Stable isotope analysis reveals prolonged drought stress in poplar plantation mortality of the Three-North Shelter Forest in Northern China

The Three-North Shelter Forest (TNSF) is an important ecological barrier in northern China. However, in the last decade, poplar tree degradation has occurred in similar to 80% of TNSF stands in Zhangbei County (Hebei Province), with about one-third of trees either dead or dying. Using isotopic techniques, we investigated the corrected delta C-13 value of tree-ring (delta C-13(cor)) and intrinsic water-use efficiency (iWUE) differences between non-dieback and dieback trees of the same age to understand the cause of poplar dieback death associated with the TNSF. We found that the diameter of poplar trees within the same age group decreased as degradation progressed. From 1997, inter-group differences of delta C-13(cor) and iWUE occurred but were not statistically significant. The differences became significant from 2002. Comparisons suggest that the continuous occurrence of positive Delta iWIJE (iWUE(dieback)-iWUE(non-dieback)) may be the threshold for subsequent divergence of the two groups. Stepwise regression revealed that the impact of groundwater depth on iWUE was stronger than that of other environmental factors (e.g., temperature and evapotranspiration) and was the leading cause of poplar degradation. IsoSource model analyses indicated that non-dieback trees took water mainly from soil 30-80 cm below the ground surface, and that dieback trees did it mainly from 0-30 cm below the surface. Non-dieback trees used more water from the 80-150 cm soil layer and groundwater than did dieback trees. Groundwater depth, which increased in the experimental area and so aggravated local droughts, was strongly related to cumulative water consumption but not to evapotranspiration (ET0), indicating that increasing groundwater depth was primarily caused by the change in land use. Therefore, poplar dieback in the TNSF in Zhangbei County was attributed to groundwater overuse associated with changed land use, which enhanced the duration and intensity of water stress on the trees.