Ecophysiological differences between fringe and dwarf Avicennia marina mangroves

In this investigation, morphological and physiological differences between fringe and dwarf Avicennia marina (Forsk.) Vierh. growing in seawater and hypersalinity were compared along a tree height and productivity gradient in Richards Bay, South Africa. Dwarf trees had thicker leaves and cuticles, lower specific leaf area and salt gland frequency, while the concentrations of total chlorophyll and chlorophylls a and b were lower by 26, 23 and 39%, respectively, compared to fringe trees. Soil psi and soil salinity were -3.04 +/- A 0.09 MPa and 36 +/- A 3 psu in the fringe zone, compared to -7.24 +/- A 0.38 MPa and 58 +/- A 5 psu, respectively, in the dwarf zone. Midday minimum xylem psi was -4.3 +/- A 0.23 MPa in the fringe zone and -6.4 +/- A 0.28 MPa in the dwarf zone. In leaves of dwarf trees, the concentration of Na+ was 30% higher, while those of K+, Ca2+ and Mg2+ were lower by 41, 38 and 55%, respectively, than fringe trees. The Na+/K+ ratio of leaves was 2.1 +/- A 0.03 for fringe and 5.6 +/- A 0.05 for dwarf trees. Rates of secretion of Na+, Cl-, K+, Ca2+ and Mg2+ over 24 h were significantly lower in dwarf trees by 44, 45, 78, 66 and 54%, respectively. In fringe trees, the rate of secretion of Na+ and Cl- was about 28% higher during the night than during the day, while in dwarf trees the corresponding increase was about 174%. CO2 exchange, leaf conductance, quantum yield of PS II, ETR through PSII and intrinsic photochemical efficiency of PS II were significantly lower in dwarf trees by 50, 83, 39, 33 and 12%, respectively.