Assessment of mangrove forest degradation by comparing multiple rivers.

NIWA Hideyuki, IMAI Yota, KAMADA Mahito

doi:10.3825/ece.19-00023

<p>We inspected rivers where mangrove forests were distributed on the main island of Okinawa, and surveyed four rivers (Okukubi River, Ooura River, Gesashi River, and Waku River) with existing relatively large mangrove forest areas. Based on the information that can be obtained with UAV, we examined a simple and versatile evaluation method that enables quantitative evaluation. The method was a combination of vegetation classificaion using ortho-mosaic image and mangrove degradation evaluation using vegetation index. We could classify vegetation by supervised object-based image classification. As a result of totaling the area for each classification class in 4 rivers, the area ratio for each class was different among rivers. The histograms of the polygon areas classified as gaps were compared in four rivers. The average value of NDVI in the polygons classified as Bruguiera gymnorhiza tended to be less than 0.5 polygons in the Okukubi River. In the Okukubi River, polygons with low NDVI tended to increase as the distance from the forest edge decreased, and the maximum NDVI tended to decrease as the distance from the forest edge increased. It was suggested that the increase in NDVI to be close to the forest edge and the decrease in NDVI inside the patch could be used as an indicator of the degradation of the mangrove forest. From the comparison of the four rivers, the characteristics of the mangrove forests of each river can be grasped, and in particular, the mangrove forests of the Okukubi River are declining.</p>

Assessment of mangrove forest degradation by comparing multiple rivers.

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