Today we will introduce you to a survivor, a species found in Hong Kong which over time has seen its population declining and which we hope will make a comeback as it represents an important component of Hong Kong forests. You may start having flashes of furs, eyes or legs but our species has none of that. It is big though, very big even, reaching 20 m in height with 7 to 15 cm long leaves, and large acorn fruits, as long as a five Hong Kong dollars coin. The Thick-leaved oak, 𝘘𝘶𝘦𝘳𝘤𝘶𝘴 𝘦𝘥𝘪𝘵𝘩𝘪𝘢𝘦, is a species described from Hong Kong in 1900 by Sidney Alfred Skan and belonging to the Fagaceae family. Some of you may know of it by its previous name 𝘊𝘺𝘤𝘭𝘰𝘣𝘢𝘭𝘢𝘯𝘰𝘱𝘴𝘪𝘴 𝘦𝘥𝘪𝘵𝘩𝘪𝘢𝘦, before the species was transferred back to the Quercus genus. The Cyclobalanopsis oaks also known as ring-cupped oaks (~90 species), however, are just a subgroup of the larger Quercus genus which includes about 430 species, with the former diverging and diversifying across SE Asia at the end of the Oligocene - early Miocene periods, about 23 M years ago. As you can see, the ancestry of our species doesn’t date back from yesterday.
𝘘𝘶𝘦𝘳𝘤𝘶𝘴 𝘦𝘥𝘪𝘵𝘩𝘪𝘢𝘦 is not restricted to Hong Kong though as its distribution extend to the southern part of China, including Guangdong, Guangxi and Hainan, and potentially in Vietnam where its presence remains to be confirmed. Yes, even tree species' presence can sometimes remain elusive. In Hong Kong, however, the species is considered an important and dominant species in the original evergreen broad-leaved forests that covered the Hong Kong territory, and the Southern part of China, before humans started to heavily modify the environment hundreds of years ago. Indeed, centuries of deforestation led the thick-leaved oak to become extremely restricted; now limited mainly to areas too high and too steep for humans to use their axes and the land cleared. If Hong Kong forests may today appear widespread, covering about 26% of the territory, it is important here to remind everyone that for a long period ending in the mid XXth century, this was a very different situation.
Those centuries of deforestation have left deep marks in today’s landscape and ecosystems, with erosion removing up to 10 m of topsoil in some areas, only to leave bedrocks emerging and now a very thin layer of humus slowly reforming thanks to the forests covering what were once tree-free areas. With this topsoil being washed away, it's not only nutrients and important organic matter that disappeared but also organisms living therein or depending on it. While the trees may have been cut, their progeny under the form of seeds and once waiting in the soil were washed away, exposed to the burning sun or consumed by pathogens (e.g. fungi) and predators. The seed of the thick-leaved oak is composed of two parts: a hat enclosing less than half of the nut (acorn) with fixed ring patterns called cupules. The nut is the part which through germination will then become a small plantule and many years later may form a beautiful tall tree. But to reach that stage, an important step is necessary: seed dispersal!
Have you realized that plants use all kinds of strategies to spread their seeds as far as possible from the parent tree and thus avoid competition with the plant where they initially grew? Some plants will use water as a way of dispersal (think about coconut seed wandering on ocean’s waves until they reach a suitable beach), wind (which is obvious with dandelion seeds), their own strength or shape such as the gyroscopic seeds of the gigantic Dipterocarpaceae trees. Nearly 40% of native plant species in Hong Kong, however, rely on us, animals, for their dispersal; sometimes even manipulating our behaviour to maximize their benefits - think of the bright colour of some fruits as nothing less than indicators to let us know when the seed is ready to be spread around. And just like the Monthy Pythons argued about, the larger the seed is, so needs the dispersal agent to be.
For the thick-leaved oak, their main dispersers are rodents from the Leopoldamys family which once were most likely found in Hong Kong. The Edwards's long-tailed giant rat, Leopoldamys edwardsi, a 400 g rodent, which is larger than most other rodent species, has gone extinct here probably as a result of deforestation and habitat fragmentation, with their ecological niche then partly filled by other introduced rats and squirrel species but unable to provide similar seed dispersal services. The scatter-hoarding behaviour of the Edwards's long-tailed giant rat is important for the dispersal and germination of acorns. When the rodents collect and bury the seeds, some storage locations will be forgotten, or when the rodent dies before using them, while the seeds germinate instead of being eaten. But with this rodent species gone, the thick-leaved oak acorns rely now almost exclusively on gravity for their dispersal and luck for germination, as being buried provides several benefits against predation and desiccation than a fall on the floor does not. As a result, the surviving populations of thick-leaved oaks have reduced chances to be renewed and colonize new areas.
Ultimately to restore healthy populations of thick-leaved oaks, we thus need to act on protecting existing tree individuals and facilitate the spread of new young plants of what is one of the most important tree species within Hong Kong natural forest. If you wish to discover the seeds of the thick-leaved oak and the stories of thousands of other amazing species, come to visit the Hong Kong Biodiversity Museum!
Text: Samuel Yeung
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