Abstract:

This study was aimed at understanding the factors influencing the poplar shelterbelts degradation in the Bashang plateau of northwest Hebei Province to provide a scientific support for reconstruction for the degraded forests.The growth,soil characteristics,species and the root vertical distribution of the degraded forests in Zhangbei Bashang region were investigated.There were relationships between mortality rate and dieback rate of poplar trees and DBH,tree species,soil hardness and soil calcium layer.The rate of mortality rate and dieback increased with increasing DBH.Three species ranked in a descending order based on the rate of mortality rate and dieback was Populus simonii,Populus cathayana,Populus beijingensis,and the forests composed of Populus simonii degraded more seriously than others.The forests that degraded more seriously were located in the sites where soil hardness was higher,special with calcium layer.The roots of poplar trees are distributed in shallow soils,and most of root biomass was located in the soil of above 50cm depth.In the recent 50 years,there had been a obvious trend of aridity for the climate of Bashang plateau due to the increasing average annual temperature.The degradation of poplar shelterbelt in the Bashang plateau was related to drought climate,forest age,soil and tree diseases.The poplar rot disease was caused by the direct factors of the mortality and dieback of poplar trees,and the drought stress led by other factors and growth potential became weak for their old ages,which caused the occurrence of rot diseases.The result contributes to the management and transformation of poplar shelter belt.

Key words: Northwestern Bashang plateau of Hebei Province, poplar, shelterbelt, degradation, drought