Structure and dynamics of Picea crassifolia populations with graze disturbance in different seasons of enclosed forest

In the present study, the structural features of Picea crassfolia populations in Haxi forest region of Qilian Mountain nature reserve in different grazing season were investigated. We surveyed the data with the method of contiguous grid quadrats in two sample plots and investigated the population age structure, population life tables and survival function of the P.crassifolia populations. The results showed that the survival rate of P.crassfolia population in Ⅰ ageclass to Ⅱ ageclass in winter grazing season was 67.5% and the highest mortality rate appeared in the Ⅳ ageclass. The population survivorship curve was close to the Deevey Ⅰ. The maximum death density function was 1.83%, which appeared in Ⅲ ageclass. The survival rate of P.crassfolia population in summer grazing season in Ⅰ ageclass to Ⅱ ageclass was only 16.5%. The maximum survival rate appeared in the Ⅲ ageclass and the highest mortality rate appeared in the Ⅰ and Ⅵ ageclass. The population survivorship curve was close to the Deevey type Ⅱ and the survival rate was relatively stable. The maximum death density function was 0.23%, which appear in Ⅴ ageclass. Our results indicated that the effect of grazing on the structure and dynamics of P.crassifolia population in winter was stronger than that in the summer. Grazing in winter has disturbed population structure seriously, leading to lack of seedlings and saplings reserves, and population structure disorders.