Abstract
Hainan Island is the second-largest island in China and has the most species-diverse mangrove forests in the country. To date, the height and aboveground ground biomass (AGB) of the mangrove forests on Hainan Island are unknown, partly as a result of the challenges faced during extensive field sampling in mangrove habitats (intertidal mudflats inundated by periodic seawater). Therefore, this study used a low-cost UAV-LiDAR (light detection and ranging sensor mounted on an unmanned aerial vehicle) system as a sampling tool and Sentinel-2 imagery as auxiliary data to estimate and map the mangrove height and AGB on Hainan Island. Hainan Island has 3697.02 hectares of mangrove forests with an average patch area of approximately 1 ha. The results show that the mangroves on whole Hainan Island have an average height of 6.99 m, a total AGB of 474,199.31 Mg and an AGB density of 128.27 Mg ha−1. The AGB hot spots are located in Qinglan Harbor and the south of Dongzhai Harbor. The proposed height model LiDAR-S2 performed well with an R2 of 0.67 and an RMSE (root mean square error) of 1.90 m; the proposed AGB model G~LiDAR~S2 performed better (an R2 of 0.62 and an RMSE of 50.36 Mg ha−1) than the traditional AGB model G~S2 that directly related ground plots and Sentinel-2 data. The results also indicate that the LiDAR metrics describing the canopy’s thickness and its top and bottom characteristics are the most important variables for mangrove AGB estimation. For the Sentinel-2 indices, the red-edge and shortwave infrared features, especially the red-edge 1 and shortwave infrared Band 11 features, play the most important roles in estimating mangrove AGB and height. In conclusion, this paper presents the first mangrove height and AGB maps of Hainan Island and demonstrates the feasibility of using UAV-LiDAR as a sampling tool for mangrove forests.
Highlights
Mangroves are special types of woody plants that grow exclusively in the intertidal zones of the tropics and subtropics, such as bays, estuaries and rivers [1]
The specific objectives of the study are to (1) estimate and map the height and aboveground biomass (AGB) of the mangroves on Hainan Island; (2) assess whether UAV-LiDAR is feasible as a sampling tool; and (3) examine which UAV-LiDAR metrics and Sentinel-2 indices are more important for mangrove height and AGB estimations
This study estimated that there is a total of 3697.02 ha of mangrove forests on Hainan Island
Summary
Mangroves are special types of woody plants that grow exclusively in the intertidal zones of the tropics and subtropics, such as bays, estuaries and rivers [1]. Huang et al [18] used Geoscience Laser Altimeter System (GLAS) LiDAR data as intermediate data to link field estimated AGB with Landsat images and Phased Array L-band Synthetic Aperture Radar (PALSAR) data to estimate and map forest AGB in China Their resultant R2 ranged from 0.54 to 0.64. The aim of this study is to estimate and map the height and AGB of mangrove forests on whole Hainan Island using a UAV-LiDAR sampling method that combines field plots, UAV-LiDAR point clouds and Sentinel-2 imagery. The specific objectives of the study are to (1) estimate and map the height and AGB of the mangroves on Hainan Island; (2) assess whether UAV-LiDAR is feasible as a sampling tool; and (3) examine which UAV-LiDAR metrics and Sentinel-2 indices are more important for mangrove height and AGB estimations
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