Analysis on the Main Causes Resulting in Vegetation Degeneration in the Heihe River Basin

Abstract

The Heihe River Basin is located in an arid area in northwest China,and its entironment is very fragile.According to the survey,the natural vegetation is seriously degenerated in the middle and lower reaches of the drainage basin.Since the 1950s,the area of the shrubbery meadow steppes with vegetation coverage higher than 70% has been reduced by 78.26%,the areas of the steppes with vegetation coverage varying in a range of 30%~70% in the lake basins and billabongs,marsh meadow grasslands and steppes with high forage yield have been reduced by about 40%,and the total area of steppes has been reduced by about 50%.The old-line Erjina Oasis has been in a deterioration trend,and the areas of woodlands and meadow vegetation have been seriously shrunk.The area of the riparian forests of Populus euphratica and Elaeagnus angustifolia has been reduced by 54% compare with that in the 1950s,and the area of riparian Tamarix ramosissima has been reduced by 33%;the flaky Achnatherum splendens meadows and Phragmites australis marshes are in a gradual disappearance,and most of them have been replaced by the communities of Alhagi sparsifilia,Karelinia caspica and Lycium ruthenicum;the riparian forests and shrubberies are seriously degenerated.There are many factors affecting vegetation degeneration,such as the arid trend of the natural environment in recent years,especially the decrease of inflow to the lower reaches of the drainage basin.Thus,a continuous drawdown of groundwater level and a seriously soil drought in vadose zone occur because of the increase of the vadose zone thickness.