Patch dynamics of Euphorbia adenochlora and relationship between patch area of E. adenochlora and disturbance elements in the Honme River.

Abstract

In order to conserve disturbance-dependent species on the floodplain, it is neces sary to make a conservation plan based on knowledge on the patch dynamics of species and their habitats. Approximately 6.3 km of Honme River, the secondary tributary of the Katsura River in the Yodogawa River system, was investigated. Aerial images obtained us ing unmanned aerial vehicles from 2016 through 2019 were utilized. Interpretation of these images allowed us to document the distribution of Euphorbia adenochlora, the submerging areas during flood events, and the area of vegetation loss caused by cutting the grass and open burning. Using the spatial analysis function of GIS, we analyzed the relationship be tween E. adenochlora and these disturbance elements, aggregated patch area of E. adenochlora. The total patch area of E. adenochlora decreased from 285 m² in 2016 to 236 m² in 2018, but it increased to 298 m² in 2019, exceeding the 2016 value. The increase in the total patch size in 2019 is attributed to both development of new as well as expansion of existing patches. Based on the results of our 4-year survey, it can be stated that the population of E. adenochlora in Honme River is sustainable. However, the data suggest that decreases in patch area are dependent on the frequency and magnitude of flooding, a natural disturbance that was found to occur on a 3-year cycle, as well as the extent of artificial dis turbances such as cutting the grass and open burning. In particular, cutting the grass was noted to occur on a yearly basis in early July. Suitable ranges for E. adenochlora were determined from the analysis results of these three disturbance elements. Thus, this study clarifies the patch dynamics of E. adenochlora over a 4-year period.