Abstract
Ecophysiological aspects of plant growth form differentiation were investigated in the three mangrove species of southern Florida: red mangrove (Rhizophora mangle L.), white mangrove (Laguncularia racemosa Gaert.) and black mangrove (Avicennia germinans (L.) L.). Both field and greenhouse studies were conducted to characterize morphologically and physiologically the two growth forms of these mangroves, scrub and tall plants, and to identify possible genetic and environmental factors responsible for the growth form differentiation in the red mangrove. Stable isotope techniques (especially analysis of carbon, hydrogen and oxygen isotope ratios) were used throughout this research.The two growth forms of all three mangrove species differed significantly in both morphological and physiological characteristics. The scrub form had an extremely low canopy height (usually about 1 m), a multi-stem crown and smaller leaves relative to the tall form. The scrub form of the red mangrove usually showed significantly lower CO$\sb2$ assimilation rate and stomatal conductance than the tall form. All scrub red mangroves studied had significantly lower foliar carbon isotope discrimination than the tall form, suggesting higher long-term water use efficiency for the scrub mangroves. Both $\delta\sp{18}$O values of stem water and predawn water potentials for the scrub form demonstrated more variation than those for the tall form, indicating more seasonal shift in water utilization between freshwater and ocean water in the scrub form.There was no obvious evidence from several greenhouse studies for the occurrence of genetic variation between the two growth forms of the red mangrove. Seedlings of the two growth forms showed similar photosynthetic and growth rate as well as foliar carbon isotope discrimination when grown under the same greenhouse conditions. The response of photosynthesis and growth to environmental stresses were similar between the two growth forms of red mangrove. In addition, the plasticity values (pl$\sp2$) were extremely high (0.75-0.98) for most of these physiological and growth characters, indicating that the phenotypic expression of these traits are strongly affected by changes in environmental conditions.Both hypersalinity and salinity fluctuation not only strongly inhibited photosynthesis and growth of seedlings grown under the greenhouse conditions, but also increased their long-term water use efficiency, while other stresses such as nutrient limitation and waterlogging did not increase water use efficiency. Thus, only hypersalinity and salinity fluctuation can account for all observed physiological differences between the two growth forms under the field conditions. In addition, differential tidal sorting of red mangrove propagules may be another important factor responsible for the growth form differentiation in the red mangrove. A hypothesized model was created to explain the factors controlling the growth form differentiation in the mangrove species of southern Florida.