Effects of Tree Density Variations on Outdoor Heritage Conservation: Numerical Study of an Ancient Brick City Wall with Four Orientations

Trees possess the potential to safeguard outdoor cultural heritage through the regulation of microclimates in their surroundings. The Leaf Area Index (LAI), a collective measure of the foliar portion within vegetation structure, reflects the regulatory ability of trees. In this study, numerical simulations were performed to compare the hygrothermal conditions of the Nanjing City Wall facing four orientations under the influence of evergreen trees with four LAI values, while also assessing the potential risks of salt weathering and algal growth. The results were as follows: 1) under the protective canopy of trees with identical LAI, the west and east walls experienced the highest temperature fluctuations, followed by the south and north sides. Wall surface temperature fluctuation was negatively correlated with LAI; 2) except for LAI = 1.5, the north wall had the least water content fluctuation under equivalent LAI levels. Water content fluctuation generally exhibited a decreasing trend with rising LAI; 3) at the same LAI level, the south wall presented the highest risk of salt weathering (including efflorescence and subflorescence), whereas the north wall faced the lowest risk. The risk of salt weathering decreased with an increase in LAI; 4) When shielded by trees with identical LAI, the north wall exhibited the greatest risk of algal growth, while the east wall faced least the risk. Higher LAI values were associated with an increased risk of algal growth on the wall surface. These results could provide a theoretical basis for large outdoor immovable heritage conservation and plant management around heritage sites.