Morphological trait as a determining factor for Populus simonii Carr. to survive from drought in semi-arid region

Widespread tree mortality induced by drought events causes significant damage to the local ecosystem and society. Populus simonii Carr. in Zhangbei, Bashang region, China, which was the dominant species of local shelter forest, experienced a wide and heterogeneous dieback, calling for an urgent need for research revealing the mechanism of this mortality event. We conducted a field survey across the county examining 443 poplar individuals with different degrees of degradation. Basic morphological tree data and tree-ring samples were obtained and analysed. The stable water isotope technique was also applied to predict the water use pattern and rooting depth. Long-term climate data were analysed and the self-calibrating Palmer drought severity index (scPDSI) was calculated. The main results were as follows: 1) The growth rate of diameter at breast height (DBH) and tree height began to decrease after the year 2000; however, this drawback effect tended to be stronger for poplars with a higher degradation degree. The results also showed that the poplars that survived were inherently larger in size. 2) Following the increase in the degree of degradation, poplars gradually lost their ability to absorb deep soil water, putting them into a more stressful water condition. We concluded that ageing and stand density were not the major effects but drought accounted for the mortality event. Larger poplars with deeper root systems tended to be more resistant to drought. Local water resource management is also vital for shelterbelt forests to survive and grow.