Vertical stratification and effects of crown damage on maximum tree height in mixed conifer-broadleaf forests of Yakushima Island, southern Japan

We investigated vertical stratification and effects of crown damage on maximum tree height in two mixed conifer-broadleaf forests in Yakushima Island, southern Japan. In both research plots, the conifer trees dominated the upper canopy while the broadleaved trees dominated the middle to lower canopy. Most broadleaved trees were shorter than the median crown-base height (H_CB) of the conifer trees. Estimates of the maximum height (H_max) of the conifer trees were greater than those of the broadleaved trees. Crown damage had significant negative effects on maximum height of the conifer trees. Crown damage was observed for 72.8-88.7% of the conifer trees, and severe types of damage such as stem breakage and top die-back were the most predominant. The H_max of the damaged conifer trees was 16-17% shorter than that of the intact trees and as much as 16-28% shorter than the potential maximum height estimated from the diameter-height relationship of the tallest intact trees. We inferred that crown disturbance is an important factor determining the maximum height of the canopy of the two mixed forests. Our results suggested that vertical stratification between conifer and broadleaved trees may be an important mechanism contributing to their coexistence and additive basal area of mixed forests on Yakushima Island.