Abstract:Based on the investigation data from a 9.6 hm2 Miyaluo dark coniferous forest dynamics plot in western Sichuan in September 2019, the diameter at breast height (DBH) size class structure of two dominant species Abies fargesii var. faxoniana and Betula utilis was analyzed. The spatial distribution, intraspecific and interspecific associations of the two dominant species with four different DBH size classes were also illustrated by using paired correlation functions g(r) and g12(r) in the O-ring statistics of spatial point pattern analysis. The DBH size class structure of the two dominant species showed continuously and a reverse "J" type distribution. The individuals of A. fargesii var. faxoniana and B. utilis with different DBH size classes mainly showed a tendency from aggregation distribution to random distribution. The interspecific relationships of A. fargesii var. faxoniana including small trees to saplings, medium trees to saplings, medium trees to small trees, big trees to medium trees showed positively spatial associations, while that of B. utilis presented a trend from significantly positive (P<0.05) to non-obvious associations (P>0.05). With the increase of spatial scale, the relationship between big trees to sapling and between big trees to small trees of A. fargesii var. faxoniana was from non-obvious to significantly positive associations (P<0.05). However, the big trees to saplings of B. utilis showed a change from significantly negative to non-obvious associations (P<0.05). In the meanwhile, the big trees of A. fargesii var. faxoniana to all DBH size classes individuals of B. utilis mainly showed a trend from significantly negative (P<0.05) to non-obvious associations. There were mainly significant negative associations (P<0.05) between individuals with big DBH size classes of B. utilis and all DBH size classes individuals of A. fargesii var. faxoniana (P<0.05). In short, the spatial distribution pattern and associations of A. fargesii var. faxoniana and B. utilis varied with the change of DBH size classes and the spatial scale.
