Geospatial Technologies for Vegetation Mapping

Introduction

Hong Kong is a special administrative region of Mainland China, located on the eastern shore of the Pearl River Estuary, having an area of ~ 1100 km2. It shares a border to the north with Guangdong province of China and is surrounded by the South China Sea on east, west and south. Hong Kong is situated between 2209ˊ to 2337ˊ latitudes and 11352ˊ to 11430ˊ longitudes on the northern margins of Asian tropics. The topography of the area is rugged with the highest point of 957 m at Tai Mo Shan in the New Territories. Due to rugged terrain, more than 75 % of the area is undeveloped and the population is squeezed into less than 25 % area (Corlett 1999).

The climate of Hong Kong includes features of both the tropics and subtropics with highly seasonal rainfall and temperature. Despite being located 100 km south of the Tropic of Cancer, the climate is best described as subtropical due to temperature and rainfall seasonality with a hot humid summer from May to September and a cool dry winter from November to February. Mean annual rainfall is 2398 mm but 80 % of this amount is confined to May to September, and mean annual temperature is 23.3C with minimum and maximum temperature in January and July, respectively (Leung et al. 2008). Temperature falls below zero above 400 m elevation, several times in a decade, and rainfall increases with elevation (Weir and Corlett 2006; Dudgeon and Corlett 2011).

Current forest cover in Hong Kong comprises Feng Shui Woods1, natural secondary forest, and plantations. Since year 1841, 390 native tree species (Corlett 1999), belonging to 61 families have been recorded in Hong Kong (Zhuang and Corlett 1996). The majority of the secondary forest has developed through structural succession on lands protected from fire since 1945. Hong Kong’s primeval vegetation was a diverse seasonal rain forest with at least 150-180 woody species per hectare as found in nearby protected areas in South China (Jingyun et al. 2004; Cao et al. 2013). Natural forest succession from grassland is said to be very slow, and dominated by hardy native pioneers and often invasive exotic species (Dudgeon and Corlett 2011).

Hong Kong’s primeval vegetation was a diverse seasonal rain forest with at least 150-180 woody species per hectare as found in nearby protected areas in South China (Jingyun et al. 2004; Cao et al. 2013). Only tidal areas, swamps, river estuaries and areas with thin soil unsuitable for tree growth, would have naturally supported grasslands and shrubby vegetation (Dudgeon and Corlett 2011). Due to human activities over hundreds of years, most of the forests were either completely lost or strongly altered. On barren land, erosion of centuries removed more than 10 m of topsoil, and together with unsustainable agricultural practices, the weathered bedrock was exposed in many places making the establishment of native trees very difficult (Lam 1977). Harsh environmental conditions such as torrential rains, strong sunshine, frequent hill fires, occasional droughts and irregular low winter temperatures often below 10C lead to the establishment of vast areas of grassland. Natural forest succession in these grasslands is said to be very slow and dominated by hardy native pioneers and often invasive exotic species (Dudgeon and Corlett 2011).

It is estimated that complete deforestation in Hong Kong was completed around 400-600 years ago. As a result, primary forest occurs only in a few patches as feng shui wood (Chinese system of geomancy) and in topographic ravines (Hau 1997; Dudgeon and Corlett 2011). In last few hundred years, the uplands of Hong Kong are assumed to have been cultivated as evident from the presence of remnant agricultural terraces on steep terrains, on historic aerial photographs of 1945 and 1963. Despite the forest climate, there is no substantial primary forest present in the landscape (Dudgeon and Corlett 2011). In the currently forested landscape of Hong Kong, most vegetation consists of semi-natural plant communities, secondary forest, successional shrublands, fire-maintained grasslands, and monoculture plantations of exotic species and mixed plantations (Leung et al. 2008). Despite total deforestation and defaunation which took place over 300 years ago, Hong Kong has a large number of tropical rainforest plant species (Turner and Corlett 1996).

1Feng shui woods are those near to villages, and preserved over centuries by villagers, often having multi-layered canopy, large trees, and large woody lianas. They are considered the closest present-day representation of primary forest in Hong Kong (Zhuang and Corlett 1997).


Reference

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