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.
The growth conditions of Hibiscus syriacus and Benthamidia florida planted on the east and west side of the street in front of Tottori Station were investigated. The soundness, flower setting and tree size were confirmed to be different between east and west in both species. The growth conditions of westward trees were worse than that of eastward trees. The survival rate of westward Hibiscus syriacus was 20% while that of eastward was 100%. It was estimated by the tree-ring analysis that the westward Hibiscus syriacus died in the same year they were planted. The insolation period was significantly shorter in the westward than eastward. The investigated trees were planted under the roofed sidewalk, which is common in snowy countries. This resulted in poor branching under the shade of the sidewalk roof. Especially, most branches of the Benthamidia florida under the sidewalk roof were dried and lost. The difference in the survival and growth of the two species is thought to be caused by the difference in shade tolerance.
Root and ectomycorrhizal density was examined in coastal Japanese black pine (Pinus thunbergii) stands. We set plots in 3 different types of understory vegetation in the stands: A-type; no-understory vegetation, B-type; herb layer less than 500 mm in height, C-type; shrub layer taller than 1000 mm and herb layer. The length of fine roots was shorter, and the number of the roots was less in the pine stands with dense understory vegetation. In such stands, the number of ectomycorrhizae per root-length (mm) and per unit volume of the soil sample (cm3) also reduced. These results suggested that vegetation succession caused decline of root and ectomycorrhizal activity. These parameters could be new simple indices for assessment of pine stand health.