Abstract
Top-dieback has often been reported in Cercidiphyllum japonicum trees planted in urban area. To clarify the mechanism of the dieback, leaf physiological and morphological characteristics of top-dieback trees were compared with healthy trees. We studied seasonal changes in leaf physiological properties and shoot morphology among upper and lower parts of the crown in healthy and top-dieback trees. In top-dieback trees, leaves at both upper and lower crown showed -0.1 MPa lower predawn water potential than in healthy trees throughout the growth period from May to September. Upper crown leaves of top-dieback trees also showed reduction in maximum photosynthetic rate and increase in dark respiration rate compared to lower crown leaves in top-dieback trees and the both leaves of healthy trees from the end of July. Individual leaf area was also smaller in the shoots of upper crown of top-dieback trees, while shoot growth was greater in lower crown in top-dieback trees than in healthy trees. These results indicated that chronic drought stress and reduction of photosynthesis in leaves at the upper crown of top-dieback trees are the major factors to cause of top-dieback of Cercidiphyllum japonicum trees in urban areas.
