Differences in morphology, carbon isotope ratios, and photosynthesis between scrub and fringe mangroves in Florida, USA

Abstract

All three mangrove species in Florida (USA), Rhizophora mangle L., Laguncularia racemosa Gaert. and Avicennia germinans (L.) L., are found frequently in scrub mangrove forests, in which individuals rarely exceed 1.5 m in height. In the present study, the differences in morphological characteristics, leaf carbon isotope ratios and photosynthetic gas exchange between individuals in scrub and fringe mangrove forests in south Florida were investigated quantitatively. Plants in the scrub forests had much lower canopy height, more main stems per tree and smaller leaves, relative to those in the fringe forests. There was a significant correlation between tree height and leaf δ 13C value, with higher δ 13C values (1–4% more positive) for plants in the scrub mangrove forests. Correspondingly, scrub mangroves showed significantly lower intercellular carbon dioxide (CO 2) concentration and higher intrinsic water use efficiency over long-term carbon assimilation, relative to fringe mangroves. Photosynthetic gas exchange measurements on R. mangle individuals showed a 15.5% lower CO 2 assimilation rate, 6.1% lower intercellular CO 2 concentration and 11.6% higher intrinsic water use efficiency in scrub mangroves, consistent with those estimated from leaf carbon isotope ratios. A higher slope for the linear correlation between CO 2 assimilation rate and stomatal conductance was observed for the individuals in the scrub mangrove forest, which is in agreement with other measurements indicating higher water use efficiency in scrub mangroves. Possible environmental factors responsible for these morphological and physiological differences between scrub and fringe mangroves are discussed.