Abstract. Large isolated trees are a common feature of the agricultural landscape in humid tropical regions originally covered by rain forest. These isolated trees are primarily used as a source of shade for cattle and people. 13 pastures (totalling ca. 80 ha) currently used as cattle pasture were studied. In them, we registered 265 isolated trees belonging to 57 species. 50 trees of the most frequent species (Ficus spp. n = 30 and Nectandra ambigens n = 20) were selected to examine the influence of isolated trees on floristic composition and vegetation structure in the pastures. At each tree, three 4–m2 quadrats were sampled: under the canopy, directly under the canopy perimeter, and beyond the canopy in the open pasture. Under‐canopy vegetation was structurally and floristically different from the other two sampling sites. Mean species richness per quadrat was significantly higher under the canopy (17.8 ± 4.3 SD) than at the canopy perimeter (11.2 ± 3.4) and in the open pasture (10.6 ± 3.6) sites. Stem density was higher at under‐canopy sites, where greater proportions of endozoochorous and rain‐forest species were found. Isolated trees function as nursery plants for rain‐forest species by facilitating the establishment of zoochorous species whose seeds are deposited under the tree canopies by frugivorous birds or bats. Our results imply that isolated trees may play a major role in seed dispersal and establishment of native species, which is of consequence for the preservation of rain‐forest species in these fragmented landscapes.

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.

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

No abstract