2017 Volume 34 Issue 1 Pages 1-21
This study clarified the differences in species composition, species diversity, and the characteristics of species in abandoned, specially cultivated, and conventionally cultivated rice fields. We also investigated the role of these types of rice fields in substituting semi-natural wetlands. Conventional cultivation used a standard amount of agricultural chemicals and chemical fertilizers, whereas special cultivation practices used only half, or even less than 50%, of agricultural chemical ingredients. Abandoned rice fields used no agricultural chemicals. For comparison, we also investigated plants on levees. There were indications that species richness and diversity were higher on levees than in rice fields. This may be due to land conditions and the moderate disturbance caused by mowing in the levees. Within the rice field category, species richness and diversity in abandoned and specially cultivated rice fields were higher than those of conventionally cultivated rice fields (three times in abandoned rice fields and two times in specially cultivated rice fields), and the difference between specially and conventionally cultivated rice fields was significant. There is the possibility that soil puddling lowered the occurrence of weeds in the rice fields, but soil puddling also occurred in the specially cultivated rice fields. Using different amounts of pesticides is likely to affect species occurrence and dominance. In special cultivation rice fields, however, the use of herbicide was comparable to that of conventionally cultivated rice fields. When comparing field conditions between conventionally cultivated and specially cultivated rice fields, specially cultivated rice fields at the foot of mountains had sandy loam soils and soil ditches, where conventionally cultivated rice fields had clay loam soils and U-shaped ditches made of concrete. Sandy loam soils would enable water leakage of dissolved pesticides, and soil ditches may enable the growth of wetland plants from upper streams. Such different conditions may explain the different weed species communities between conventional and specially cultivated rice fields. We postulated that reduced insecticides use had positive effects for pollination with insects, but there was no evidence of this: the rates for entomophilous flowers did not differ. In abandoned rice fields, we found many wetland species, suggesting that abandoned rice fields play a role in contributing to the habitat of wetland species, and that cultivated rice fields with sandy loam soils and soil ditch may have positive effects on wetland species.
