The Mediterranean area is regarded nowadays as one of the hot-spots of world biodiversity. The Baetic ranges in Spain have such a large number of endemic plant taxa that the territory has been recognized as a well-defined biogeographical unit. These endemic taxa tend to concentrate on disjunct highland areas that have been described consequently as 'highland islands'. Despite the importance and complexity of these sites, the research carried out so far has produced only descriptive classifications. These approaches overlook the relationships between the sites and do not serve to identify the main centres of endemism. Herein, multivariate analysis techniques (cluster analysis and reciprocal averaging) and the parsimony analysis of endemicity (PAE), applied to the 222 orophilous endemic taxa of the Baetic ranges, have been used to reveal the floristic similarities between the areas involved, to identify the centres of endemism and to re-assess the previous classifications. Four centres of endemism are defined, one siliceous and three calcareous. The most outstanding feature in the Baetic ranges is precisely the floristic difference between siliceous and calcareous mountains. Calcareous territories extend along a SW-NE axis in line with the oceanic-continental gradient. The nature of the soil in the vast siliceous and calcareous-dolomitic territories of the Sierra Nevada is probably the cause of the remarkable floristic diversity. Not surprisingly, in our analysis these areas stand out as centres of endemism. We conclude that previous biogeographical classifications of the ranges are too rigid and do not properly reflect the floristic similarities of the area under study.

The presence of trees or shrubs, usually legumes, in xeric environments, promotes the existence of patches with higher concentrations of resources, such as nutrients released from litter decomposition, and favorable microenvironments that support higher plant diversity. This work contributes to the recognition of microcosmos flora of Prosopis laevigata. Also, species diversity/richness-tree structure relationships in Tehuacán-Cuicatlán Valley are reported. Seventy trees were labeled, and their canopies' areas were measured. Finally, we made a taxonomic recognition of vegetation under the canopies in the rainy and dry seasons. We found 76 and 62 plant species in rainy and dry seasons respectively. Asteraceae, Cactaceae, and Leguminosae were the families with more plant species at both seasons. Nevertheless, the number of individuals differs among seasons. Canopy cover and height of trees had a direct influence on richness of protected species and diversity. This is consistent with the general hypothesis that shadow is an important factor for the establishment of species under canopies in the facilitation process. Our results support the widespread hypothesis that show the legumes as nurse plants; as well as their crucial and potential role for plant resources conservation in arid and semiarid environments.

Background: Understanding changes in local community composition along environmental gradients is essential for studying the long-term metacommunity dynamics. The metacommunity structure depends on the distribution of species along environmental gradients in terms of their coherence (continuity in their distribution range), species turnover and grouping of their range limits. A Clementsian structure would be defined by coherent ranges, significant turnover and sharp limits between local communities. All other things equal, a Gleasonian structure is distinguished by the absence of clear boundaries between local communities. Questions: The structure of a scrubland/semiarid/xeric metacommunity changes 23 years after its first characterization? Do environment and spatial variables determine the metacommunity structure? Species studied: 104 perennial-plant species. Study site and dates: Zapotitlán semi-arid valley, Puebla, in 1980 and 2003. Methods: Metacommunity structure and its relationship to environmental (edaphic) and spatial (altitude, slope and geographical location) variables were analyzed using data from the two historic surveys. Results: In 1980 a Clementsian structure was determined, which remained unchanged after 23 years. The importance of environmental filters decreased from 1980 to 2003. Conclusions: The prediction that, due to stochastic dispersion of propagules, the metacommunity would tend toward a Gleasonian structure was not fulfilled. There was no evidence for homogenization, although local tetechera communities (with dominance of the giant columnar cactus Cephalocereus tetetzo) had been invaded and transformed into shrubland communities. Local communities and the metacommunity should be monitored continuously to understand of the long-term structuration of these systems.

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