Spatial Variation Patterns Of Leaf Phenology Of Common Deciduous Trees In Shanghai And The Influencing Factors

Plant leaf phenology includes leaf-out,leaf color changes,and leaf off.Plant leaf phenology not only reflects adaptation strategies to the environment,but also has important effects on microclimate regulation,species distribution and carbon cycling processes.The high environmental heterogeneity and frequent anthropogenic disturbances result in extensive spatial variability in leaf phenology.So far,a large number of studies have focused on vegetation phenology,while the dynamics of leaf phenology of species and the influencing factors are not sufficiently understood.In this study,we selected 10 common deciduous trees in Shanghai,and investigated 654 pure forest plantation sample plots.Their enhanced vegetation index(EVI)time series from 2019 to 2021 was obtained by Sentinel-2 satellite data.Using derivatives method,threshold method and dynamic threshold method,extracted leaf phenology metrics such as leaf-out start date,leaf off start date and position of peak value to clarify the leaf phenology variation patterns and differences among trees under different environmental gradients in cities.We further analyzed the relationship between leaf phenology and environmental factors such as climate and urbanization,and compared the response of different tree species to the environment,aiming to elucidate the leaf phenology variation patterns of different species in the city and their influencing factors.The main results are as follows:(1)At species level,leaf-out start date and leaf off start date extracted by dynamic method from remote sensing data were positively correlated with leaf-out start date and leaf off start date of ground phenological observations respectively(R~2=0.66,P<0.05;R~2=0.57,P<0.05).Among10 deciduous tree species,Metasequoia glyptostroboides leafed out first(94±11th)(mean±sd),Triadica sebifera leafed out last(103±11th),Populus×canadensis leafed off first(278±22th),Sapindus Saponaria leafed off last(302±23th).In brief,the leaf-out start date and position of peak value of temperate species,single-leaved species,and flower-with-leaf species got earlier.The leaf off start date of species with smaller tangential diameter of vessel lumina got earlier.At the urban-rural gradient,except for Jinshan District in the distant suburbs,the closer to the central urban area,the earlier the leaf-out start date and leaf off start date of trees.The leaf-out start date,leaf off start date and position of peak value of trees in Chongming District in the distant suburbs were later.(2)Different tree species exhibited variations in their response to the leaf-out start date regarding pre-season(December of the previous year to March of the current year)environmental factors.Taxodium distichum was positively correlated with the average monthly light brightness and negatively correlated with the average monthly surface temperature before the season.Taxodium distichum var.imbricatum,Triadica sebifera and Koelreuteria bipinnata were negatively correlated with the average monthly precipitation before the season.Similarly,different tree species exhibited variations in their response to the leaf off start date regarding pre-season(June to September of the current year)environmental factors.Populus×canadensis was negatively correlated with the average monthly light brightness before the season.Metasequoia glyptostroboides was negatively correlated with the average monthly light brightness before the season and the distance from the coastline,Ginkgo biloba were negatively correlated with the distance from the coastline.Furthermore,different tree species exhibited variations in their response to the position of peak value regarding pre-season(May to June of the current year)environmental factors.Liquidambar formosana was negatively correlated with the average monthly surface temperature before the season.Taxodium distichum was negatively correlated with the average monthly precipitation and the distance from the coastline.Koelreuteria bipinnata was positively correlated with the distance from central city,and negatively correlated with the distance from coastline.Sapindus Saponaria was negatively correlated with the average monthly precipitation before the season.In summary,there were significant inter-and intra-species differences in leaf phenology of common deciduous trees in Shanghai.The leaf-out start date and leaf off start date of deciduous trees in urban centers occurred earlier than those in distant suburbs.The leaf phenology of different species were influenced by different environmental factors and showed complex response patterns.Therefore,urban forest planning and construction should be reasonably configured according to the sensitivity and adaptability of different species to the environment,so as to give full play to their role in the ecosystem.