Study On Hygrothermal Regulation Of Trees To Buffer The Weathering Of City Wall

Nanjing City Wall,one of the most important cultural heritages in China,has a long history that can be traced back over 600 years.However,Nanjing City Wall has been exposed for centuries to climatic variations such as the temperature,relative humidity,and solar radiation,and been deteriorated by efflorescence,peeling,cracking and so on.Weathering not only damages the aesthetic value of the heritage,but also causes the loss of historical and cultural information,and may even threaten the structural stability of the wall.Therefore,it is critical to control the dependent environment for the conservation of the site heritage.As an effective measure to alleviate the urban heat island and improve the urban hygrothermal environment,vegetation has been widely concerned.However,in the field of heritage conservation,the relationship between trees and weathering of cultural heritage has not been fully understand and quantified.Therefore,taking Nanjing City Wall as an example,based on field investigation and hygrothermal simulation,the influence of trees around the City Wall on the dependent environment and hygrothermal characteristics of the City Wall were analyzed,and the influence of trees on wall surface weathering were evaluated.Based on the investigation and analysis of the environment of the Nanjing City Wall,The deterioration conditions of the City Wall were surveyed and a hygrothermal measurement was conducted.Measured results showed smaller temperature fluctuations on shaded surface,which seems to decrease the temperature stress on the City Wall by the tree.Then a coupled two-dimensional hygrothermal ‘City Wall-plant' model was developed by FORTRAN language and validated with measured values.The validated model was used to clarify the influence of the roadside trees on the temperature and humidity distribution of City Wall.The simulated results obtained using ‘City Wall-plant' model showed that the trees play a positive role in mitigating the weathering of the wall surface.The evergreen trees can reduce the risk of freeze-thaw cycle,efflorescence,and subflorescence of the City Wall.Finally,based on the varied solar radiation amount of wall orientation,the influence of trees on the weathering of wall surface were evaluated.The results show that the risk of freezing-thawing cycle risk was found higher on north orientation,and the risk of efflorescence and subflorescence risk were higher on south wall.The evergreen trees can effectively alleviate salinization,freeze-thaw cycle damage and salt crystal damage of all orientations.Based on the influence of trees on the surface weathering of the City Wall,the optimization strategy of tree management around the City Wall was proposedThe results of this paper show that the hygrothermal regulation of trees plays a positive role on the weathering of the City Wall,which will provide reference for the landscape design around the Nanjing City Wall and the protection of the City Wall.