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 specia l reference to riparian disturbance regime. Though the dbh distribution of the whole community was L-shaped, those of several component tree populatio ns had several modes, suggesting intermittent regeneration periods in the p ast. Correlation of spatial distributions among tree populations and subpop ulations showed 6 major groups reflecting riparian disturbances in the past and different establishment patterns among species. A cluster of small-siz ed tree populations (Salix sachalinensis, Alnus hirsuta and Populus maximow iczii <30 cm dbh) were distributed on the lower terrace along the active ri ver channel, while large-sized subpopulations (dbh greater than or equal to 60 cm) of Quercus crispula and Ulumus davidiana. var. 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 less than or equal to dbh < 60 cm) of Q. crispula and U. davidiana var. japonica, tog ether with Betula and Acer spp. populations occurred on the intermediate te rrace. Dendrochronological analyses indicated that the large and intermedia te-sized tree groups were established about 330 and 90 years ago, respectiv ely, while the small-sized tree group established about 35 years ago. A sur vey of historical disturbances showed that these periods of establishment o i 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 closel y related to the frequency and scale of riparian disturbances. Longevity of trees and the preferred conditions for seed germination and/or seedling es tablishment were particularly important for the guild structure in this for est community.