Ecological characteristics of the alien tree Sapium sebiferum community and the causal factors of its establishment at deforested sites on the Awaji Island, Japan

Abstract

Sapium sebiferum, an alien tree species that originated in China, has been naturalized in many areas of Japan. We examined the ecological characteristics of S. sebiferum community and the causal factors for its establishment at deforested sites, which were formed by clear-cutting the Quercus phillyraeoides community, on the Awaji Island in Japan. We recorded the locations of escaped S. sebiferum individuals of ≥1-m height in a 4-ha sampling area; 1118 individuals were found at the deforested sites, but none were found at the neighboring sites covered by the Q. phillyraeoides community. The S. sebiferum community mostly developed within 100 m of mature S. sebiferum individuals that were planted as ornamental roadside trees. Major component species of the Q. phillyraeoides community (e.g., Q. phillyraeoides and Q. serrata) and native pioneer tree species (e.g., Mallotus japonicus and Aralia elata), which are strong competitors of S. sebiferum, were rare or absent at the deforested sites because of browsing by sika deer. This suggests that browsing on native plant species by sika deer, clear-cutting of the Q. phillyraeoides community, and the presence of seed sources of S. sebiferum in the vicinity of the deforested sites greatly contributed to the establishment of the S. sebiferum community. The S. sebiferum community was similar to the Q. phillyraeoides community with regard to soil depth, although soil erosion was clearly caused by sika deer in areas of bare ground at the deforested sites. The native plant species were much more abundant in the S. sebiferum community than in the native plant communities established at the deforested sites. The maximal height of escaped S. sebiferum individuals was 7.6 m, whereas that of planted S. sebiferum individuals was 19.0 m. For escaped S. sebiferum individuals, the height highly correlated with the stem diameter at breast height and the height growth rate did not show a decreasing trend. Thus, the S. sebiferum community is likely to continue growing and exist for a long period. This may pose various problems for the conservation of indigenous ecosystems.