A STUDY ON ANNUAL CHANGES IN SURFACE COVER OF FLOATING-LEAVED PLANTS IN A LAKE USING AERIAL-PHOTOGRAPHY

A simple method of aerial photography was used to investigate annual and between-year changes in surface cover of the floating-leaved plants, Nymphoides peltata, Nelumbo mucifera and Trapa natans, in natural stands in Lake Kasumigaura. Oblique photographs of vegetation were taken with a 35-mm camera with real color film from airplanes between 1983 and 1989. Vegetation maps were traced on the positions after projective transformation to vertical photography. The distribution of the stand of Nymphoides peltata seemed to be restricted to a region of less than 1.5 m water depth. A rise of 1.0 m in the water level after a typhoon in 1986 caused a subsequent decrease in vegetation area of Nelumbo nucifera. The vegetation recovered in shallower regions after one year in 1987. The expansion rate of Nymphoides peltata and Nelumbo nucifera in one direction was in the range 3.8-10.0 m yr-1 and 4.1-20.8 m yr-1, respectively. Nelumbo nucifera, Nymphoides peltata and Trapa natans, having high leaf biomass to total biomass may therefore be more vulnerable to large flood and waves. Fluctuation in water level and spatial difference in water depth may also be important factors determining the vegetation area and survival of Nymphoides peltata, Nelumbo mucifera and Trapa natans.