PRIMARY PRODUCTION AND NUTRIENTS IN A HONG KONG SHRUBLAND

Abstract

Secondary shrubland is widespread in southern China. This study is a pioneer one of this plant community's distribution and seasonal dynamics of phytomass and nutrients, production of dry matter and nutrients, cycling and use of nutrients. These were studied for a period of two years in the shrubland at sites in Hong Kong's New Territories. Above-ground phytomass, including standing dead matter and litter, averaged 2 239 g/m2 and below-ground phytomass averaged 1 408 g/m2. Foliage production of the evergreen shrub Rhodomyrtus tomentosa, the dominant species, was continuous and gradual. The foliage phytomass depended upon a balance of production of new foliage and senescence and abscission. The phytomass of reproductive structures of R. tomentosa tended to increase regularly from April to July and exhibited a sudden decline after September. A study of the nutrient distribution showed that the distribution patterns of phytomass and nutrient resources are significantly different. For foliage and reproductive structures, the distribution ratios of nutrients were larger than those of phytomass. It is inverse for standing dead matter and stems and branches. The seasonal pattern of N, P and K contents in the foliage and reproductive structures of R. tomentosa paralleled that of the phytomass. R. tomentosa contained a relatively large amount of nutrients in the leaves throughout the year. Mean net primary production (NPP) of the shrubland was 972 g/m2/yr. The mean above-ground dry matter production was 757g/m2/yr which made up 77.9 per cent of the total NPP. The mean production of N, P and K in the shrubland were, respectively, 6 726, 464 and 4 282 mg/m2/yr. Litterfall occurred throughout year, but there was a marked seasonal pattern in the rates of litter production. Climatic factors have an important influence on litter fall seasonality. High litter production is associated with a period of dry weather or a typhoon. The mean return of dry matter by litter fall for two years was 477 g/m2/yr; the N, P and K returns were respectively 2 815,164 and 892 mg/m2/yr. This study showed that 45.1 per cent of the N requirement in NAPP and 51.3 per cent of the P requirement in NAPP came from internal cycling of the plants. Four definitions of nutrient-use efficiency are discussed and compared. An assessment of nutrient-use efficiency through the litter fall mass and concentration data indicated that efficiency for the use of nutrients in the shrubland was greater than the value for an efficient nutrient economy in tropical forest. A conservative utilization of phytomass resources based on reduced nutrient losses is stressed. It is suggested that the foliage, branchlets, bark and reproductive structures from shrubs and trees are not removed.