Guidance for mangrove replanting: 1. Interspecific variations in responses of mangrove saplings to two contrasting salinities

Abstract

The early growth of seven species of true mangroves representing all the categories relevant to viviparity (i.e. true viviparous species, crypto viviparous species and non-viviparous species) and including pairs of species which are closely related as well as species commonly used in replanting, was studied in response to two contrasting salinity regimes, low saline (i. e. 3-5 ppt) and high saline (i.e. 25-27 ppt). Growth performance of the seven species (i.e. Avicennia marina, A. officinalis, Bruguiera gymnorrhiza, B. sexangula, Rhizophora apiculata, Rhizophora mucronata, and Sonneratia caseolaris) in terms of plant dry weight, Relative growth rate (RGR), leaf area and shoot height was assessed. The percentage water content of plants under the two salinity levels was also assessed. Performances of all the aspects of A. marina under the low saline and high saline conditions were not significantly different implying that this species has the highest salinity tolerance among the seven species. S. caseolaris did not survive under high saline conditions proving that it is the lowest in salinity tolerance. The performances of the other five species were in between these two ends, and showed considerable variation. The RGR of each of A. officinalis, B. gymnorrhiza, B. sexagula and R. apiculata was significantly lower under high saline conditions, with reductions in growth compared with low salinity conditions of 51%, 40%, 64% and 32% respectively. By considering variations in the performance of all the factors assessed, it was possible to arrange the seven species in descending order of salinity tolerance as A. marina > R. mucronata > B. gymnorriza and R. apiculata > A. officinalis, and B. sexangula > S. caseolaris, showing that even taxonomically similar species may be distant in salinity tolerance. The percentage water content of the least saline tolerant mangrove species, i.e. A. officinalis, B. sexangula, Rhizophora apiculata, was higher when they were grown under low saline conditions, implying that species with less tolerance to salinity may opportunistically absorb and keep more water when the salinity is low. As salinity of the habitat appears to be a primary factor controlling the survival and growth of seedlings planted, these interspecific variations in salinity tolerance of species should be taken into consideration in mangrove replanting.