Riparian disturbance and community structure of a Quercus-Ulmus forest in central Japan

Population structure and ecological characteristics of component species of a riparian Ulmus-Quercus forest in central Japan were analyzed with special reference to riparian disturbance regime. Though the dbh distribution of the whole community was L-shaped, those of several component tree populations had several modes, suggesting intermittent regeneration periods in the past. Correlation of spatial distributions among tree populations and subpopulations showed 6 major groups reflecting riparian disturbances in the past and different establishment patterns among species. A cluster of small-sized tree populations (Salix sachalinensis, Alnus hirsuta and Populus maximowiczii <30 cm dbh) were distributed on the lower terrace along the active river channel, while large-sized subpopulations (dbh ≥60 cm) of Quercus crispula and Ulumus davidiana vat. japonica occurred on the higher terrace. The Phellodendron amurense population also occurred on the higher terrace in small clumps though the trees were small (less than 55 cm dbh). Subpopulations of intermediate-sized individuals (30 ≤ dbh < 60 cm) of Q. crispula and U. davidiana var. japonica, together with Betula and Acer spp. populations occurred on the intermediate terrace. Dendrochronological analyses indicated that the large and intermediate-sized tree groups were established about 330 and 90 years ago, respectively, while the small-sized tree group established about 35 years ago. A survey of historical disturbances showed that these periods of establishment of the former two groups almost coincided with the historically major floods occurring in 1662 and 1902. However, the disturbance that resulted in the establishment of the youngest group could not be precisely identified. Thus, the forest is a mosaic of three differently-aged patches, which is closely related to the frequency and scale of riparian disturbances. Longevity of trees and the preferred conditions for seed germination and/or seedling establishment were particularly important for the guild structure in this forest community.