Asymmetric growth of belowground and aboveground tree organs and their architectural relationships: a review

The issue of forest trees biologically designed to withstand wind and slope stresses for improving tree anchorage and stability has been of interest to many forest ecologists for over 200 years. Asymmetry in architecture and function is a typical effect of biomechanical design. This review tried to find the architectural connectivity between belowground and aboveground organs of a tree based on a summary of observations of the asymmetric growth of crown, trunk, and root system. The asymmetrical aboveground growth is influenced by a complex interaction of tree species, age, neighborhood competition, wind, lighting, slope, and elevation. The asymmetrical belowground development is dependent on tree species, age, trunk leaning, wind, soil, and slope. Uneven water conduction, nutrient allocation, hormone content, and photosynthesis rate can influence the relationship of architectural mechanisms between the belowground and aboveground organs. The contradictory observations on the directional deformations of the root system (buttresses) reveal the particular prominence of combined effects of multiple factors. Future research should focus on the comprehensive understanding of the belowground and aboveground architectural relationships of different tree species. Our review provides novel insights into the connotations of root-shoot balance in biomass distribution of the individual plant organs.