The spatial distribution patterns and associations of plant species result from both abiotic and biotic interactions, which may reflect factors and processes that nearly all plants experience; namely resource availability constraints, limits to propagule dispersal, and intra-or inter-specific competitive interactions. In the karst area of Guilin, China, Alchornea trewioides (Benth.) Muell. Arg. is a common pioneer species; to determine the importance of its ecological function in this community, it is critical to analyze the spatial patterns of its individuals. To this end, the spatial patterns of A. trewioides were investigated in situ and analyzed with three objectives: (1) to describe the spatial patterns and individual-association changes among the different diameter classes; (2) to quantify and reveal the relationship between the spatial scale and the spatial patterns for this species; and (3) to analyze the effects of different diameter classes on the spatial patterns and the individual associations. In this study, we set up six plots, each 10 m × 10 m, in the karst habitat (i.e., calcium-rich, alkaline soil with low water-holding capacity, high ratio of exposed(80%), and prone to periodic flooding), wherein we recorded and mapped all live individuals of A. trewioides. To identify the differences and associations between these individual plants, they were divided into three diameter classes Ⅰ (diameter ≤ 0.5 cm), Ⅱ (0.5 cm < diameter ≤ 1.0 cm), and Ⅲ (diameter > 1.0 cm). The point-pattern analytical approach has proven effective and useful elsewhere to study the plant population characteristics and the community structure underlying ecological processes. On the basis of Ripley's K function, we used the g(r) function statistic to explore differences in the distribution patterns. We also used the L(r) function statistic to examine the spatial associations among the three diameter classes. The results showed that the spatial patterns and their individual associations were closely related to the spatial scale and diameter class tested: (1) At the scale of 2 m, the A. trewioides population had a clumped distribution, whereas at a much larger scale it had a random distribution. (2) The different diameter classes were another important factor that affected the spatial associations. As the diameter classes of individuals were gradually increased, the changing spatial associations between individuals went from positive, to independent, and then to a negative correlation. (3) On a large spatial scale, the mother plants Ⅲ played a significant role in inhibiting the cloning of other smaller ramet plants. However, as the differences among the three diameter classes increased, the spatial association gradually showed a correlation that was independent, or even a negative. Comparing the spatial distribution patterns among diameter classes Ⅰ and Ⅱ, Ⅱ and Ⅲ, and Ⅰ and Ⅲ, the last class showed the strongest negative association in the population that we studied.