@phdthesis{Tayierjiang2015, author = {Tayierjiang, Aishan}, title = {Degraded Tugai Forests under rehabilitation in the Tarim riparian ecosystem, Northwest China: monitoring, assessing and modelling}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:824-opus4-2938}, school = {Katholische Universit{\"a}t Eichst{\"a}tt-Ingolstadt}, pages = {XIX, 130 Seiten : Illustrationen, Diagramme, Karten}, year = {2015}, abstract = {The Tarim River, located along the northern and eastern borders of the Taklamakan Desert in Xinjiang, Northwest China, is about 1320 km long and thus one of the longest continental inland waterways in the world. In an undisturbed state, it is accompanied on both sides by floodplain forests of Euphrates Poplar (Populus euphratica Oliv.), also known as "Tugai Vegetation" or "Tugai Forests". These forests form typical desert riparian forests as the so-called "Green Corridor" and prevent the two deserts, the Taklamakan and the Kuruk-Tagh, from merging together. At present, large areas that were originally P. euphratica riparian forests have disappeared, with the remaining areas in critical condition. Specifically, the greatest threats over the past few decades have been the rising water consumption in the upper and middle reaches and massive hydro-technical interventions (construction of embankments and dams) in the development of uncultivated land (including the direct clearance of forests) for cotton production. These have led to widespread destruction of the natural ecosystems, particularly in the lower reaches of the river where about 320 km of the floodplain forests are either highly degraded or dead as the result of nearly 30 years of river desiccation. As a consequence of these deteriorations, land desertification has been aggravated and has expanded in the region. Because of the challenges facing this ecosystem, restoration measures with the diversion of "Emergency Ecological Water" have been implemented in the affected areas since 2000. This new process of recovery measures needs efficient and extensive long-term monitoring. In this thesis, current restoration measures as implemented to date are presented and hydrological dynamics as a function of water diversion are analysed. The structure, distribution and health statues (referred to as "vitality") of the dominant species Populus euphratica have been monitored by measuring Eco-morphological parameters of more than 5000 trees in two sampling sites (a 100-ha permanent plot and five random sampling plots of 50-m radii) at Arghan village in the lower reaches of the Tarim River in the Xinjiang Uyghur Autonomous Region of China. The effects of restoration measures on the revitalization of degraded trees have been analysed and assessed by using individual-based repeated measurements of the tree data. From a long-term point of view, and taking labour intensity and the high cost of tree height measurements into account, a suitable height-diameter model for obtaining the tree height (TH) of P. euphratica based on the diameter at breast-height data (DBH) has been developed and evaluated. The average annual groundwater depth within 300 m of the river channel at different transects (Yingsu, Karday, Arghan) recovered significantly after a second water diversion in 2001. The same rising trend in groundwater level was retained until 2004, started to decline until 2009, and then rose again until the end of the water diversion in 2011. The Mann-Kendall statistical trend test was applied to the mean monthly and annual groundwater depth of six groundwater wells at different distances from the river on the Arghan transect. The trend test for the whole time series period from 2003 to 2011 indicated that there was a statistically significant decreasing trend for all wells, but that from January 2010 to December 2011 the groundwater level in all wells had a statistically significant increase. With the exception of a relatively favourable groundwater depth for floodplain forests within a 150 m distance of the riverbed, the groundwater depth at larger distances remained far below 5 m, causing most forest plants (with the exception of the Tamarix sp.) to suffer from water scarcity. Analyses of forest structure, distribution and health status suggested that a maximum TH of 19 m was found within 0-20 m and 100-200 m of the river. By contrast, a maximum DBH of 126 cm was found within 500-600 m of the river. Trees with a TH of over 15 m and a DBH of over 60 cm appeared sporadically. The maximum CD (crown diameter) of 16.58 cm (one tree) was found next to the river bed. There was a relatively great variation in DBH when compared to TH and other eco-morphological parameters. On average, stand structure tended to be simpler as distance from the river increased. Trees classified as healthy and good accounted for approximately 40 \% of all sample trees, with slightly and highly degraded trees representing nearly 60 \%. Some of the eco-morphological parameters of the trees vary differently with respect to the ecological water diversion measures. In particular, in the vertical (north-south) direction, the farther the trees are from the river, the weaker the response of ecological indicators. These parameters include: tree vitality classes (VS), crown diameter (CD), height to crown (HC), frequency of root suckers, number of young seedlings, and shoot growth, as well as newly developed crown types of P. euphratica trees. Generally, the floodplain forests within 200 m of the riverbed recovered remarkably, and those between 200 and 800 m from the river showed a medium response to the water transfer, while forests further than 800 m away from the riverbed showed lesser signs of recovery. Following the evaluation and comparison of the results from ten commonly used models by means of multiple-model performance criteria (such as asymptotic t-statistics of model parameters, standardized residuals against predicted height, the root-mean-square error (RMSE), Akaike's information criterion (AIC), mean prediction error (ME) and mean absolute error (MAE)), Model (1): TH=1.3+a/(1+b×e^(-c×DBH)) and Model (6): TH=1.3+DBH^2/(a+b×DBH+c×DBH^2) are recommended as suitable choices for approximating the height-diameter relationship of P. euphratica. The limitations of the other models, resulting in poor performance for predicting TH accurately, are also discussed.}, subject = {Tarim}, language = {en} }