Allometry and Competition between Saplings of Picea jezoensisand Abies sachalinensisin a Sub-boreal Coniferous Forest, northern Japan

Abstract

The crown shape and the mode of competition between saplings (<2m in height) of the two conifers, Picea jezoensisand Abies sachalinensis, of a sub-boreal forest, northern Japan, were investigated based on the diffusion model. A model for individual sapling growth considering both inter- and intraspecific competition was developed. The effect of species-specific crown shape on the sapling growth and competition of the two species were examined. Picea jezoensisand Abies sachalinensissaplings had deep conic and shallow flat crowns, respectively. Picea jezoensishad more foliage mass than Abies sachalinensisof the same sapling mass. It was suggested that the Picea jezoensissapling has a high cost for assimilation–respiration balance under dark conditions of closed canopies, whereas the Abies sachalinensissapling maintains effective assimilation even under suppressed conditions. Widely spaced saplings, such as gap successors, of Picea jezoensishad a greater relative growth rate ( a 0) than widely spaced Abies sachalinensis. The crown shape of saplings of the two species shows different adaptations for efficient persistence in the sub-boreal forest. Saplings of Picea jezoensisand Abies sachalinensiswere not uniformly distributed, but aggregated in different sites as the saplings grew, indicating habitat segregation between the two species at the sapling stage. Intraspecific sapling competition was one-sided in each of the two conifers. Interspecific sapling competition was one-sided in the direction only from Abies sachalinensisto Picea jezoensis. Therefore, asymmetric competition prevailed at the sapling stage of the two species. These results contrast with weak symmetric competition or the almost absence of competition between trees (≥2m in height) of the two species (Kubota and Hara, Annals of Botany 76: 503–512, 1995). The mode of competition changed with the life-history stage from the sapling (intense and asymmetric) to the tree (weak and symmetric or almost absent). In conclusion (1) asymmetric and intense competition between saplings brought about habitat segregation between the dominant species, Picea jezoensisand Abies sachalinensis, in the early stage of life-history; (2) therefore, the coexistence of Picea jezoensisand Abies sachalinensisof the sub-boreal forest was determined by the boundary conditions for the growth dynamics of the trees, as segregation of establishment sites resulting from asymmetric and intense competition between saplings; (3) then the species composition of the forest was maintained by weak symmetric competition or the almost absence of competition between trees.