The Impacts Of Land Cover Change On The Spatial Extent Of Mangroves In China

Mangrove forests,the ecosystem with high ecosystem service value in coastal zones,have decreased at an alarming rate over the past decades due to anthropogenic forces.However,due to the complexity at a large scale of human impact,there is a lack of quantitative understanding between it and China’s mangrove loss at large spatial and temporal scales.To address the above issue,this study used satellite remote sensing（RS）,geographic information system（GIS）analysis,and other technical means to carry out the following work.We first proposed a method of Spatio-temporal Detection Frequency Threshold（STDFT）for mangrove mapping and produced China’s mangrove maps from 1990 to 2020using Google Earth Engine（GEE）.Secondly,to analyze the land cover change in the southern coastal zone of China from 1990 to 2020,we mapped the aquaculture ponds using the GEE and constructed a set of land cover data more suitable for analyzing the ecological environment in coastal zone.Finally,we used the association rule mining（ARM）method to analyze the impact of anthropogenic forces on the spatial extent of China’s mangroves from the perspective of land cover change and investigated the effectiveness of China’s conservation policies.The results show that:（1）In the absence of a large number of training samples,the method of STDFT has successfully mapped China’s mangroves from 1990 to 2020,with an overall index accuracy of more than 90%,and a Kappa coefficient of more than0.9.（2）The growth rate of China’s mangrove area was 329.6 ha?a^-1 from 1990 to2020.The change of mangrove can be divided into two stages:from 1990 to 2000,the mangrove area decreased by 597.4 ha and accounted for 4.7%of the area in1990;From 2000 to 2020,the mangrove area increased significantly,with a total increase of 10490 ha and accounted for 87.2%of the area in 2000.On the whole,China’s mangrove forests are in a period of significant expansion.（3）Driven by socio-economic factors,land cover changes in the southern coastal zone of China are intense.From 1990 to 2020,the area of coastal aquaculture ponds increased from 280275.7 ha to 471562.7 ha,and the area of impervious surface from 899647.6 ha to 2883957.6 ha.The change can be divided into two stages:the change from 1990 to 2010 was drastic,which was the period of rapid development,and during the period from 2010 to 2020,the land cover change decreased due to the development of environment-friendly policies in China.In particular,the land cover changes around mangrove wetlands are more dramatic.（4）Experimental results of the ARM method indicated that the impact of land cover change on the spatial extent of China’s mangroves can be divided into two stages.During the period from 1990 to 2000,the loss of mangrove forests was correlated with the expansion of coastal aquaculture ponds,farmland,and urban,which indicated that the main causes of mangrove degradation are large-scale land cover changes driven by anthropogenic factors.Moreover,the establishment of protected areas did not effectively prevent the loss of mangrove forests during this period.From 2000 to 2020,due to the effective implementation of protection and restoration policies,the area of mangrove showed a significant upward trend.The impact of land cover change on the spatial extent of China’s mangroves gradually weakened,indicating that high-intensity coastal zone development in recent decades does not necessarily damage mangrove forests.These findings provide important clues for understanding the relationship between human impact and mangrove forests to support scientific references for formulating and improving mangrove conservation and restoration policies.