雅鲁藏布植被类型及与侵蚀类型的关系

Typical Types of Vegetation and Erosion in the Yalutsangpo Basin

雅鲁藏布流域发育高山垫状植被、干旱河谷植被、沙洲植被、亚热带植被和高山冰川植被等多种类型的植被,与各种侵蚀类型相互克制又相互适应。通过2年的野外调查和采样分析研究植被分布规律和与各种侵蚀的相互关系。峡谷亚热带植被的物种多样性、覆盖度和植被厚度都很高,水力和风力侵蚀完全被克制,但是重力侵蚀和泥石流侵蚀发育。高山垫状植被由几十种耐寒草本和苔藓物种组成,植被仅1～3 cm厚,但是却有半米多厚的密集根系,侵蚀率极低。干旱河谷植被覆盖度仅1%～10%,由阳生草本和地衣苔藓组成,承受各种水力侵蚀。雅鲁藏布和拉萨河河谷有很多风沙沙丘,风力侵蚀率极高,只有极少不怕风沙的物种构成极稀疏的沙丘植被。植被平均厚度随高程降低而增加,在5 000 m高山只有2 cm,到2 000 m的大峡谷增加到3 m。物种多样性和覆盖度在干旱河谷最低,到5 000 m高山和低于2 000 m大峡谷都增加到最大值。

Various types of vegetation and erosion occur in the Yalutsangpo （Brahmaputra） River Basin. Field investigation was carried out in 2009 and 2010 studying the relation between the typical types of vegetation and ero- sion and the distributions of vegetation and erosion along the elevation. An alpine mat vegetation develops on the mountains with altitude over 4 000 m, with a coverage over 90% but a vegetation thickness of only 1 - 3 cm. The river valleys in the west part of the Yalutsangpo and the Lhasa River are very dry with low precipitation and high e- vaporation. Poor vegetation develops in the dry valleys and water erosion and aeolian erosion occur. In the great Yalutsaangpo Canyon subtropical vegetation with coverage of almost 100% and highest vegetation thickness controls all forms of water erosion and aeolian erosion. Nevertheless, riverbed incision causes very intensive gravitational e- rosion, with main forms of avalanches and landslides. Sclerophyllous forest grows in glacial gullies and moraine fans. The vegetation thickness increases from 2 cm in mountains of elevation of 5 200 m to 350 cm in the gtand canyon of elevation of less than 2 000 m. The taxa richness, or the number of species, and the vegetation coverage is extremely low in the dry valleys at elevation of 3 000 ~ 3 500 m, and inexease with both increasing and decrea- sing elevation. The richness and coverage reach the highest for the subtropical vegetation and for the alpine mat vegetation.