A classification is presented of community types found in the coastal sand dune systems along the Gulf coast and the Caribbean Sea of Mexico. Twenty-eight dune systems were sampled along transects using the Braun-Blanquet approach. A total of 4444 relev6s were subjected to agglomerative classification and table arrangement (program TABORD). Synoptic species values were calculated for the 396 clusters obtained. They were arranged in two new data sets, one comprising the material from Tamaulipas and Veracruz and the other Yucatan and Quintana Roo. These data sets were again subjected to classification and ordination (detrended correspondence analysis) programs. Final community types are based on the basic clusters, 59 types for the Gulf area and 31 for the Caribbean. Their geographical and ecological distribution is interpreted.The description of types includes information on differential and dominant, as well as frequent accompanying species, and also on some structural characters. The results were compared with a parallel study of the central Gulf area, including Tabasco and Campeche.Six main distribution patterns of the community types were found: 1) along the coast (mostly pioneer communities), 2) northern section of the Gulf, 3) northern section of the Gulf and the Caribbean, 4) mainly restricted to the Gulf, 5) intermediate patterns within Tabasco and Campeche and 6) restricted to Yucatan and Quintana Roo.The ordinations showed both local zonation patterns directly related to dune physiography and regional patterns produced mainly by soil and climate differences. Both for the Gulf and the Caribbean material axis 1 reflects a sea-inland gradient from pioneer types towards thickets on the fixed dunes. Axis 2 shows a geographical transition from Tamaulipas to Veracruz types in the Gulf material and from Yucatan to Quintana Roo types in the Caribbean case.The Gulf types can be grouped into: 1) Tamaulipas types with species in common with Texas and the south-** This research received support from CONACYT PCECBNA 005223 and CONACYT PCECBNA 005238. We are grateful to

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Distribution patterns of 237 species belonging to sixty-three families of the coastal dunes of the Yucatan Peninsula were analysed. Cosmopolitan families like Gramineae, Compositae and Leguminosae are best represented. Other families are mainly tropical. For a geographical synthesis the coast of the Yucatan Peninsula was divided into five natural sections according to differences in climate, substrate, water-table, distance to the sea and proximity of adjacent backdune vegetation. These factors explained the presence or absence of particular species which were related to the different physiognomy of the stabilized dune vegetation. The five sections are Campeche and Quintana Roo with a humid climate and West, Middle and East Yucatan where a semi-dry climate prevails. A higher floristic richness was found on the coasts facing the Caribbean Sea where hurricanes occur and neighbouring tropical rain forest influences the floristic composition of the dune vegetation. The zonation of species along the sea-inland gradient showed differences in morphology and life history. The species attributes characterize the different habitat zones. Species adapted to salt-spray and exposure are widely distributed herbs with abiotic dispersal modes. The sheltered habitats of the dunes are densely covered by palms, trees with broad entire leaves, epiphytes and armed and succulent shrubs. These species show narrow distribution patterns and zoochorous dispersal modes. Herbs are most tolerant to saline habitats, whereas shrubs and trees thrive in the nonsaline dune environments. The flora of the sand dunes of the Yucatan Peninsula is equally characterized by two elements: (a) Caribbean species and (b) South Mexican and Central American species which together form the Meso-American element.

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Question: How do differing social and economic systems affect the dynamics and trajectory of land cover / land use change on similar, neighbouring ecosystems in a time span when an economic industrialization program was enforced? Location: Tijuana River watershed, located on the border between Baja California, Mexico and California, United States. Methods: We quantified land use changes between 1970 and 1994 in the Tijuana River watershed. Using aerial photographs and geographic information systems, we elaborated land‐cover/use maps and calculated transition probability matrices to describe natural land‐cover changes at the landscape level on both sides of the border. Results: Land cover / land use transitions are mainly driven by urban development on both sides of the border, but exhibit different patterns in each country. The processes seem to be more complex in the Mexican part of the basin, where itinerant land use may revert induced grasslands and rain‐fed agriculture into natural communities, than on the US side, where the transition pathways are few and unidirectional. Conclusions: Despite the need for an integrated planning and management of binational basins and shared water resources, in practice, these goals may be hampered by different economic and social factors triggering land use change within each country.

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Antecedentes y Objetivos: Se presenta el primer listado florístico de las playas y dunas costeras de México, actualizado y respaldado por ejemplares de herbario.Métodos: Se revisaron 14 herbarios nacionales y extranjeros. Se sobrepuso una retícula de 2 × 2 km al mapa de la costa y de las dunas costeras de México para referir todos los registros que corresponden a 2180 sitios con 12,419 ejemplares de plantas. Se calcularon índices de diversidad, similitud, diversidad taxonómica y redundancia.Resultados clave: La flora registrada consta de 153 familias, 897 géneros y 2075 especies de plantas vasculares, las cuales representan 9.5% de la flora vascular de México. Las cinco colecciones con índices de redundancia altos IR≥0.7 son los herbarios de la Universidad Nacional Autónoma de México (MEXU, IR=0.9), del Centro de Investigación Científicas de Yucatán (CICY, IR=0.8), y los del Centro de Investigaciones Biológicas del Noroeste (HCIB), el Instituto de Ecología, A.C. (XAL) y el del Museo de Historia Natural de San Diego (SD), cada uno con un IR de 0.7. Se reconocen cinco regiones florísticas que corresponden a los mares de México. El Pacífico Norte tiene mayor diversidad taxonómica y el Pacífico Sur menor diversidad taxonómica. El Golfo de California, Golfo de México y Mar Caribe tienen diversidades taxonómicas similares. Las especies con más registros son Trianthema portulacastrum (165), Croton punctatus (107), Echites umbelllatus (106) e Ipomoea pes-caprae (90). Por el carácter de ecotono de las dunas costeras, la mayoría de las especies se comparten con los tipos de vegetación vecinos (selvas, matorrales, y humedales como las marismas y manglares).Conclusiones: Esta primera lista de la flora en playas y dunas de México es la base para múltiples estudios florísticos regionales y locales, biogeográficos, y ecológicos; además, que será importante para su uso en temas de impacto ambiental y manejo costero.