Net aerial primary productivity, litter production and decomposition of the reef Phragmites communis in a nature reserve in Hong Kong: management implications

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Abstract
Mean net aerial primary productivity of Phragmites communis (L.) Trin. amounted to 2198.3 .+-. 416.7 g dry wt m-2 yr-1 (8.24 .+-. 1.56 .times. 107 kcal m-2 yr-1) in the Mai Po Marshes, a nature reserve for migrant birds in Hong Kong. Success of P. communis is also reflected by the high mean aerial standing biomass of 1250.5 .+-. 309.3 g dry wt m-2 and > 50% areal coverage increase over a 2 yr period. Litter production was estimated as 504.9 .+-. 212.7 g dry wt m-2 yr-1. Export of litter was only 0.06% of production; the bulk of litter was thus accumulated and decomposed in situ. Decomposition of litter was influenced by temperature and inundation regimens, the rate of 52.5% air exposure being significantly higher than at 13.5 and 2.5% exposure. Cumulated day-degree and cumulated immersion time were better predictors of decomposition rates than time alone. Higher decomposition rates at intermediate exposure were probably related to phylloplane algal growth, which gave the litter very low C:N ratios, attracting grazing gastropods. Success of P. communis is probably related to the high accretion rate and to traditional management practices which are complementary to the seasonal growth cycle. Rapid spread of the read may decrease the open area available to waterfowl and their prey, and may also encourage further accretion. Clipping at peak biomass in autumn coupled with hydroperiod and water level management are possible natural ways of control.