Wider crown shyness between broad-leaved tree species than between coniferous tree species in a mixed forest of Castanopsis cuspidata and Chamaecyparis obtusa

摘要

Persistent openings (gaps) between adjacent crowns at similar heights can be found in tall forests. Such openings are called "crown shyness" and are formed through the collision of tree crowns during high winds. While crown shyness has been studied in plantations, much less studied in natural forests despite its importance for light acquisition, competition, and coexistence. In this study, we focused on a mixed forest in Japan, and examined a hypothesis that broad-leaved tree species, which often have top-wide crowns, have wider crown shyness than coniferous tree species, which often have narrow cone-shape crowns, because top-wide crowns may be more exposed to wind and more likely to collide with their neighbors. Crown shyness, which was measured by drone imagery, was clearly found between individuals of the broad-leaved tree species Castanopsis cuspidata but not between the coniferous tree species Chamaecyparis obtusa. This interspecific difference may be associated with differences in crown shape; Castanopsis had wider (1.57-fold) and shallower (0.64-fold) crowns than Chamaecyparis. Within Castanopsis, the size of crown shyness tended to negatively correlated with stem diameter and positively correlated with stem slenderness. Our study showed that the difference in crown shape among species is one of the key factors in determining the size of crown shyness. Top-wide crowns of broad-leaved trees compared to conifers may have a competitive advantage in light interception, but it may come at the cost of crown collisions with adjacent crowns and resulting in persistent openings at the canopy layer.