Spatial-temporal NDVI pattern of global mangroves: A growing trend during 2000-2018.

Mangroves are coastal vegetation with high ecological and economic value that are mainly distributed in tropical and subtropical intertidal zones. In the past, they have been degraded by extensive deforestation for agricultural and aquatic land. In recent years, mangroves have been protected and sustainably used through considerable measures of conservation, restoration and afforestation, but the health trends of mangroves during this process are not clear. To identify the mangrove health conditions and dynamics, we investigated the spatial-temporal trends of global mangroves using the Moderate Resolution Imaging Spectroradiometer (MODIS) normalized difference vegetation index (NDVI) dataset during 2000-2018. The results illustrated that 1) Asian mangroves had the highest NDVI values, especially in Southeast Asia (0.80), while the average NDVI of African mangroves was the lowest (0.67). NDVI values higher than 0.80 were mainly located in Southeast Asia and South America, which accounted for 24.0 % and 7.1 % of the global mangrove area, respectively. 2) Globally, the proportion of mangrove forests that increased significantly (23.6 %, p value < 0.05) was approximately twice as large as the significant decrease (10.7 %, p value < 0.05). Asia, where mangroves are widespread, accounts for nearly half of the world's significant increase (10.8 %) and decrease (4.6 %). Generally, the annual average NDVI for global mangroves exhibited a slow increasing trend from 2000 to 2018 (p value = 0.13). 3) The global mangrove NDVI showed a positive correlation with precipitation (R = 0.79, p value < 0.01) and temperature (R = 0.37, p value < 0.01), while it was inhibited by sea surface salinity (R = -0.45, p value < 0.01) on a scale of 1° of latitude. 4) The results of the overall growth trend of mangroves indicated that global mangrove conservation appeared to achieve initial success, but direct or potential factors, such as salinity stress, natural disasters, small-scale deforestation, construction of coastal facilities, and sea level rise, still threaten the survival of mangroves, leading to a decline in their health status. This study provides information on the health status of mangrove ecosystems and can assist in formulating subsequent conservation and management measures.