This study investigates whether plant richness or soil conditions affect the productivity of silver birch (Betula pendula Roth.) stands naturally regenerated in central Poland to a greater extent. We analysed 31 plots with birch regeneration aged from 2 to 19 years. At each plot, we determined the features of birch stands, vegetation cover, and soil. Relevés were elaborated according to the Braun--Blannquet procedure and ecological indicator values were calculated according to Zarzycki et al. To describe soil characteristics, we sampled its upper 50 cm layer. In the laboratory, texture, reaction, total carbon, total nitrogen (C and N, respectively), and base cation contents were determined. These features served to calculate the soil fertility index. Tree height measurements were applied to determine the site index (i.e., height at the base age of 25 years) for the analysed stands with a previously developed formula. The stands were also characterised with density and volume. We found that the site index depended significantly on the C:N ratio and soil fertility index (r = 0.36, p = 0.045 and r = -0.49, p = 0.005, respectively), while the stand density and volume were driven by both soil features, namely pH and loam content (r = 0.44, p = 0.013 and r = 0.36, p = 0.044, respectively) and plant richness, i.e., the number of trees and ingrowths (r = -0.52, p = 0.003 and r = -0.37, p = 0.038, respectively). At the early stage of silver birch regeneration, soil conditions play a more important role in stand development than the vegetation cover, which has the negative effect on birch productivity.