Stand structure and growth patterns of understorey trees in a coniferous forest, Taisetsuzan National Park, northern Japan

Stand structure was studied with special reference to growth and mortality patterns of sapling and understorey trees in a coniferous Picea jezoensis and Abies sachalinensis forest in Taisetsuzan National Park, Hokkaido, northern Japan. Picea jezoensis was dominant in the basal area, while A. sachalinensis was abundant in large numbers in the canopy. Estimated mortalities increased significantly with diameter at breast height (DBH) for both P. jezoensis and A. sachalinensis in the canopy, but the tendency was different between the two species. Picea jezoensis had a lower mortality rate than A. sachalinensis, especially at small DBH classes. The spatial distribution of understorey individuals of A. sachalinensis did not show any significant correlation with the spatial distribution of canopy gaps, but that of P. jezoensis showed: a significant correlation. Abies sachalinensis can grow higher than P. jezoensis under suppressed conditions; while P. jezoensis requires canopy gaps for steady height growth. This growth pattern leads to a different waiting height in the understorey (>= 2 m in height and 10 cm in diameter at breast height). Abies sachalinensis waited for an improvement in light conditions at higher strata (max. 7 m), while P. jezoensis waited at lower strata (max. 3 m). The estimated mortality of understorey A. sachalinensis increased with size, while that of understorey P. jezoensis decreased. Therefore, R jezoensis gives priority to survival while A. sachalinensis gives priority to understorey growth. The difference in the 'waiting pattern' between the two species in the understorey was considered a significant feature for the canopy recruitment process of P. jezoensis and A. sachalinensis.