Effects of light and intertidal position on seedling survival and growth in tropical tidal forests

Abstract

Reciprocal transplant experiments were used to study the effects of tidal inundation and light level on growth and survival of four species of mangroves in Australia: Avicennia marina (Forsk.) Vierh., Bruguiera gymnorrhiza (L.) Lam., Ceriops tagal C.T. White, and Rhizophora stylosa Griff. Seedlings were planted in the high or low intertidal and in light gaps or under the shade of a closed canopy. Survival and growth of the seedlings were monitored for 30–36 months. Significant differences in survival were found among species, between intertidal zones and due to light level. Averaged across intertidal zones and light level, survival was greatest for Rhizophora and decreased in the order Ceriops, Avicennia, and Bruguiera. For all species survival was greater (P ⩽ 0.001) in the high than in the low intertidal treatment, regardless of light level. Within the high intertidal all species survived better in light gaps than under the canopy. Relative growth of Rhizophora stylosa, Avicennia marina, and Ceriops tagal were greater in the high versus low intertidal and in gaps versus under the canopy. For Bruguiera gymnorrhiza growth was not significantly different between gap and canopy or high and low intertidal. Although Avicennia marina, Bruguiera gymnorrhiza, and Rhizophora stylosa survived and/or grew best in the high intertidal they reach maximum abundance in the low intertidal. Ceriops tagal, however, performed best in the region where it is most abundant, yet even there, it was out-performed by the former three species. These results indicate that the species zonation patterns often observed across the intertidal cannot be explained by physiological adaptation alone. Factors such as propagule dispersal, competition and predation on propagules may also be important.