ABSTRACT
Hundreds of trees grown in an arboretum for 70 years under identical environment and management practices were found to exhibit an extreme variation in vegetational composition around their trunks. Comparison of vegetation and soil properties under trees of 200 different woody species in the arboretum was conducted, and the results were evaluated from an allelopathic viewpoint. Differences in existing vegetation surrounding tree trunks were similar to that in emergence/growth from soil seed banks of the same areas, suggesting species-depending allelopathic influences of the trees. Most coniferous species were categorized to the most inhibitory group, as compared with broadleaf species. The most promotive group only consisted of broadleaf species. Growth of test plant (lettuce) differed greatly, ranging from 0 mg to 358 mg/pot in dry weight, depending on the tree species/soil association. Soil pH and soil water EC also varied widely between 3.7 and 7.7, and between 0.06 mS and 2.02 mS/cm, respectively. The tendencies found for the arboretum trees coincided with those for tree species tested in a forest. On an average, growth of the test plants and soil pH were lower in conifer soils than in broadleaf soils. A high positive correlation was shown between growth of lettuce plants and soil pH for coniferous species. These results suggest that substances consecutively egressing into stem flow contribute to large variations in vegetation, that soil properties are specific to taxonomic groups, and at least in coniferous trees, substances which can lower soil pH appear to be the cause of growth inhibitory activities.
