The cumulative contributions of 3 .EGA(PCR)s selected according to eigenvalue were 63.8%, 84.3% and 96.0%, respectively.

• 对白榆全分布区20个群体的样点6维环境因子PCA求算的EGA(PC_r)；取r=1；2；3；建立3个生态梯度轴；累计贡献率依次为63.；8%25；84