Density-dependent effects in Picea schrenkiana forests in Tianshan Mountains

Abstract

The density-dependence hypothesis is one of the most important mechanisms proposed to explain species coexistence in natural forest communities. Picea schrenkiana is the single dominant species of the forest communities in Tianshan Mountains, which plays a very important role in temperate forests of China. We used reexamined data with a five-year interval on an 8-ha P. schrenkiana forest dynamics plot to illustrate the dynamics of species composition. Pointpattern analysis was conducted to reveal the spatial structure of P. schrenkiana individuals of different ages, and the effects of the density-dependence hypothesis on different ages of P. schrenkiana was discussed. The results were as follows: (1) Woody plants with DBH ≥ 1 cm were 11,835 trees in 2009 and 11,050 in 2014. The mortality rate was 8.82%, the recruitment rate was 2.19%, and the net decrease was 6.63%; (2) Young and medium trees of P. schrenkiana presented an aggregation distribution across a scale of 0–40 m, and the aggregation intensity decreased as the scale increased. Mature trees presented a random distribution on scales of 0 m ≤ r ≤ 2 m and r = 4 m, and an aggregation distribution on other scales with a low density. (3) Using a “case-control” design method and eliminating habitat heterogeneity, the distribution of young and medium trees were taken as examples of case. The distribution pattern of mature trees was used as a comparison and represented habitat heterogeneity, and was compared with that of young and medium trees. Young and medium trees of P. schrenkiana were observed to present ·研究报告· 第 3 期 王慧杰等: 天山雪岭云杉森林群落的密度制约效应 253 more considerable gathering than was observed for mature trees on scales of 0–40 m, and density decreased while the diameter class increased. These results indicate that the density-dependence hypothesis has an influence on distribution patterns of different age classes. This study reveals that P. schrenkiana was affected by habitat heterogeneity and presented a significant aggregation effect. The spatial pattern of P. schrenkiana is revealed to be affected by the density-dependence hypothesis after eliminating the effects of habitat heterogeneity. The results of this paper support the Janzen-Connell hypothesis.