Variation in stand structure and tree species composition across topographic gradients in a warm-temperate secondary oak forest of western Japan

Understanding the structural and compositional characteristics of secondary forests is essential for evaluation of their successional dynamics, which are critical for development of forest management strategies at both local and regional scales. This study aimed to investigate the variations in stand structure and tree species composition under different topographic conditions within a secondary deciduous oak forest in a hilly area of the lower warm-temperate zone in western Japan. A tree census was conducted using 100 m² plots across four topographic conditions—crest slope, upper sideslope, lower sideslope, and colluvial slope—with ten plots established for each condition. The results indicated that the dominant canopy deciduous oaks, <i>Quercus serrata</i> and <i>Quercus variabilis</i>, exhibited either a unimodal or flattened unimodal diameter at breast height distribution across these conditions. In contrast, the evergreen oak <i>Quercus glauca</i> displayed an inverse J-shaped distribution on the upper sideslope, lower sideslope, and colluvial slope. These structural patterns suggest that <i>Q. glauca</i> is likely to increase in dominance across most of the hilly area, potentially driving a transition from a deciduous to an evergreen broad-leaved forest. However, tree species composition varied among topographic conditions, with certain deciduous broad-leaved species and evergreen coniferous species characterizing the crest and colluvial slopes. These findings underscore the importance of vegetation monitoring to clarify topography-related successional patterns in this oak forest, which would also be essential for evaluating the necessity of implementing management practices for forest and biodiversity conservation.