Abstract: Based on the data from 100 m×100 m fixed plots in Cuigang Forest Farm of Xinlin Forestry Bureau, we investigated the impacts of horizontal and vertical dimensions on community assembly processes and species abundance at different successional stages in the Greater Khingan Mountains. These plots were established in communities at three different successional stages, including Betula platyphylla forest, Larix gmelinii-Betula platyphylla mixed forest, and Larix gmelinii forest. Different species abundance models were used to fit species abundance distributions of large tree layer, sapling layer, and regeneration layer at each successional stage and under different sampling scales (ranging from 100 to 10000 m²). The χ² test was employed to determine the optimal model, and then the community assembly mechanisms of each community under different spatial dimensions were quantified. The results showed that: (1) The optimal species abundance models varied under different spatial and temporal dimensions. The Niche Preemption Model and the Zipf-Mandelbrot Model exhibited the best fitting effects. (2) The neutral processes in the local communities were relatively strong. As the sampling scale increased, the neutral processes weakened while the ecological processes strengthened. The species abundance patterns of the large tree layer in the Betula platyphylla forest and the mixed forest were less affected by the sampling scale. As the forest layer became smaller, the influence of the sampling scale on it gradually increased. The species abundance pattern of the large tree layer in the Larix gmelinii forest was greatly affected by the scale, while the species abundance pattern of the regeneration layer was less affected by the scale. (3) The larger the forest layer in the community, the stronger the ecological processes of community assembly and the weaker the neutral processes.

Key words: successional stage, tree layer, observation scale, community assembly mechanism