Asymmetric crown spread of street tree Cinnamomum camphora in Shanghai metropolis

The camphor tree crown tends to develop asymmetries, and wind and motorway-related asymmetric roots are dominant factors that help understand the aboveground responses to the belowground in architecture.

The below- and above-ground morphological responses of forest trees have long been observed and their adaptive growth has been found related to tree stability. Some external forces, such as wind loading, can be transferred through the trunk to the root system, causing the root architecture to be affected by the aboveground architecture. Due to the different functions and manner of resource uptake of below- and above-ground organs, it is necessary to understand the aboveground responses to the belowground in architecture. Urban street tree roots are particularly susceptible to stresses from massive impervious surfaces, so the responses of crown architecture should be based on the combined effects of internal and external factors. This study used 190 street trees of Cinnamomum camphora at a coastal site in Shanghai and the architectural data of each tree’s crown were collected. Uncrewed aerial vehicles were used to determine crown volume and its orientational partitioning, flow design was used for wind velocity data, and ground-penetrating radar (GPR) was used to detect the data of root distribution. The results showed: (1) bidirectional crown asymmetry index (bCAI) shows the crown bias toward the road regardless of road orientation; (2) there is an increasing trend of bCAI with DBH; (3) orientation of tree toward road becomes a dominant factor influencing bCAI; (4) the relative directions between trees toward roads and wind show additive effects in consistent situations and restraining effects when in opposing directions. These findings explain the crown architectural traits of street trees and contribute to understanding root–shoot balance in biomass distribution.