| Urban forest is the most vital infrastructure in a city,and its canopy coverage plays an important role in the service function of urban forest ecosystem and the maintenance and strengthening of urban sustainability.In the coming decades,many cities in China will face the environmental pressure caused by urbanization,and urban forest will become an important guarantee for urban ecological security.However,the relevant knowledge about the impact of urbanization on the spatial and temporal dynamics of urban forests is still lacking to a great extent.It is urgent to pay more attention to the spatial pattern and temporal dynamics of urban tree canopy cover and the mechanism for explaining the formation of these patterns,so as to help urban forest managers determine the appropriate urban forest development strategy.In this paper,based on three high-resolution remote sensing images:true color aerial photos with 0.5m resolution in 2002,World View-2 panchromatic and multispectral fusion images with 0.5m resolution in 2013 and Beijing No.2 panchromatic and multispectral fusion images with 0.8m resolution in 2019,the urban tree canopy/land cover within the sixth ring road of Beijing was extracted by using object-oriented interpretation technology on the platform of e Cognition Developer 9.0.The temporal and spatial dynamic changes of urban tree canopy cover in the study area from 2002 to 2019 were studied from three scales:regional entirety,village(street)and urban tree canopy patch.Furthermore,based on the data of 820urban forest structure quadrats obtained from field investigation,the spatial visualization of DBH,tree height,crown width and species diversity structure indicators of urban forests in the study area was carried out by using geostatistical Kriging method,and then the quantitative dynamics and quality of urban forests were linked to get a deeper understanding of urban forest dynamics.At the same time,we analyzed the reasons for the dynamic changes of urban forests in the study area.(1)In 2019,the areas of urban tree canopy,impervious surface,grassland,bare land,water and farmland within the sixth ring road in Beijing are 92 332.09 hm~2,89 801.01 hm~2,13054.09 hm~2,13 842.95 hm~2,5 147.71 hm~2 and 12 147.91 hm~2 respectively.Their area percentages are 40.80%,39.68%,5.77%,6.12%,2.27%and 5.37%,respectively.The spatial distribution of urban tree canopy in the whole study area is uneven,and the advantages of urban tree canopy over impervious surface are mainly reflected in the area within the 5th to 6th ring road of the suburbs.(2)The urban tree canopy coverage in 2002,2013 and 2019 are 19.94%,36.24%and40.80%respectively.The growth rates of the two periods are 8.17%and 2.10%respectively.During 2002-2013,the growth are mainly the transformation of impervious surface,bare land and farmland,and during 2013-2019,the growth are mainly the transformation of impervious surface and grassland.The proportion of impervious surface in three years are 38.65%,45.50%and 39.68%respectively,and the change rates in two periods are 1.77%and-2.13%respectively.The growth in the previous period mainly come from the transformation of urban tree canopy,farmland and bare land.Grassland and bare land are the two most dramatic land types in two time periods.The change rates of the two land types are 34.24%and-8.74%respectively from 2002 to 2013,and are-3.84%and 36.93%respectively from 2013 to 2019.(3)From 2002 to 2013,the stability order of land cover types are impervious surface(71.15%),urban tree canopy(59.33%),water(54.06%),grassland(26.01%),farmland(21.96%)and bare land(2.81%).From 2013 to 2019,By 2013-2019,the order changed to urban tree canopy(75.50%),water body(71.67%),impervious surface(69.13%),farmland(62.86%),grassland(35.26%)and bare land(16.13%).Except for impervious surface,the stability of other land cover types has been improved.Urban tree canopy have become the most stable land cover types in the study area.The regional stable index increased from 54.98%in 2002-2013 to69.86%in 2013-2019.(4)The average urban tree canopy coverage at village(street)scale increased from 21.37%in 2002 to 32.99%in 2013 and reached 39.57%in 2019.From 2002 to 2013,the increase of urban tree canopy mainly occurred outside the second ring road,while the area within the second ring road showed a decreasing trend;On the contrary,in the previous stage,the concentrated distribution area which decreased in the center of the city began to increase,while the periphery of the city began to decline.The Morans I index of urban tree canopy in 2002,2013 and 2019 were 0.49,0.52 and 0.42,respectively,which indicated that the spatial distribution of urban tree canopy is clustered,and the development of urban tree canopy in various villages(streets)is uneven,but the imbalance tended to improve in 2019.The average impervious surface coverage at the village street scale increased from 48.29%in 2002 to53.79%in 2013 and decreased to 46.61%in 2019.In two periods,the impervious surface increased continuously,mainly in the suburbs,including the Houshan of Haidian and the Hexi area of Fengtai,while the area decreased continuously,mainly within the fifth ring road.During the period from 2002 to 2019,the strength space of impervious surface is shrinking,and its dominance over urban tree canopy is gradually weakening.The game between impervious surface and urban tree canopy at village(street)scale tends to develop in a balanced way in most areas.(5)The average urban tree canopy patch area gradually increased from 0.07 hm~2 in 2002 to0.18 hm~2 in 2013 and reached 0.28 hm~2 in 2019.During 2002-2013,the areas of canopy patches that no change,expansion,shrinking,newly created and totally lost are 26.72%,49.18%,14.45%,5.94%and 3.70%respectively,and the areas of five types of changes in2013-2019 are 55.30%、24.54%、16.30%、2.22%、1.64%respectively.The change of patch scale are mainly expanding and keeping stable.(6)According to the cumulative curve of patch area and quantity in three years,a new grade standard of urban forest patch size is put forward:small patch(≤100 m~2),medium patch(100-1000 m~2),large patch(1 000-10 000 m~2),large patch(10 000-100 000 m~2)and huge patch(>100 000 m~2).The change of urban tree canopy patches of different grades shows that small patches change dramatically in the process of urban development,and easily disappear completely,while large patches have higher stability.(7)During the period from 2013 to 2019,the probability of aggregation of patches within the 5th ring road increased,and the fragmented patches began to spread from the city center to the periphery of the city,and dispersed from a single center to a multi-center.Compared with the previous period,the degree of fragmented is moderate.(8)Small trees with DBH below 20 cm,tree height 5-10 m and crown width 4-6 m are the main urban forests in Beijing,and large trees are mainly distributed in the city centre.The overall species diversity H value and evenness index J value are 3.65 and 0.75,respectively.The high-value areas of species diversity are mainly distributed in the northwest and southwest of the 3rd-5th ring road,while the low-value areas are distributed in the suburban transition zone beyond the 5th ring road.The spatial distribution pattern of DBH and species diversity are consistent with the spatial distribution pattern of urban tree canopy coverage in 2002,the fragmentation degree of patches from 2002 to 2013 and the spatial distribution of patch aggregation density from 2013 to 2019.(9)The influence of urbanization on the dynamics of urban tree canopy is the result of the interaction of many factors. |
PDF Full Text Request