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The xerophytic scrub located on the lava field produced by the Xitle volcano has been almost completely destroyed by the urban sprawl of Mexico City. The Pedregal de San Ángel Ecological reserve (1.77 km2) offers protection to one of the most important remnant portions. Despite such protection status, this plant community is presently still affected by invasion of exotic species, pollution, illegal extraction of selected species, and recurrent fires. The aim of this study was to update the floristic knowledge of the reserve and to analyze possible changes in the floristic richness and composition that have taken place in the last 50 years. Out of the total 337 species that are included in the present checklist (193 genera, 74 families), 152 species had already been reported by Rzedowski (1954) in a pioneer study that covered the entire lava field (80 km2). Contrastingly, 166 species recorded by him for this xerophytic scrub were not encountered in this study, although 21 of them do occur in another ecological reserve (Lomas del Seminario) located on the same lava field but at a higher elevation. The analysis of the distribution of species of the present checklist by vegetation type showed that only 34 of them occur exclusively in xerophytic scrub, whereas the remaining also occur in other plant communities. The information provided in this study is expected to serve as the basis for future monitoring studies aimed at assessing the dynamics of this flora through time.

Land use change (LUC) has important effects on the aboveground biota, mainly by altering richness and diversity, as well as ecosystem functioning. However, the effects of LUC on abiotic and biotic soil properties need to be assessed to determine recovery potential when the original vegetation is restored. The Los Tuxtlas tropical rain forest landscape in Veracruz, Mexico, offers a suitable framework for testing this. To assess the effect of LUC on the microbial community of this tropical rain forest landscape, we analyzed the whole-cell fatty acid profile of the microbial community and physicochemical properties of soils from four types of land use: crops, pastures, secondary forest, and primary tropical rain forest for the dry and rainy seasons. Regardless of season, the microbial data grouped according to land use. The composition of the microbial community was correlated with soil pH in the dry season, and with nitrate, soil organic matter, and available phosphorus concentration in the rainy season. Land use affects the abundance of the microbial community as a function of seasonal variation. In comparison with the microbial community in primary and secondary forest, that of crops and pastures was more greatly affected by seasonal variation. In fact, there were no significant differences between the primary forests or among secondary forests of different ages. These results may indicate a capacity for fast recovery (5 years or less) by the microbial community upon forest regeneration.

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