Abstract
This paper describes the forest structure and the tree form and growth of Picea jezoensis and Larix gmelinii forests on the Shmidt Peninsula, northernmost Sakhalin, and clarifies present conditions for their coexistence on the Shmidt Peninsula. From these results, past conditions for the coexistence of these two species dominant in Hokkaido during the Last Glacial Age are deduced, and reasons for the extinction of Larix gmelinii from Hokkaido in the Holocene are discussed. The present conditions permitting the coexistence of these two species on the Shmidt Peninsula are two-fold: reduction of suitable habitats for the establishment of Picea jezoensis associated with the reduction of height growth of this species, and frequent disturbances producing open habitats suitable for Larix gmelinii. Past conditions for their coexistence in Hokkaido differred between two periods of the Last Glacial Age, the Last Glacial Maximum and the latest Late Glacial. In the Last Glacial Maximum, dry climate could promote dominance of Larix gmelinii, while in the latest Late Glacial, a significant increase of disturbance events promoted the shift of forest composition from Picea jezoensis through Larix gmelinii to Quercus mongolica. The key factor to the extinction of Larix gmelinii from Hokkaido in the Holocene was probably the prevalence of deciduous broad-leaved species such as Quercus mongolica throughout Hokkaido.
