Conservation of an Endemic Plant, Croton hancei in the Hong Kong Special Administrative Region
Volume 3 Number 2 - July 2006
Joseph K. L. Yip, Eric Y. H. Wong and Patrick C. C. Lai
In 1997, staff of the Hong Kong Herbarium collected an unusual shrub of the family Euphorbiaceae from Tsing Yi Island, Hong Kong Special Administrative Region (HKSAR), China. After thorough examination by the experts from the South China Institute of Botany, the shrub was confirmed to be Croton hancei Benth. (Hong Kong Croton) which has been lost to botany for over hundred and fifty years.
C. hancei is a shrub about 2 m tall. The young branches and inflorescence are densely covered with stellate hairs which are absent in other parts of the plant. The oblong leaves are net-veined, with short petioles and serrulate margins, and usually densely crowded at the branch tips. The inflorescence is a raceme, each female flower has 3 styles and the upper part of each style is 2-lobed. The flowers appear in spring and the globular fruits set in summer.
This species was first discovered about 1850 by H. F. Hance on Hong Kong Island, but the specific location where he collected the type specimen is unknown British botanist George Bentham described it as a new species in Flora Hongkongensis in 1861, based on the specimen collected by Hance. Since then, the species had not been found until 1997 when it was rediscovered at a site on Tsing Yi Island. The species is considered endemic to Hong Kong as it is not known in any other locality.
The re-discovery of C. hancei contributes to the study of the Hong Kong flora and the genus Croton. For instance, Chang and Kiu (2003) examined the material from Tsing Yi Island and compared it with specimens from Guangxi which were identified earlier as C. hancei. Chang and Qiu noticed differences in the male and female floral morphology and native habitat, and therefore described the Guangxi specimens as a new variety, C. hancei var. tsoi H. S. Kiu. This illustrates that the re-discovery is of scientific interest and value.
The Natural Population and its Habitat
Currently, the only known site of C. hancei is a native woodland of approximately 0.4 ha on a steep slope facing northeast in the southern part of Tsing Yi. This woodland is isolated from the surrounding grassland by an array of giant rocks and the ridge of the slope. Field observations and analyses of the floristic composition of the existing woodland indicate that it might be the remnant of a primary forest which may have been preserved from the damage of occasional hill fires and other human activities in the past centuries by the shelter of the giant rocks and the steep and rugged topography.
In order to estimate the population size of C. hancei at the site, a field survey of the species was carried out in December 2005 (Figure 3). As no C. hancei had been recorded outside the woodland at the site, the woodland on the slope was considered the typical habitat of the species. Three permanent quadrats of 5m by 5m were randomly selected within the woodland to define the sampling area for the estimation. The population was estimated to be approximately 1700 individuals. Among the individuals surveyed, 53% and 14% of them were flowering and fruiting respectively and young seedlings were also observed in situ which demonstrated that the population is in a healthy condition.
In situ conservation
In view of the special botanical interest, the site has been listed as a Site of Special Scientific Interest (SSSI). The SSSI is about 1.1 ha, covering the woodland which contains the C. hancei population, the surrounding grassland and an array of giant rocks as a protection buffer to the population. By listing the site as a SSSI, relevant government departments responsible for planning and development of the HKSAR are made aware of the special scientific value of the site and due consideration of its conservation should be given. When developments in or near the site are proposed, an environmental impact assessment would be required and the Agriculture, Fisheries and Conservation Department (AFCD) would be consulted about the potential impacts on the SSSI.
Ex situ Conservation and Reintroduction
The AFCD has attempted to propagate C. hancei since its rediscovery in 1997. Because the site is difficult to access and the population is small, only limited material has been collected. Some juvenile plants were collected and raised under nursery conditions. They have been kept in AFCD’s greenhouse and raised in Shing Mun Arboretum for ex situ conservation. Cuttings were also taken from the natural population which resulted in an 80% survival rate. More encouragingly, the individuals kept in the greenhouse produce flowers and fruits with viable seeds. The seeds were subsequently sown with a 65% germination rate. Trial plots were also set up for reintroducing the species to suitable natural habitats and it is hoped that more populations of the species will become established in the wild.
Because of its conservation importance and botanical interest, C. hancei is recorded in Rare and Precious Plants of Hong Kong (Hu, 2003). Using the 2001 IUCN Red List Categories and Criteria, the status of C. hancei in China is “Critically Endangered (CR)”. It is hoped that the above measures will guarantee its continued existence in nature.
Bentham, G. 1861. Flora Hongkongensis. L. Reeve, London.
Chang, H. T. & Kiu, H. S. 2003. Noteworthy taxa from China (continued II). Guihaia 23(2): 97-101.
Hu, Q. M. (ed.) 2003. Rare and Precious Plants of Hong Kong. AFCD, Hong Kong.