Abstract: Picea crassifolia is an important constructive and dominant species of subalpine mountain forest vegetation in the Qilian Mountains. This paper takes the Picea crassifolia natural secondary forest as a research object which grows in Dayekou Basin of the Qilian Mountains, by setting up a major quadrate method investigation and analyzing its population structure and spatial distribution pattern. The method of adjacent grid method was used to analyze the entire individual[JP8]’s[JP] data of 100 m×100 m simple investigation, the population dynamics and [WTBX]DBH[WTBZ], tree height and crown breadth frequency distribution and 6 kinds of aggregation intensity indexes were used to analyze the structure and spatial distribution pattern of Picea crassifolia population. The results show that population dynamics of Picea crassifolia population belonging to increasing type. DBH size class frequency distribution of Picea crassifolia population is “pour J” type, DBH size class with the change of individual level is quite fit for the logarithmic equation y=219.32 ln (x) + 482.67 (R²=0.9638, P< 0.01), differentiation index of DBH is 0.48, the differences of DBH in the population for each individual is obvious; Frequency distribution of the tree height is “intermittent” type, Tree height size class and individual level can be explained by quadratic equation y = 0.795 x²-31.23 x + 285.1 (R² = 0.603, P < 0.01), differentiation index of tree height is 0.55, tree height difference is evident; The logarithmic equation y = 5.912 ln (x) 4.2493 (R² = 0.603, P < 0.01) can be used to show the relation between tree height and DBH; Crown breadth size class and individual are accorded with equation y = 5.3176 x³-91.759 x² + 408.88 x-173.87 (R² = 0.8355, P < 0.01), differentiation index of crown breadth is 0.53, the differentiation crown breadth of population individual is apparent. In general, Picea crassifolia seedlings is relatively rich, natural regeneration ability is strong, at present the performance of Picea crassifolia population is the mature stable populations. On the spatial distribution pattern, Picea crassifolia population space distribution pattern is gathered as plaques form, and the diffusion coefficient, bushes index, index of patchiness, average crowded index, negative binomial distribution parameters, Cassie index are 1.162, 2.285, 0.162, 85.802, 1.002 and 0.026 respectively. Distribution pattern for different development stages are different, Ⅰ level and Ⅱ level seedlings are in aggregate distribution, the young tree, middle size tree and big tree distribute uniformly, with the age increase, the population gathering level reduce, which means that the aggregate distribution turns into uniform distribution, this population displays obviously diffusion trend. This study can provide the theoretical proofs to administration and management of Picea crassifolia forest.

Key words: Qilian Mountains, Picea crassifolia, population structure, differentiation indexes, spatial distribution pattern