During the past five decades, a significant decrease occurred in the river discharge of the middle Yellow River main stream mainly because of climate changes and human activities in the Loess Plateau. Previous studies on the attribution of river discharge changes usually focused on the lumped effects of climate changes and vegetation growth on hydrological processes, neglecting the contribution by the vegetation growth caused by climate changes. This paper analyzed the variation trends of several ecohydrological variables through a case study of the Wuding River watershed. Two types of simulations, assuming static and dynamic modes of vegetation changes separately, were carried out during the period 1961-2012 using a newly developed physically-based eco-hydrological model, CLM-GBHM. Results show that relative to the 1961-1971 discharge, 79.8% of the decrease in the 1972-2000 discharge was contributed by climate change itself while 74.6% by climate change and vegetation growth. And relative to 1972-2000, vegetation growth over 2001-2012 reduced the increasing trend in river discharge that would be produced directly by climate change, thereby leaving human activities as the major factor in the discharge reduction. All this shows a buffer effect of vegetation growth in the Wuding River watershed.