Trees and Canopies

Mangrove trees and forests have obvious similarities to their terrestrial counterparts, yet other attributes of mangroves, especially physiological and morphological specializations, make them structurally and functionally unique. These attributes include:

• Aerial roots

• Viviparous embryos

• Tidal dispersal of propagules

• Frequent absence of an understory

• Frequent absence of annual growth rings

• Highly efficient mechanisms for nutrient retention

• Physiological mechanisms to tolerate salt

Because of the high irradiances and temperatures that occur in the tropics, one would expect that mangroves are C₄ (CAM) plants. However, isotopic ¹³C signatures for a number of species show only C₃ carbon fixation (Andrews and Muller 1985). Some work suggested a modified C₄ pathway in Rhizophora, but all physiological studies indicate that mangroves obtain water, prevent desiccation, and closely coordinate stomatal conductance and CO2 assimilation, without use of a C₄ pathway (Joshi et al. 1984; Martin and Loeschen 1993). Other C₃ characteristics include a temperature-dependent CO₂ compensation point of 40–90 μl ⁻¹, and a temperature optimum for photosynthesis below 35°C (see review of Clough 1992).