Comparative Study On Vitality Dynamics Of Populus Euphratica Oliv. Riparian Forests In The Lower Reaches Of Tarim River By Ecological Water Transportation

Desert riparian forest is the pillar of the desert ecosystem, also is a main protection system for ecological stability and biodiversity of river in arid region. Populus euphratica Oliv. is the only dominant tree species to form large-scale riparian forests in the continental arid regions of NW China, mainly distributed in the flood plain areas of Tarim and Heihe River basin. Populus euphratica has multiple ecological functions such as wind reducing and sand fixation, soil and water conservation, climate regulation etc. The rising water consumption in the upper and middle reaches and, in the past decades, massive hydrotechnical interventions during the development of uncultivated land use (cotton growing), led to the widespread destruction of the natural ecosystems, particularly the Populus euphratica riparian forests are either highly degraded or dead in the lower reaches of Tarim River. As a result, protection and reconstruction of the "green corridor" has become an urgent priority task. In order to avoid the further expansion of the desert, it is absolutely essential to preserve the floodplain forests and to revitalize the variously degraded P. euphratica stands. Since 2000 the Chinese government has made efforts to restore these ecosystems and conducted man-made ecological water transfer. The riparian forest is being regenerated and improved in different degrees, As the result of ten water diversion to the lower reaches of Tarim River, but it is not far enough to gurantee the overall recovery of the downstream ecological system. Practice has proved that it was quite difficult to recover the ecological system of damaged desert riparian forest.In this paper, positive response of growth status of Populus euphratica to ecological water transfer was analyzed based on large number of data obtained from field investigation in the past five years along the Argan section in the lower reaches of Tarim River. The results show that Ecological water diversion into the lower reaches of Tarim River has a positive impact on the recovery and regeneration of desert riparian vegetation. With the continuous and expanding of water diversion, groundwater depth nearby the riverbed has returned to some level that riparian vegetation can completely use. Ecological parameters of the Populus euphratica forests have different response to the ecological water transfer. Generally, in the vertical direction, the farther the coverage is from the river, the weak response the ecological parameters are. These parameters include the canopy density, crown size, crown loss as well as the initiation sticks of Populus euphratica seedlings and the frequency of delivery. The growth volume of DBH and height of tree near the riverbed is bigger compared to the growth volume of tree growing in far from the riverbed. In the meanwhile, the growth volume of tree height and DBH show up a spatial and temporal response. From 2004 to 2007, annual DBH average growth volume of trees at middle age is 0.81cm,0.74cm,0.65cm,0.41cm,0.19cm, respectively. From 2007 to 2010, annual DBH average growth volume of trees at middle age is 0.66cm,0.64cm,0.57cm,0.32cm,0.10cm, respectively. Along with the increasing of the distance from the riverbed, average growth of Populus euphratica began to decline. Quantity of Populus euphratica in Different growing ranks has different trend of fluctuations, and the fluctuation mainly took place between trees ranked by VS0/VS1/VS2/VS3, the quantity of Populus euphratica ranked by VS1/VS2 in research area has increased gradually.