Hills and Streams: An Ecology of Hong Kong

3. Climate and the Hong Kong Biota [Includes Image Plates]

3 Climate and the Hong Kong Biota Is Hong Kong tropical? The simplest answer is yes, it is more than 100 km south of the Tropic of Cancer and thus well within the tropics. However, if the question is 'Does Hong Kong have a tropical climate?', the answer is less obvious. Hong Kong's climate has features which are not typical of the tropics as a whole. As noted in Chapter 2, temperatures below 10°C - in the range known to cause chilling damage to sensitive plant species - occur at least a few days every winter, while temperatures below SoC are recorded several times each decade. A sea-level frost (O°C) has been recorded only once on Hong Kong Island (in 1893) but frosts occur more frequently at higher altitudes and in the northern New Territories. Both climatologists and plant geographers have tended to define the tropics in a way that excludes Hong Kong, placing it instead in the subtropics or an intermediate 'transitional tropics' zone. In marked contrast, zoogeographers have had no doubts that Hong Kong is well within the tropics. For example, the vast majority (990/0) of the 202 butterfly species recorded from Hong Kong originate in the Oriental tropics; only three are species with a primarily north-temperate range. The moth fauna, too, is dominated by tropical representatives. Similarly, of the 21 bat species found locally, only one (the noctule, Nyctalus noctula) can be described as being temperate or non-tropical. The different views of plant and animal geographers probably reflect real differences in how plants and animals respond to brief periods of extreme (for the tropics) cold. Plants are immobile and 25 Hills and Streams: An Ecology of Hong Kong mostly long-lived: in the above-ground parts, at least, minimum cell temperatures are likely to equal minimum air temperatures. Only buried seeds and underground storage organs can escape. In contrast, most terrestrial animals are mobile and can move to favourable microhabitats, while birds and mammals can thermoregulate by using more energy to maintain body temperature. The temperature of streams is buffered by heat from the soil, with only the very smallest bodies of water freezing even at high altitudes. In most parts of the world, there is a belt of drier climates, with non-forest vegetation, between the forests of the tropics and those of the temperate zone. Only in East Asia was forest cover, at least until the human disturbance of the past millennium, continuous from near the equator to the Arctic treeline. The absence of a sharp break in the vegetation (or in the associated fauna) makes it particularly difficult to say where the tropics ends and the subtropics begins. In any case, arguments about where to place a boundary along a gradient of continuous, gradual change are neither interesting nor informative. The real question is how Hong Kong's biota differs from that of areas to the north and south and to what extent these differences can be explained by differences in climate. Vascular plants The majority of Hong Kong's flora consists of genera in which most species are found within 10-15° of the equator, and only a minority of genera have their predominant distribution to the north. That is to say, Hong Kong's flora is largely tropical at the generic level. However, several major, strictly tropical families of Asian plants do not penetrate as far as Hong Kong (e.g., Burseraceae, Dipterocarpaceae, Myristicaceae), and a number of largely extra-tropical families or genera (e.g., Ericaceae, Machilus and Ilex) are better represented here than in the lowland tropics further south. Hong Kong's flora is tropical, but less so than places nearer the equator. The best comparison is with the mid-altitude forests of equatorial mountains, where the families Fagaceae, Lauraceae, Myrtaceae, and Theaceae are also prominent, while lowland families, such as the Burseraceae, Dipterocarpaceae and Myristicaceae, are rare or absent. But it must be borne in mind that, while mean annual temperatures on equatorial mountains are similar to those in Hong Kong, the montane climate resembles the equatorial lowlands in its lack of seasonality. 26 Climate and the Hong Kong Biota The probable influence of cold extremes on plant distributions in Hong Kong was graphically illustrated by the impact of the exceptional low temperatures which occurred on 28 December 1991, when a surge of cold air from the north produced temperatures below 5...