken from garnet-bearing rocks across the northern and eastern portion of the region’s Main Central Thrust (MCT) shear zone. The MCT separates units affiliated with the Greater Himalayan Crystallines (GHC) in its hanging wall from the Lesser Himalayan Formation (LHF). Late Miocene monazite ages are reported from the LHF (10–14 Ma), whereas those from the GHC are Miocene (18–20 Ma). Some paths from the LHF and GHC show a P decrease before burial, consistent with erosion before compression. MCT shear zone and GHC rocks show a P increase and then decrease over a short T interval. This hairpin P-T path is consistent with an imbrication model for the Himalayas. LHF P-T path conditions and those obtained using conventional thermobarometry are best in agreement. These paths also are consistent with observed mineral assemblages and garnet zoning. Although we have the most confidence in LHF results, MCT shear zone and GHC P-T path shapes suggest processes to establish imbrication tectonics may have occurred here as early as the Miocene. Full article

The Sundarbans is a deltaic mangrove forest, formed about 7000 years ago by the deposition of sediments from the foothills of the Himalayas through the Ganges river system, and is situated southwest of Bangladesh and south of West Bengal, India. However, for the last 40 years, the discharge of sediment-laden freshwater into the Bay of Bengal through the Bangladesh part of the Sundarbans Mangrove Forests (BSMF) has been reduced due to a withdrawal of water during the dry period from the Farakka Barrage in India. The result is two extremes of freshwater discharge at Gorai, the feeding River of the BSMF: a mean minimum monthly discharge varies from 0.00 to 170 m³·s⁻¹ during the dry period with a mean maximum of about 4000 to 8880 m³·s⁻¹ during the wet period. In the BSMF, about 180 km downstream, an additional low discharge results in the creation of a polyhaline environment (a minimum of 194.4 m³·s⁻¹ freshwater discharge is needed to maintain an oligohaline condition) during the dry period. The Ganges water carries 262 million ton sediments/year and only 7% is diverted in to southern distributaries. The low discharge retards sediment deposition in the forestlands’ base as well as the formation of forestlands. The increase in water flow during monsoon on some occasions results in erosion of the fragile forestlands. Landsat Satellite data from the 1970s to 2000s revealed a non-significant decrease in the forestlands of total Sundarbans by 1.1% which for the 6017 km² BSMF is equivalent to 66 km². In another report from around the same time, the estimated total forestland loss was approximately 127 km². The Sundarbans has had great influence on local freshwater environments, facilitating profuse growth of Heritiera fomes (sundri), the tallest (at over 15 m) and most commercially important plant, but now has more polyhaline areas threatening the sundri, affecting growth and distribution of other mangroves and biota. Landsat images and GIS data from 1989 to 2010 at the extreme northern part of Khulna and Chandpai Ranges revealed the formation of a large number of small rivers and creeks some time before 2000 that reduce the 443 km² forestland by 3.61%, approximately 16 km², and decreasing H. fomes by 28.75% and total tree cover by over 3.0%. The number of the relatively low-priced plants Bruguiera sexangula, Excoecaria agallocha and Sonneratia apetala, has, on the other hand, increased. Similar degradation could be occurring in other ranges, thereby putting the survivability of the Bangladesh Sundarbans at risk. The growing stock of 296 plants per ha in 1959 had been reduced to 144 by 1996. Trend analysis using “Table Curve 2D Programme,” reveals a decreased number of 109 plants by the year 2020. The degradation of the Bangladesh Sundarbans has been attributed to reduced sediment-laden freshwater discharge through the BSMF river system since commissioning the Farakka Barrage on 21 April 1975 in India. To reduce salinity and forestland erosion, the maintenance of sediment-laden freshwater discharge through its river system has been suggested to re-create its pre-1975 environment for the growth of H. fomes, a true mangrove and the highest carbon-storing plant of the Sundarbans. This may possibly be achieved by proper sharing of the Ganges water from the Farakka Barrage, forming a consortium of India, Nepal, Bhutan and China, and converting parts or whole of the Ganges River into water reservoir(s). The idea is to implement the Ganges Barrage project about 33 km downstream, dredging sediments of the entire Gorai River and distributaries in the Ganges floodplain, thus allowing uniform sediment-laden freshwater flow to maintain an oligohaline environment for the healthy growth of mangroves. The system will also create healthy hinterlands of the Ganges floodplain with increased crop production and revenue. The expenditure may be met through carbon trading, as Bangladesh is a signatory of the Copenhagen Accord, UN Framework Convention on Climate Change. The total carbon reserve in the BSMF in 2010 was measured at about 56 million metric tons, valued at a minimum of US$ 280 million per year. The forest is rich in biodiversity, where over 65 species of mangroves and about 1136 wildlife species occur. The BSMF acts as a natural wall, saving property as well as millions of lives from natural disasters, the value of which is between 273 and 714 million US$. A 15 to 20 km band impact zone exists to the north and east of the BSMF, with a human settlement of about 3.5 million that is partly dependent on the forests. Three wildlife sanctuaries are to the south of the BSMF, the home of the great royal Bengal tigers, covering a total area of about 1397 km². Construction of a coal-fired power plant at Rampal will be the largest threat to the Sundarbans. It is a reserve forest, declared as a Ramsar site of international importance and a UNESCO natural world heritage site. Full article