Effects of sea level rise on mangrove Avicennia population growth, colonization and establishment: Evidence from a field survey and greenhouse manipulation experiment

Mangroves will either face longer tidal inundation or retreat landwards in response to on-going accelerated sea level rise. However, little is known about the growth, regeneration or colonization of mangrove seedlings under the different tidal inundation regimes associated with accelerated sea level rise. In the present study, a field survey and a greenhouse mesocosm experiment were conducted to evaluate possible effects of accelerated sea level rise on colonization, establishment and seedling growth of a mangrove pioneer species, Avicennia marina. Avicennia populations at different elevations of the intertidal zone on Xiamen Island in Fujian Province, China showed distinctive patterns in both seedling density and plant age. The seedlings at lower elevations had less annual biomass accumulation and population productivity, but higher shoot to root ratios, suggesting that elevation has positive effects on seedling growth. The greenhouse mesocosm experiment with 1-year-old A. marina seedlings utilized five inundation periods (0, 2, 4, 6 and 12 h in a semidiurnal tidal cycle) and two inundation depths (root immersion and canopy immersion). Both inundation period and depth exerted significant and negative effects on biomass accumulation, photosynthetic rate, leaf electron transportation and water use efficiency. However, the negative effects of canopy immersion were more profound than root immersion. Canopy immersion exacerbated the effects of prolonged inundation, with no seedlings surviving under canopy immersion at the 12-hour treatment. These results suggest that at lower elevations with higher sea level, canopy immersion will have greater negative effects on seedling colonization, establishment and early growth of A. marina. This finding is instrumental in predicting the future dynamics of mangrove forests under increasing sea levels.