Hills and Streams: An Ecology of Hong Kong

5. Succession and Climax

5 Succession and Climax Succession Compared with most animals, vascular plants are long-lived, sedentary organisms. It is therefore tempting to view vegetation as merely the static matrix through which animals move. Plants are not immortal, however, and the seed stage is highly mobile. Vegetation does change and often with surprising rapidity. Seasonal changes in vegetation and individual plants have been discussed in Chapter 4. Here we are concerned with longer-term changes in the structure and species composition of vegetation and the associated animals. These changes are termed succession. It is most convenient to restrict the term succession to unidirectional vegetation change, using other terms for fluctuations and cyclic changes. Ecology textbooks distinguish between primary succession on surfaces, such as newly-exposed rock, which have not previously borne vegetation, and secondary succession in areas where the previous vegetation, but not the soil, has been destroyed. In practice, however, this distinction is far from absolute. Succession on highly-eroded hillsides, for instance, shows some features of both types. Primary succession on totally new substrates is unimportant and unstudied in Hong Kong and will not be considered further. Early studies of succession emphasized the orderliness and predictability of the changes. The distinguished American ecologist, 63 Hills and Streams: An ,-r,,,,,,-,,, Frank Clements, compared primary succession to the life history of an organism and secondary succession to the healing of a wound. Current ecological theory, in contrast, emphasizes the role of individual species biology and of chance. Succession occurs because every local flora consists of plant species with many different combinations of ecological characteristics. When a site is made available for colonization by the destruction of the previous vegetation, different species have different probabilities of arriving at the site because of differences in dispersal ability or in their ability to persist through the disturbance. Among the species that do arrive, there will be variations in establishment success, growth rate, competitive ability, shade tolerance and so on. The result of all these differences between species is the progressive change in structure and species composition that we recognize as succession. In parts of the world where the local flora consists of only a few plant species, succession will proceed in an orderly and predictable manner. In areas such as Hong Kong, however, with a rich flora and many plant species sharing similar ecological characteristics, the particular species present will depend to a considerable extent on chance and on the composition of the surrounding vegetation. In these circumstances, detailed predictions are impossible. If the physical environment remained constant and there was no further major disturbance, a site would eventually be occupied by all the species in the local flora which could persist indefinitely under such ,:onditions. This final, stable plant community can be termed the climax community for that particular site. In practice, even in the absence of man, long-term changes in climate and rare natural catastrophes, combined with the slowness of plant migration, prevent this final equilibrium from ever being reached. The theoretical climax under a given set of environmental conditions may still, however, be a useful concept for the description and understanding of vegetation change. Before the arrival of man, Hong Kong was covered in a dense, species-rich rain forest, inhabited by a diverse vertebrate and invertebrate fauna. We can only speculate about the detailed floristic composition of this forest because no large remnants survive, either in Hong Kong or in adjacent parts of South China. The tree families Lauraceae and Fagaceae were probably dominant, judging by their prominence in the regional flora. The importance of the Fagaceae is supported by the prominence of genera of this family (Castanopsis, Lithocarpus and Quercus) in samples of fruits and seeds from an archaeological site at Penny's Bay, Lantau Island, dated at about 6000 64 Succession and Climax years ago. The samples consist of only exceptionally woody fruits and seeds so the absence of the typically thin-walled seeds of the Lauraceae does not mean that this family was not present near the site. The other species identified were Choerospondias axillaris (the hog plum), an unidentified species of Elaeocarpus, Gnetum montanum, Pinus massoniana and Schima superba, all of which can still be found in Hong Kong today. Hong Kong is well within the typhoon belt and this probably had a significant influence on forest structure. The impact of typhoon winds is variable: temporary defoliation is the most widespread type of damage...