Adaptive management approach to increasing the diversity of a 30-year-old planted forest in an urban area of Japan

Thirty years have passed since the completion of land reclamation and the planting of broad-leaved evergreen trees at the Expo’70 Commemorative Park, which was developed in an urban area of Osaka, the second largest city in Japan. Canopy closure and biomass development of the forests in the park are proof of the success of the revegetation of laurel forests. However, because low penetration of solar radiation to the forest floor seemed to restrict the diversity of forest floor vegetation, we carried out adaptive forest management to increase the diversity. To improve the light environment on the forest floor, we created four artificial gaps, each 15 m×15 m. Further, to establish a new diverse population from seed, we spread forest topsoil from a secondary forest containing a diverse seedbank over these gaps. The average gap fraction under the intact canopy was 0.09. Partial removal of trees (52% of basal area) increased the average gap fraction to 0.19 and heavy removal (more than 88% of basal area) increased it to 0.29–0.54. Species number and density of germinated seedlings became significantly greater in the gaps than under the intact canopy. Many seedlings arose in the gaps from the seedbanks that have built up over the last 30 years in the topsoil of the park forest. Only 1 year after creation of gaps, the spread of forest topsoil had resulted in the germination of six species that did not exist in the park before, indicating the possibility of creating diverse populations. In the partial removal plot, the seedling density was the highest, but the seedling death rates were also large and the seedlings grew the least. Differences in the light environment as determined mainly by removal intensity might affect patterns of revegetation from a seedbank.