Root biomechanics in Rhizophora mangle: anatomy, morphology and ecology of mangrove’s flying buttresses

Background and Aims Rhizophora species of mangroves have a conspicuous system of stilt-like roots (rhizo-phores) that grow from the main stem and resemble flying buttresses. As such, the development of rhizophores canbe predicted to be important for the effective transmission of dynamic loads from the top of the tree to the ground,especially where the substrate is unstable, as is often the case in the habitats where Rhizophora species typicallygrow. This study tests the hypothesis that rhizophore architecture in R. mangle co-varies with their proximity to themain stem, and with stem size and crown position. Methods The allometry and wood mechanical properties of R. mangle (red mangrove) trees growing in amangrove basin forest within a coastal lagoon in Mexico were compared with those of coexisting, non-buttressedmangrove trees of Avicennia germinans. The anatomy of rhizophores was related to mechanical stress due to crownorientation (static load) and to prevailing winds (dynamic load) at the study site. Key Results Rhizophores buttressed between 10 and 33 % of tree height. There were significant and directscaling relationships between the number, height and length of rhizophores vs. basal area, tree height and crownarea. Wood mechanical resistance was significantly higher in the buttressed R. mangle (modulus of elasticity,MOE¼18 162GPa)thaninA. germinans (MOE ¼12 160 5GPa). Slenderness ratios (total height/stem diame-ter) were higher in R. mangle, but there were no interspecies differences in critical buckling height. When in prox-imity to the main stem, rhizophores had a lower length/height ratio, higher eccentricity and higher xylem/bark andpith proportions. However, there were no directional trends with regard to prevailing winds or tree leaning. Conclusions In comparison with A. germinans, a tree species with wide girth and flare at the base, R. mangle sup-ports a thinner stem of higher mechanical resistance that is stabilized by rhizophores resembling flying buttresses.This provides a unique strategy to increase tree slenderness and height in the typically unstable substrate on whichthe trees grow, at a site that is subject to frequent storms.Key words: Allometry, Avicennia germinans, biomechanics, flying buttresses, mangroves, Mexico, modulus ofelasticity, Rhizophora mangle, root anatomy, rhizophores, thigmomorphogenesis