Age structure and response to fine scale disturbances of Abies sachalinensis, Picea jezoensis, Picea glehnii, and Betula ermanii growing under the influence of a dwarf bamboo understory in northern Japan

We studied regeneration patterns of the four canopy dominants, Abies sachalinensis (Fr. Schm.) Masters, Picea jezoensis (Sieb. et Zucc.) Carr., Picea glehnii (Fr. Schm.) Masters, and Betula ermanii Cham., of an old-growth (>400 years) boreal coniferous forest in northern Japan. Age and size structure, height growth, and diameter growth of tree populations in a 40 X 40 m plot were analyzed. Seedling establishment was restricted to nurse logs and mounds (98%), where the influence of dwarf bamboo (Sasa senanensis (Franch. et Sav.) Rehd.) was limited. Abies sachalinensis had a high density (10 263/ha) but a high mortality (8.4%/year for saplings, 1.7%/year for canopy trees) and a short canopy residence time. It also showed a more rapid tree-ring width increase after disturbances. Picea glehnii had a low density (1450/ha) but a low mortality (6.9%/year for saplings, 0.1%/year for canopy trees) and a longer residence time in the canopy. Picea jezoensis had an intermediate density (8206/ha) and its mortality rate for canopy trees (0.9%/year) was lower than that of A. sachalinensis, although its sapling mortality rate (8.1%/year) was similar to that of A, sachalinensis. Betula ermanii had a higher mortality (14.1%/year for saplings, 2.4%/year for canopy trees) than the conifers. On the other hand, B. ermanii had the highest height growth and P. glehnii had the lowest. The mortality and average height growth of saplings showed a trade-off relationship. The trade-off relation in life history strategy may contribute to the coexistence of these species. The proportion of the gap area created in the forest was 1.2-2.4%/year, and a forest turnover time calculated from aboveground volumes was 87-99 years. These values were similar to a weighted mean of the estimated lifetimes of the constituent species, 71 years for A. sachalinensis, 123 years for P. jezoensis, and 49 years for B. ermanii.