Factors Affecting the Species Diversity of Understory Plants in Abandoned Terraced Paddy Fields on Sado Island, Niigata Prefecture

The effects of microtopography, soil water, and light conditions on the species diversity of understory plants were studied in abandoned terraced paddy fields on Sado Island, Niigata Prefecture, Japan. The study area was abandoned about 40 years ago and plant succession has progressed to various types of plant community since that time. A wide variety of soil water conditions formed in the abandoned terraced paddy fields, according to the microtopography, namely, paddy plane, levee, and levee slope. The number of species of both woody and lianous plants decreased significantly as the soil water content increased. Conversely, many herbaceous species adapted to various soil water conditions. In particular, many herbaceous plants grew in the understory in good light conditions, even in soils with high water contents. The understory of the forest community was shaded, whereas various light conditions occurred in the understories of the shrub and reed communities. Such mosaic arrangements of various soil water and light conditions among different landscape elements maintained a high species diversity of understory plants in the terraced paddy fields, abandoned many years ago.