| In recent decades,global climate change and its impact on terrestrial ecosystems have received widespread attention from the academic community.Evergreen forests are a unique vegetation type in the world with very important ecological service functions and research value.Evergreen forests have more significant biochemical feedback to climate change and are vulnerable to disturbance by human activities.The exploration of spatial and temporal patterns of evergreen forests is an urgent need for scientific research and production practice,however,there is a great lack of annual fine-resolution evergreen forest maps.Therefore,it is necessary to map timely and accurate evergreen forest maps with high spatial resolution and time series to provide thescientific basis for evergreen forest protection,management and sustainable development.Dabie Mountain is located in the north-south climate transition zone,with the typical north-subtropical to warm temperate transition climate characteristics,its plant communities have obvious transition characteristics,very sensitive to climate change and external disturbances.The zonal vegetation in the region is mixed deciduous broad-leaved and evergreen broad-leaved forest,but the native vegetation has been destroyed.It is not clear how the forest ecosystem of Dabie Mountain has evolved in the last 20 years due to natural and anthropogenic factors.In particular,it is not clear how the Dabie Mountain evergreen forest has evolved in the context of global warming.Therefore,a systematic study of the effects of climate change and human activities on the evergreen forests of Dabie Mountain is of great significance for the ecological security and sustainable economic and social development of the Dabie Mountain region.This paper takes Dabie Mountain as the study area,all available Landsat-5/-7/-8 and Sentinel-1/-2 remote sensing images were acquired based on the GEE cloud computing platform from 1999-2022,constructs an algorithm for extracting evergreen forests and tea plantations based on phenology characterustucs in combination with multi-source and multi-temporal remote sensing images,and evaluates the recognition accuracy of the mapping algorithm.The spatiotemporal distribution pattern of evergreen forest were explored in the Dabie Mountains.The spatial expansion patterns,areas and speeds of tea plantations were revealed in different periods.The impact of tea plantation expansion was quantified on the natural evergreen forest.The response mechanism of natural evergreen forest areas(Evergreen forest after the exception of tea plantations)to air temperature and precipitation was revealed.The main conclusions of this paper are as follows:(1)The spatiotemporal pattern of the evergreen forest was elucidated in the Dabie Mountains from 1999 to 2022.In this paper,a fast,convenient,and robust algorithm for extracting phenology was constructed from evergreen forests in the Dabie Mountains.The overall accuracy(OA)of the evergreen forests is higher than 85.70%in 1999-2022.Based on this method,the spatiotemporal pattern of the evergreen forest was estimated in the Dabie Mountains from 1999 to 2022,that is,the evergreen forest area showed an increasing trend in different altitude gradients and provinces as a whole.Among them,the area of the evergreen forest shows a decreasing trend in the altitude range below 100 m from 1999 to 2010 and an increasing trend from2010 to 2022;the area of the evergreen forest all shows a fluctuating increasing trend in the altitude range of500-900 m and 1000-1700 m;the area of the evergreen forest all showed a stable increasing trend in the altitude range of 100-500 m,900-1000 m and>1700 m.(2)The law of spatial expansion of artificial evergreen forests(tea plantations)is revealed in the Dabie Mountains from 1999 to 2022.In this paper,a high-precision,fast,convenient,and robust phenology algorithm was constructed for extracting tea plantations in Dabie Mountains.The OA of extracting tea plantations is 92.45%based on the physical characteristics,that of the random forest algorithm is 92.91%,and that of the support vector machine algorithm is 90.59%.Secondly,based on the optimal algorithm,the law of tea plantation expansion was estimated in the Dabie Mountains from 1999 to 2022:(i)The total area of tea plantation expansion was 983.65 km~2,and the expansion speed of tea plantations showed a dynamic trend;(ii)The spatial expansion pattern of a tea plantation was dominated by outlying expansion,and the area of outlying expansion generally showed a fluctuating upward trend,and the proportions of edge expansion and filling expansion area showed a fluctuating downward trend.(3)The impact of tea plantation expansion on the landscape pattern of natural evergreen forests was clarified.Evergreen forests can be divided into natural and artificial evergreen forests,and the artificial evergreen forests are mainly tea trees in the Dabie Mountains.At the class level,the dominant landscape types of the Dabie Mountains as a whole and in each province are natural evergreen forests,and the dominant patch types of natural evergreen forests form good connectivity.The scale of tea plantation land has been expanding in each province and in the whole,and the status of tea plantations has been gradually highlighted in the landscape pattern,and the patch types of tea plantation have been gradually clustered in each province,among which,the tea plantation category in Anhui Province has a more fragmented and complex spatial structure than the natural evergreen forest category,and the tea plantation and natural evergreen forest patches tend to be irregular in Henan Province.At the landscape level,with the expansion of tea plantations,the landscape pattern within the Dabie Mountain region first shows fragmented and irregular changes,the fragmentation of the Dabie Mountain landscape gradually increases,the shape of the patches tends to be complex,and the internal heterogeneity of the landscape types also increases,and then gradually tends to regular changes under the impetus of relevant national policies,which in turn leads to an increase in the overall degree of agglomeration.(4)The natural evergreen forest changes in Dabie Mountain are influenced by a combination of temperature and precipitation factors,but they are more sensitive to temperature changes compared to precipitation.The area of natural evergreen forest in Dabie Mountain is positively correlated with annual average temperature,annual minimum temperature,and annual precipitation,and negatively correlated with annual maximum temperature.The mechanisms by which temperature and precipitation affect natural evergreen forests area under different elevation gradients are different.In the range of elevation below 100m and above 1700 m,annual maximum temperature,and annual average temperature are the main drivers;in the range of 100-300 m,annual minimum temperature,annual average temperature,annual maximum temperature,and annual precipitation are the main drivers;in the range of 300-500 m and 600-1300 m,annual minimum temperature,annual average temperature,and annual precipitation are the main drivers;in the range of 500-600 m,annual average temperature,and annual precipitation were the main drivers;in the range of 1300-1700 m,annual minimum temperature,and annual average temperature were the main drivers. |
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