| The numerous benefits of urban trees and forests are being increasingly recognized globally but grossly understudied in developing countries in Africa.Studying the size characteristics,health status,vertical structure,and spatial distribution functions of street trees is still crucial to improving urban forest greening,scientific planning and management of urban green infrastructure,and maximizing its ecological service.This study aims to verify the feasibility of Google Street View images in assessing the size characteristics,health status,vertical structure,and spatial distribution of street trees in five cities of Africa.A total of 58 363 street trees in 4098 sample plots were investigated from Kampala,Nairobi,Bloemfontein,Johannesburg,and Pretoria in Africa.This method includes measurement indexes of size characteristics of diameter at breast height(DBH),Under-branch height(UBH),tree height(TH),Canopy size(CS),Health status(tree supporting ratio,Dieback ratio,Unhealthy leaf color ratio,and Dead ratio),vertical structure grade of vegetation and features of the street trees(lane number,roadside shops,parking vehicle,and the number of pedestrians),as well as coordinates(latitude,longitude).The descriptive statistics parameters such as frequency,averages,and spatial distribution were computed for each street trees parameter.Redundancy ordination analyses were performed to detect responsible road development parameters for forest variations.We have gotten the following conclusions.1)The results showed that,in terms of flora composition,the five cities were dominated by broadleaved trees,branch tops dominated trees,and flowering trees dominated three-quarters of cities.2)Size parameters for Kampala,Nairobi,Bloemfontein,Johannesburg,and Pretoria were DBH at21.69 cm,41.30 cm,31.39 cm,38.49 cm,and 30.46 cm,respectively,UBH at 2.75 m,3.81 m,2.42 m,3.65 m,and 3.02 m,respectively,were TH at 7.82 m,7.80 m,6.78 m,7.72 m,and 7.20 m respectively and CS at 4.01 m,4.61 m,4.12 m,4.49 m,and 4.12 m,respectively.3)Street trees were mainly middle-sized,with less proportion of large trees and small sapling trees.In the five cities,the DBH(20-40cm)accounted for 45%,42%,70%,45%,and 56%,respectively;the UBH(2-4m)reached 69%,53%,60%,54%,and 48%,respectively;the TH(5-10cm)accounted for 65%,61%,60%,64%,and 49%,respectively.The CS(3-6m)of 57%,62%,53%,61%,and 47%,respectively.4)In terms of health status,the health status of street trees in five cities was relatively excellent,and the proportion of stand unhealthy is generally at a low level.We did not find any trees supported by tripod branches,a tiny percentage of dead trees,unhealthy leaf color,and dieback trees.5)The vertical structure vegetation types in each city are dominated by trees Arbor,shrub,herb structure,accounting for 44%,41%,40%,42%,and 41%,respectively.We also found many bare soils,showing the necessity for plantation afforestation in these areas.6)RDA ordination analysis showed that street tree structure(size,vertical structure)and health are closely associated with road developments.With the road development of more pedestrians,cars,and shops,the health risk of street trees increased,the tree size increased,and the complex community structure appeared.Our findings strongly suggested a high potential of Google Street View utilization in urban street trees evaluation of Africa,both at the individual level,city statistics level,and spatial distribution.Comparing studies in these cities previously,we have found that trees size and health status are a basis for many evaluations of urban tree functions.Our new dataset is a vital addition to such data shortage in these cities and possibly favors their evaluation of the function of street trees in local development.This research establishes baseline information for future monitoring,assessment by using big data of Street View pictures and guiding urban landscape planning and management,which can benefit to decision managers. |
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