Invasion of the palm Trachycarpus fortunei in Japan
Trachycarpus fortunei (Arecaceae) originates from China and was introduced for ornamental purposes in several countries. But in some cases, T. fortunei has been able to invade adjacent areas. For example, the plant has escaped from gardens in Switzerland (Ticino) and in Japan.
In Japan, it was often planted in gardens because of its strong cold tolerance, and escaped and naturalized in adjacent fragmented forests. So far, the species has not been recorded as naturalized in large, continuous forests. The plant only reproduces by seeds which are dispersed by birds. The plant is usually dioecious, but sometimes bears both male and female flowers on one plant.
Distribution of the species was studied in a forest in upper Morito River in Miura Peninsula (Honshu), where annual mean temperature is 16.1°C and annual precipitation is 1634 mm. The area is hilly, with altitude varying from 20 to 200 m above sea level. The vegetation consists of planted Cryptomeria japonica (Taxodiaceae, evergreen tree) and of abandoned coppice forest of the deciduous tree Quercus serrata (Fagaceae).
Results showed that the palm usually forms a trunk after the leaf length has attained 120 cm. Juveniles without trunks made up 72% of the population. No plants taller than 6 m were found in the studied area, but many dead trunks of less than 6 m height were observed. Palms with stems taller than 2 m bore flowers and were considered adult. In the year of study, about 60% of adult plants flowered.
Distribution of adults and juveniles showed that the plant is spreading from adjacent residential and agricultural areas. Juveniles without trunks are likely to have strong shade tolerance, since they grow on the forest floor. However, the palm is considered to require light in its flowering stage. Since the estimated observed height of the palm is 6 m, which is lower then the height of the native Japanese forest’s trees (composed for instance of Eurya japonica, Theaceae), it is expected that the palm will not become the dominant species in natural forests. The palm can nevertheless reproduce and colonize sites that are at least slightly open: in disturbed forests, under deciduous canopy, along riversides, in forests close to cliffs, and at forest edges. Although the palm will not dominate in natural forests, their species composition will be affected by the alien palm, and a serious reduction in native plants may be observed.
It is advised to remove adult palms by cutting their trunk, since adult plants do not resprout. However, juvenile plants produce new leaves when cut, and removing them is very labour intensive.
Sources
Koike F (2006) Invasion of an alien palm (Trachycarpus fortunei) into a large forest. In: Koike F, Clout MN, Kawamichi M, De Poorter M, Iwatsuki K (eds) Assessment and control of biological invasion risks. Shoukadoh book sellers, Kyoto, Japan, the World Conservation Union (UICN), Gland, Switzerland. 200-203.