Effectiveness and challenges of ecological engineering for desert riparian forest restoration along China's largest inland river

Environmental flow, one of the main restoration measures for regulated rivers, aims to restore and sustain degraded riparian ecosystems and the human wellbeing that depends on these ecosystems with a reasonable magnitude, frequency, duration and timing of water flow. However, efficacies of environmental flows are often restrained by other demands along the river and inappropriate water management, leading to only spatially limited habitats being suitable for ecosystem functioning. This paper presents current restoration measures in the lower reaches of the Tarim riparian ecosystem in Northwest China and analyzes ecohydrological dynamics as a function of environmental flows. The groundwater level oscillation before/after and during the environmental flows (i.e., manmade water diversion) was examined, and the Mann-Kendall test was applied to identify the overall trend of change in groundwater depth from 2003 to 2011. The growth, distribution, health status (referred to as "vitality") and renewal potentials of the dominant tree species Populus euphratica were quantified by using individual-based repeated measurements of P. euphratica morphology in two sampling sites (a 100-ha permanent plot and five random sampling plots with 50-m radii) at Arghan village in the lower reaches of the Tarim River. Based on these measurements, the effects of restoration measures on the groundwater recharge and the revitalization of degraded trees were assessed. The main contribution and achievements of the water diversion project on rehabilitation of the degraded riparian forests were evaluated; further, the limitations and challenges faced by of the restoration project were examined. The results of this research could help establish a reference for monitoring changes in riparian forests in Central Asia and possibly recommend restoration measures for riparian forest ecosystems in arid regions worldwide.