Provincial Scale Monitoring of Mangrove Area and Smooth Cordgrass Evasion in Subtropical China Using UAV Imagery and Machine-Learning Methods

The survival and growth of mangrove along the coastal China was threatened by the invasive smooth cordgrass (Spartina alterniflora). Due to the high mortality and frequent replanting of mangrove trees and impacts of invasive smooth cordgrass, it is still unclear about the exact mangrove forest area in Zhejiang Province, China. Based on provincial scale UAV imagery and large numbers of field survey plots, this study classified the area and distribution of mangroves and the invasion status of smooth cordgrass using the identified machine-learning method. The accuracy assessment indicated that the overall accuracy and Kappa coefficient were 97% and 0.96, respectively for land cover classifications. The total area of mangrove forest and smooth cordgrass was 140.83 ha and 52.95 ha, respectively in Zhejiang Province. The mangrove forest area was mostly concentrated in Yuhuan, Dongtou, Yueqing and Longgang districts. The overall survival rate of mangrove trees was only 36.41%, with lower than 20% survival rates in all northern and some central districts. At spatial scale, the mangrove trees showed a scattered distribution pattern, and over 70.04% of the planting area has canopy coverage lower than 20%, indicating a high mortality rate. Smooth cordgrass has widely invaded in all 11 districts, accounting for about 13.7% of the total planting area of mangrove trees. Over 67.3% and 85.4% of the planting area has been occupied by smooth cordgrass in Wenling and Jiaoxiang districts, respectively, which calls for an intensive anthropogenic intervention to control the spreading of smooth cordgrass in these districts. Our study provides a more accurate monitoring of the mangrove and smooth cordgrass distribution area at a provincial scale. The findings will help guide the replanting and management activities of mangrove trees and the control planning of smooth cordgrass, and also provide data basis for accurate estimation of carbon stock for mangrove forests in Zhejiang Province.