Mangroves of the United Arab Emirates: ecotypic diversity in cuticular waxes at the bioclimatic extreme

Scattered populations of Avicennia marina can found in the Arabian Gulf, oc cupying one of the driest mangrove habitats in the world, in which salt con centrations may reach levels that are beyond the physiological limit for ot her species. These ecosystems an poorly known, in particular, what levels o f productivity can be attained and whether there are any indications of gen etic adaptation to these exceptionally seven conditions. We have estimated standing biomass as varying between 70 and 110 t ha(-1) for the tallest sta nds and between 14 and 65 t ha(-1) for shortest stands. Our estimates for l itterfall are 7.4-8.5 t ha(-1) yr(-1) in the tallest stands and 5.1-6.9 t h a(-1) yr(-1) in the shortest stands. These values of biomass are intermedia te between levels reported for the same species in New Zealand and in Austr alia and the estimate of litterfall is comparable with that obtained from s tands in Australia. Thus, despite the harsh ecological conditions, A. marin a forms highly productive stands. We have analyzed leaf cuticular hydrocarb on composition, by gas chromatography, as a marl;er of population different iation. The dominant alkanes were C-33 and C-31, which are unusually long a s compared with the other widespread species Avicennia. germinans. Since lo nger carbon chain lengths appear to offer an adaptation to minimize cuticul ar water loss, it would appear that A. marina has evolved under more arid c onditions than A. germinans. Our sampled population from the Inner Gulf dif fered in quantitative cuticular wax composition from the Outer Gulf populat ion, suggesting a possible genetic differentiation between the two populati ons. Our data indicate the need for more detailed analyses of the possible role of genetic adaptation in the productivity of mangrove ecosystems at th e margins of their ecological range. (C) 1999 Elsevier Science B.V. All rig hts reserved.