The Dynamics Of Tea Plantation Encroachment Into Forests And Effect On Forest Multi-scale Landscape Pattern In Yunnan Province

High demand for tea has driven the expansion of tropical and subtropical tea plantations over the past few decades.While the planting of tea plantations brings economic benefits,it also has a huge impact on biodiversity,ecosystems and forest landscape patterns.With the development of remote sensing technology,the identification algorithm of tea plantations has become more and more mature,especially based on the unique artificial pruning phenological characteristics of tea plantations,which are considered to be effective in identifying tea plantations.In addition,in the main tea-producing of Yunnan Province,China,in the past 30 years,the expansion of tea plantations has caused forest destruction while affecting the forest landscape pattern,but the impact of tea plantation expansion on the forest landscape pattern is still unclear.Therefore,this study uses the US Landsat ETM+/OLI and DEM as the data source,Yunnan Province as the research area,and the research period from 1991 to 2021,using the Google Earth Engine(GEE)platform to achieve the spatiotemporal analysis of the historical encroachment of tea plantations into the forest and using the Fragstats software realizes the multi-scale quantitative research on the forest landscape pattern of tea plantations encroachment into forest.First,in the tea plantations pruning phenological period determined,based on time series analysis,the Normalized Difference Vegetation Index(NDVI)(300-345d),Land Surface Water Index(LSWI)(30-90d)and the Difference Between NDVI and LSWI(DBNL)(60-120d)for the tea plantation to extract the best index and the corresponding time window,complete the1991-2021 tea plantations history encroachment into the forests maps.Then we select the Growth Area of Tea plantations Encroachment into Forests(GATEF(km~2)),the Growth Index of Tea plantations Encroachment into Forests(GITEF(km~2/t))to analyze the characteristics of the tea plantations encroachment in Yunnan Province at different periods.Moreover,we select the Tea Plantations Expansion Index(TPEI)to quantitatively identify the spatial pattern of tea plantation encroachment.In addition,based on the database of the historical encroachment of tea plantations into the forest,a quantitative study on the impact of tea plantation encroachment on forest landscape patterns was carried out at multiple scales such as typical tea planting areas,different planting level zones and different buffer distances of tea plantations.First,combined with the actual situation of the study area,the index that can effectively reflect the characteristics of landscape density,shape,connectivity,diversity and aggregation is selected from the class metrics and landscape metrics to quantify and compare the impact of tea plantation encroachment on forest landscape patterns at different scales.In class metrics,Percentage of Landscape(PLAND),Patch Density(PD),Largest Patch Index(LPI),Landscape Shape Index(LSI),Patch Area Distribution(AREA＿MN),and patch cohesion index(COHESION)are selected.In landscape metrics,we selected LPI,Contagion(CONTAG),Landscape Division Index(DIVISION),Splitting Index(SPLIT),Aggregation Index(AI),Edge Density(ED)and Shannon Uniformity Index(SHEI).To explore the change characteristics of the landscape index caused by the encroachment of tea plantations at different scales,the change rate of the landscape index in different periods was analyzed,and the trend analysis was used to quantitatively analyze the change trend of the landscape index at the landscape and class metrics.And we analyze the relationship between tea Plantation Encroachment into forests Level(P＿EL)and forest landscape pattern,as well as the relationship between different buffer distances and forest landscape pattern.The main conclusions are as follows:(1)The spatial and temporal distribution of tea plantation encroachment in various cities in Yunnan Province from 1991 to 2021 was different.The increased area of tea plantations in the main tea-producing areas in Yunnan Province in the six time periods accounted for 87.13%,85.61%,74.96%,71.56%,83.09%and 85.49%of the increased area of tea plantations in Yunnan Province respectively,and the area proportions were all above 70%.The encroachment of tea plantations in Yunnan Province is mainly concentrated in the main tea-producing areas in Yunnan Province.From 1991 to 2021,the prefectures with the rapid encroachment of the GITEF(km~2/t)in Yunnan Province were mainly concentrated in the main tea-producing areas in Yunnan Province.The average value of GITEF(km~2/t)at different periods in Yunnan Province from 1991 to 2021 shows that the overall encroachment rate of tea plantations in Yunnan Province first increased rapidly,and then decreased.The overall spatial pattern of tea plantation expansion in Yunnan Province is that the tea plantation out-lying expansion mode shows a downward trend,while the edge expansion mode and the in-filling expansion mode show an upward trend.(2)Simao District,Pu’er City,Yunnan Province,as a typical tea area,experienced the encroachment of tea plantations from 1991 to 2021.At the class metrics,from 1991to 2021,the changes of tea plantations and forests showed an opposite trend.The changes of PLAND,PD and COHESION of tea plantations were greater than those of forests,and the changes of landscape indices,such as LPI,LSI and AREA＿MN were smaller than those of forests.At the landscape metrics,CONTAG and LPI of Simao District showed a significant downward trend,indicating that during the entire study period,although forests were still the dominant type,due to the encroachment of tea plantations,there were many small patches in the Simao.(3)In the process of exploring the influence of different tea plantations encroachment into forests(P＿EL)levels on forest landscape pattern.In the landscape metrics,CONTAG,DIVISION,SPLIT,ED,SHEI and AI indices were all significantly correlated with PE＿L.Among them,ED,DIVISION,SPLIT,SHEI and PE＿L were significantly positively correlated.CONATG,AI and PE＿L were significantly negatively correlated.(4)In the process of exploring the influence of tea plantations on forest landscape pattern under different buffer distances.In class metrics,the partial correlation coefficient between the distances and the PLAND of the tea plantations is-0.332,and the partial correlation coefficient between the distances and the PLAND of the forest is0.326,indicating that as the distance increases,the proportion of the area in the tea plantation area is getting smaller,while the forest area is keeps rising.In the landscape metrics,the partial correlation coefficient between distances and CONATG,DIVISION,and AI are 0.683,0.418,and 0.859,respectively,indicating that with the expansion of distance,there are dominant patch types with extremely high connectivity in the regional landscape,the probability that two random pixels in the region are located in the same patch will decrease,and the regional landscape becomes more agglomerated.After the buffer distance of 2000 meters,the influence of tea plantation expansion on the forest landscape pattern slowed down and stabilized.