Questions: Are there interspecific differences in mortality and recruitment rates across life stages between two shade-tolerant dominant trees in a sub-alpine old-growth forest? Do such differences in demography contribute to the coexistence and co-dominance of the two species?
Location: Sub-alpine, old-growth forest on Mt. Ontake, central Honshu, Japan.
Methods: From 1980 to 2005, we recorded DBH and status (alive or dead) of all Abies mariesii and A. veitchii individuals (DBH ≥ 5 cm) in a 0.44-ha plot. Based on this 25 year census, we quantified mortality and recruitment rates of the two species in three life stages (small tree, 5 cm ≤ DBH < 10 cm; subcanopy tree, 10 cm ≤ DBH < 20 cm; canopy tree, DBH ≥ 20 cm).
Results: Significant interspecific differences in mortality and recruitment rates were observed in both the small tree and sub-canopy tree stages. In this forest, saplings (< 5 cm DBH) are mostly buried by snow-pack during winter. As a consequence, saplings of A. mariesii, which is snow and shade tolerant, show higher rates of recruitment into the small tree stage than do those of A. veitchii. Above the snow-pack, trees must tolerate dry, cold temperatures. A. veitchii, which can more readily endure such climate conditions, showed lower mortality rate at the subcanopy stage and a higher recruitment rate into the canopy tree stage. This differential mortality and recruitment among life-stages determines relative dominance of the two species in the canopy.
Conclusion: Differential growth conditions along a vertical gradient in this old forest determine survival of the two species prior to reaching the canopy, and consequently allow co-dominance at the canopy stage.