Growth and physiological responses of saplings of two mangrove species to intertidal elevation

Differences in growth and physiological characteristics of saplings of Avicennia marina and Kandelia obovata were compared among 3 intertidal elevations. Growth performance of A. marina generally followed the order of the middle > lower > upper elevation. At the middle elevation, A. marina had maximum values of tree height, branch number, crown length and diameter, shoot biomass and shoot to root biomass ratio (S:R). Differently, K. obovata exhibited best growth at the upper elevation, followed by the middle and lower elevation. Except for shoot biomass and S:R, all growth parameters of K. obovata showed a tendency to decline continuously with decreasing elevation. These different responses between the 2 species indicated that A. marina is more tolerant to low elevation than K. obovata. Decreasing elevation did not compromise gas exchange of A. marina leaves, while net photosynthetic rate, transpiration rate, stomatal conductance and intercellular CO₂ concentration of K. obovata leaves declined sharply as the elevation decreased. Leaf chlorophyll (chl) a:b ratio and contents of chl a, total chl and total carotenoid showed no significant differences among elevations for both species. With decreasing elevation, activities of superoxide dismutase and peroxidase increased significantly for both species, while significant increase in malonaldehyde contents was only recorded for K. obovata. With decreasing elevation, proline accumulation in leaves was accelerated for A. marina but not for K. obovata. These physiological responses also confirmed that A. marina is more tolerant to low intertidal elevation than K. obovata.