Net ecosystem CO2 exchange in the “Coeur de Voh” mangrove, New Caledonia: Effects of water stress on mangrove productivity in a semi-arid climate

• Mangrove-atmosphere CO2 exchanges were studied using an eddy-covariance system.
• Net ecosystem exchanges are affected by air temperature, VPD, PAR and tidal cycles.
• Water availability is a major controlling factor of CO2 fluxes in this dwarf mangrove.
• Monthly rainfall strongly affected monthly net ecosystem production.
• During rainy season, ecosystem respiration counterbalanced ecosystem photosynthesis.

In New Caledonia, a world-renowned mangrove, the “Coeur de Voh”, grows in a semi-arid climate and at a high elevation in the intertidal zone. Net CO2 exchange (NEE) between the Avicennia marina shrub mangrove and the atmosphere was studied using an eddy-covariance system. The NEE and its biophysical controls were investigated, and net ecosystem production (NEP) was determined for one year. The daytime NEE ranged from −10.7 μmol m−2 s−1 to +6.9 μmol m−2 s−1 and the nighttime NEE ranged from +0.38 μmol m −2 s−1 to +8.80 μmol m −2 s−1. Both minimum values of daytime NEE and maximum values of nighttime NEE were measured during the wet, and warm season. Solar radiation, temperature and vapor pressure deficit were the controlling factors of NEE variations on both diel and seasonal scales. Water availability from tides and rainfall appeared to be key factors driving mangrove productivity. The tidal cycle also had pronounced effects on the mangrove NEE. Our results suggest that submerged conditions during high tide decreased the ecosystem respiration contribution and may increase the rate of tree photosynthesis. The absence of flooding spanning a few days rapidly decreased the NEE, suggesting that the trees experience water stress. The NEP was significantly and negatively correlated with rainfall and showed high seasonal variation. During the wet season, the NEP was very low and even negative, whereas the ecosystem photosynthesis was the greatest of the year. Indeed, the low NEP during the wet season was related to a significant increase in ecosystem respiration, whereas the low NEP during the dry season was linked to a decrease in ecosystem photosynthesis due primarily to an absence of freshwater from rainfall and seawater from tides. We estimate that the annual NEP was 73.8 gC m−2 and may be lower if carbon tidal exports are significant. Consequently, the NEP of this dwarf mangrove in a semi-arid climate was much lower than that of mangroves in humid climates.