Carbon, Water and Heat Fluxes Comparison between Two Subtropical Mangroves Sites, Southeastern China

Abstract:

Due to the numerous ecological services provided by mangroves and its vital ecological role, the monitor of mangrove ecosystem in China receives a growing concern. We deployed eddy covariance system and meteorological instrument to continuously monitor the exchange of CO2 flux, water vapor, heat flux and meteorological factors of mangrove ecosystem on Guangdong and Fujian province in 2012, namely GDGQ and FJYX. The major species of two sites were similar, and by 2012 were on average 2.8~3.3 m in height. Climatically, temperature, net radiation and rainfall have significant seasonal difference, all reaching peak values during the summer wet season. Based on the results available, two sites were strong carbon sink in annual scale. The cumulative NEP in GDGQ (667.92 g C m^-2 year^-1) was lower than that in FJYX (848.31 g C m^-2 year^-1), but respiration (R_d) was opposite, 1433.80 g C m^-2 year^-1 for GDGQ and 1345.13 g C m^-2 year^-1 for FJYX. Tidal inundation decreased nighttime R_d by ~0.82 μ mol m^-2 s^-1 in GDGQ and ~0.99 μ mol m^-2 s^-1 in FJYX. The diurnal patterns of sensible (H_s) and latent heat fluxes (LE) of two sites were both single peak, and peak values both occurred at 12:00~14:00. H_s of GDGQ was higher than FJYX during nighttime and lower in daytime, but LE of GDGQ was lower than FJYX during nighttime. Evapotranspiration (ET) of two sites presented similar seasonal pattern, reaching highest value in July and lowest value in January. ET of whole year were 892.66 mm and 1051.76mm for GDGQ and FJYX. Daily WUE was strong negatively correlated to salinity in in FJYX, but the pattern of GDGQ was less distinct. Due to its high salinity stress and long-time inundation, water use efficiency (WUE) of GDGQ was higher than FJYX in all months. Soil heat flux (G) was quite small when compared to other heat flux, but both had obvious diurnal pattern in two sites. G was positively correlated to air temperature and G variation range of GDGQ (-8.68 ~5.51 w m^-2) was greater than FJYX (-1.27~1.11 w m^-2) because the latter was rarely flooded by tide. Mangroves have such high NEP was attributed to year-round productivity and low ecosystem respiration, which can be partly explained by regular tidal inundation. Salinity influences must also be considered when involving ET and WUE of mangroves.