Abstract
We studied the spatial distribution of two alien species, Podocarpus nagi (Thunb.) Zoll. et Moritz. (Podocarpaceae) and Sapium sebiferum (L.) Roxb. (Euphorbiaceae), in Kasugayama Forest Reserve (KFR), a warm-temperate evergreen forest that is designated a Special Natural Monument of Japan and a UNESCO World Heritage site. The deer population, which was designated a natural monument in 1957, recently attained a size of approximately 1200-1300 head in Nara Park near KFR. Local increases in mammalian herbivores might contribute to the establishment or spread of alien species in the forest. Thus, we focused on interactions between invasive trees and mammalian herbivores and their effects on the original floral composition of KFR. We conducted field surveys and distribution mapping of the two alien species with the aid of geographic information system (GIS) technology from July to October 2002 to determine their distributions and characterize their invasion patterns. We recorded the locations of 6300 P. nagi individuals and 4543 S. sebiferum individuals using a global positioning system (GPS) in a 45 ha sampling area. Most S. sebiferum individuals were aggregated in canopy gaps (54.4%), whereas P. nagi tended to occur in closed canopies (57.9%). The density of P. nagi tended to decrease along a west to east gradient; however, that of S. sebiferum indicated no clear directional gradient. These two alien species, which first established in Mt. Mikasa and Nara Park adjacent to KFR, have expanded markedly because of their unpalatability to sika deer, although they differ greatly in invasion history and spreading behavior.
