The Nationally Rare and Endangered Plant, Aquilaria sinensis: its status in Hong Kong
Joseph K. L. Yip and Patrick C.C. Lai, Hong Kong Herbarium (Updated : 15 April 2005)

Introduction

The English common name of Aquilaria sinensis is “Incense Tree”, which is a direct translation of its Chinese name – “Heung” (香) tree. Incense is made from any wood that releases fragrance when it is burnt. The species was once widely planted in Hong Kong as the raw material of incense exported to China, Southeast Asia and places as far away as Arabia. The Hong Kong region was well known for this, thus giving it its Chinese name – “harbour exporting incense” (Iu, 1983; Hong Kong Herbarium, 2003).

In recent field surveys of local fung shui woods by AFCD in 2003, Aquilaria sinensis was recorded at 84 sites out of a total of 116 sites surveyed. This species is locally common in lowland habitats, particularly in fung shui woods behind villages where it is most commonly found. The species is considered vulnerable in mainland China due to its over exploitation there. Recently, illegal exploitation of the species has spread into Hong Kong – and so the situation of Aquilaria sinensis in Hong Kong calls for our attention.

Characteristics

Aquilaria sinensis belongs to the plant family Thymelaeaceae. It is an evergreen tree, 6 to 20 m tall. The smooth bark is grayish to dark grey (Figure 1), and the wood is white to yellowish – so giving it another Chinese name “Pak Muk Heung” (White Wood Incense). Its branchlets are sparsely covered with hairs when young. Its leaves are alternate, leathery, obovate to elliptic, generally 5 to 11 cm long and 2 to 4 cm wide, with 15 to 20 pairs of inconspicuous and nearly parallel lateral veins which is a helpful diagnostic feature in the field (Figure 2). The apex of each leaf is short acuminate and the base is broadly cuneate, with entire and smooth margins. Its flowers are yellowish green, fragrant, in a terminal or axillary umbel. The fruit is a woody obovoid capsule with an outer covering of short grey hairs (Figure 3), 2.5 to 3 cm long, opening in two flat valves when ripen. When the fruit is open, a silky thread from the base of the fruit holds the single seed (or two) in the air, very much alike a caterpillar hangs from a tree twig with its silk (Figure 4). Whether this characteristic has any biological interest is yet to be explored.

Uses

The tree produces a valuable fragrant wood used for incense and medicine. Previously, the wood was used to make joss sticks and incense, but in Hong Kong this industry has died out.

The balm (resin) produced and accumulated from the wood is used as a valuable Chinese medicine called “Chen Xiang” (沉香) for clinical use. According to Chinese medicinal literature, the resin can be extracted in large quantities by natural fungal infection or by external wounding (up to 5 cm into the bark). Sustainable harvesting of the resin of one tree can be induced by opening a wound 3 to 4 cm into the bark, and with the resin collected a few years later after accumulation. Or a small quantity of resin can be extracted from wood blocks by heating or burning, so that the resin liquefies and seeps from the wood blocks.

Sections of trees trunks or branches that contain patches of fragrant, resinous wood enter into the trade under the name “agarwood”. The resin is probably produced by the plant as a reaction against fungal infection or external wounding. Resin impregnated fragrant wood is usually found in trees older than 20 years. Although not all trees are infected, with increasing harvest pressure, harvesters in some regions often fell trees indiscriminately in search of infected wood.

Good quality “Chen Xiang”, derived mostly from a related species Aquilaria malaccensis, was formerly imported from the Asian tropics into China but the supply of such quality products is now depleted. The resin produced by Aquilaria sinensis has been used as a substitute (named as “Tu Chen Xiang 土沉香” i.e. local Chen Xiang) to the former and thus also under threats.

Conservation status

The species is chiefly distributed in South China including Hong Kong. However, owing to the intensive use of the species, wild populations outside Hong Kong have become rare and large trees are also uncommon. The 2000 IUCN Red List of Threatened Plants published by The World Conservation Union (IUCN) listed the threat status of Aquilaria sinensis as vulnerable. Regarding its conservation status, the plant is considered “an important source of medicine” and is “restricted to Jinghong in Yunnan, Guangdong including Hainan Island and Guangxi” (i.e. endemic to China). It is “mainly found in semi-evergreen monsoon forest up to altitudes of 400 m”. There is concern over the rates of exploitation and the damage to trees incurred during the harvesting of the medicinal balm. Habitat loss and clearance are also frequent (Oldfield et al., 1998). Aquilaria species are also regulated under the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES).

In mainland China, the species is listed as a category II protected plant in the “List of Wild Plants under State Protection” (1999), part of the Chinese legislation promulgated by the State Council. It is considered vulnerable in the China Plant Red Data Book, as it has become depleted due to severe damage of trees caused by indiscriminate collection of the balm used in Chinese medicine (China National Environmental Protection Agency & Institute of Botany, Chinese Academy of Sciences, 1992).

A recent book, Rare and Endangered Plants in Guangdong Province (South China Institute of Botany & Wild Animal and Plants Protection Office of Guangdong Province, 2003) reported that in Guangdong province over-exploitation and over-cutting of numerous large trees of the species have made it rare and threatened. Due to its conservation status in Mainland China, the species is also considered precious in Hong Kong, and it has been included in the book Rare and Precious Plants of Hong Kong (South China Institute of Botany & AFCD, 2003).

Status in Hong Kong

As mentioned above, the species is common in lowland areas in Hong Kong, particularly in fung shui woods behind rural villages. The species has also been found in Country Parks. It is known that the species was once wildly cultivated in South China and Hong Kong for the production of incense sticks, although no large scale plantation is known nowadays in Hong Kong. This may explain why the species is abundant around villages in modern times. Regardless of the long history of cultivation in Hong Kong, the local populations are well within its range of distribution where it regenerates in wild with vigour, and the species is therefore considered a native plant of Hong Kong (Iu, 1983).

Exploitation of this plant species for agarwood has been rare in Hong Kong, but there are recent reports that the species has been illegally exploited in our countryside. Although these incidents have not caused serious threat to the overall survival of local Aquilaria populations, the trunks of the affected trees are being cut or felled for extraction of agarwood, to an extent that the trees may not be able to recover (Figures 5 & 6) .Under the Forest and Countryside Ordinance (Cap. 96), all plants within forests and plantations on Government land are under protection. The maximum penalties for contravening the Ordinance are $25,000 fine and one year imprisonment.

Conclusion

From the point of view of flora conservation in Hong Kong, and because in other parts of China the species is over-exploited and depleted, the local populations of Aquilaria sinensis represent some of the best remaining healthy populations in China. The plant communities to which local populations of Aquilaria sinensis belong (lowland broadleaved forests and fung shui woods) have been well-preserved by both former villagers and local legislation, including the Country Parks Ordinance (Cap. 208) and Forests and Countryside Ordinance (Cap. 96). Most illegal exploitation of the species occurred recently involve unsustainable harvesting, which has caused undesirable impacts to the local populations of Aquilaria sinensis. These incidents show that the conservation of this nationally rare and endangered species deserves attention.

Figures

Figure 1. Bark of Aquilaria sinensis.

Figure 2. Leaves and flowers of Aquliaria sinensis.

Figure 3. Fruits of Aquilaria sinensis.

Figure 4. Seeds of of Aquilaria sinensis.


Figure 5. Cuts near the bases of tree trunks of Aquilaria sinensis (suspected for agarwood extraction).

Figure 6. Damages to a large individual tree of Aquilaria sinensis (suspected for agarwood extraction). Most part of the tree trunk was cut away, making the tree unlikely to survive.

 

References