Role Of Species And Planting Configuration On Transpiration And Microclimate For Urban Trees

Urban trees play an essential role in reducing the city’s heat load.However,the influence of urban trees with different configurations on the urban thermal environment has not received enough attention.Herein we show how spatial arrangement and foliage longevity,deciduous versus evergreen,affect transpiration and the urban microclimate.We analyzed the differences between physiological parameters(transpiration rate,stomatal conductance)and meteorological parameters(air temperature,relative humidity,vapor pressure deficit)of 10 different species of urban trees(five evergreen and five deciduous tree species),each of which had been planted in three configuration modes in a park and the campus green space in Xi’an.By manipulating physiological parameters,crown morphology,and plant configurations,we explored how local urban microclimate could be altered.Results:(1)Microclimate regulation capacity:group planting(GP)>linear planting(LP)>individual planting(IP).The diurnal variation trend of temperature and humidity in the three configuration modes are the same,which rose to the peak first and then decrease.(2)Deciduous trees(DT)regulated microclimate better than evergreen trees(ET).Significant differences between all planting configurations during 8 to 16 h were noted for evergreen trees whereas for deciduous trees,all measurement times were significantly different.The microclimate effects of urban trees were influenced by tree species type and configuration mode.(3)Transpiration characteristics:GP>LP>IP.The transpiration rate(E)and stomatal conductance(Gs)of GP were the highest.Total daily transpiration was ranked as group planting of deciduous(DGP)>linear planting of deciduous(DLP)>group planting of evergreen(EGP)>linear planting of evergreen(ELP)>isolated planting of deciduous(DIP)>isolated planting of evergreen(EIP).The transpiration characteristics of urban trees are affected by the type of tree species and the arrangement mode.(4)The microclimate effects of different tree species and configuration modes were positively correlated with E,Gs,and three dimensional green quantity(3DGQ),but weakly correlated with vapor pressure deficit(Vpd _L).(5)E and 3DGQ significantly affected the difference of microclimate regulation ability among the three configuration modes.(6)A microclimate regulation capability model of urban trees was developed.E,Gs,and 3DGQ could explain 93%variation of cooling effect,while E,Gs,Vpd _L,and 3DGQ could explain 85%variation of humidifying effect.The transpiration characteristics and microclimate effects of urban trees are largely determined by tree species type and configuration modes.Therefore,this study demonstrated that the urban heat island could be mitigated by selecting deciduous broadleaf tree species and planting them in groups.