为了解影响冀西北坝上地区杨树(Populus)防护林退化的因素,为坝上地区退化防护林的改造提供科学依据,对张北坝上地区不同退化程度杨树防护林的林分生长状况、土壤、树种及其根系分布进行了调查。防护林的枯稍率和死亡率与林木胸径、树种、土壤硬度及钙积层具有相关关系。林分平均胸径越大,年龄越大,林木枯梢率及死亡率越高;小叶杨的枯梢率和死亡率最高,青杨居中,北京杨最低,由小叶杨构成的防护林退化更为严重;土壤硬度较高、具有钙积层的土壤上,杨树的退化较为严重;杨树的根系分布土层较浅,根系生物量主要分布在0~50cm深土层中;近50年来,受气温上升的影响,张北坝上地区气候具有日益干旱化的趋势。坝上地区杨树防护林的退化是气候干旱、林分年龄、土壤及林木病害等因素共同作用的结果,杨树枯稍及死亡的直接原因是杨树腐烂病,各种因素引起的干旱胁迫及林木年龄过大导致的生长势衰弱则促进了腐烂病的发生。研究结果可为该地区的防护林经营及改造提供科学依据。

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.