The semi-mobile coastal sand dune system E1 Morro de La Mancha at the Gulf of Mexico was studied phytosociologically and ecologically. The floristic gradient resulting from a relev6 ordination was correlated with the amount of sand movement occurring on the site of each relev6, and the topography of the dune.
The gradient manifested in the ordination represents the transition from the more mobile parts of the dunes to the more stable ones. It can be interpreted as a progressive stabilization during which sand movement decreases. Chamaecrista ehamaecristoides, Palafoxia lindenii and Croton punctatus were found to inhabit the sites with considerable sand movement. The first species tolerates both erosion and accretion; the other two were only found in sites of sand accumulation. On the other hand grasslands and scrub were found on sites without any sand movement. Sand movement should be considered among the most important factors that affect the distribution of communities in sand dunes. * Nomenclature follows MEXU, the National Herbarium at UNAM. ** This research received support from CONACYT PCECB-NA 005223. *** I am grateful to S. Castillo for her help in the field work.
I also want to thank E. van der Maarel and J. P. Grime for their valuable suggestions and critical revision of the manuscript.