Abstract: Asymmetric competition plays a crucial role in the spatial distribution of populations, community structure transitions, and stable forest succession. Based on survey data from fixed 1hm2 plots in each of three different succession stages (Betula platyphylla forest, B. platyphylla-L. gmelinii mixed forest, and Larix gmelinii forest) in Daxing’an Mountains, we quantified the spatial distribution and competition asymmetry of different tree species (Betula platphylla and Larix gmelinii) and different size classes (saplings, medium trees, and large trees) using univariate and bivariate pair-correlation functions g(r) and univariate and bivariate mark variogram γ(r). The results showed that dominant tree species exhibited significant aggregated distribution at multiple scales. Symmetric competition was mainly found within species. Significant asymmetric competition was only found at the 0-1 m scale in the Betula forest. In the mixed forest, there was a significant symmetric competition between Betula and Larix at the 2-3 m scale, with spatial negative correlation at the 0-3 m scale. As tree size increased, the spatial distribution patterns of trees in each successional stage showed a transition from clustered distribution in saplings to random distribution in large trees. In Betula forest, individuals of the same and different classes mainly exhibited symmetric competition, with asymmetric competition only found between medium and large trees at the 10-12 m scale. The spatial correlation between size classes was mainly negative and uncorrelated. In the mixed Betula-Larix forest, only saplings showed significant asymmetric competition at the 10 m and 20 m scales. Large trees exhibited significant symmetric competition with medium and sapling trees, with positive spatial correlations between size classes. In Larix forest, individuals within and among different classes primarily engaged in symmetric competition, with significant asymmetric competition between saplings and large trees at multiple scales, and the spatial correlations between classes were mainly unrelated. Symmetric competition within the same species and size classes was an important factor influencing the spatial distribution patterns of trees in natural forest communities at different successional stages.

Key words: Daxing’an Mountains, symmetric competition, asymmetric competition, mark variogram, natural forest