Las especies de las familias Agavaceae, Cactaceae, Fouquieriaceae, Nolinaceae, por referir algunas, se restringen principalmente a ambientes áridos o semiáridos. Por su tamaño, muchas de esas especies son sobresalientes fisonómicamente y, otras, aunque tienen tallas pequeñas, son abundantes. Ambos escenarios contribuyen a la riqueza florística y paisajística de su área de distribución.El caso de las cactáceas es particularmente interesante, ya que, además de habitar en zonas áridas, las especies de esta familia han podido establecerse en otros tipos de vegetación como matorral de dunas (Torres et al., 2010), bosque tropical caducifolio, bosques templados, entre otros (Rzedowski, 2006).La familia Cactaceae es originaria del continente americano (Anderson, 2001). Sin embargo, presenta su máxima diversidad e importancia en el territorio mexicano (Hernández y Godínez, 1994) con alrededor de 670 especies, de las cuales cerca del 80% son endémicas del país (Guzmán, 2009). Tal riqueza no se distribuye homogéneamente; por lo tanto, existen zonas específicas que ostentan una notable riqueza. Ejemplo de ello es el Valle de Tehuacán-Cuicatlán (VTC), el cual cuenta con una alta diversidad biológica (Dávila et al., 2002) y también es una de las 3 zonas semiáridas de México con mayor número de elementos endémicos (Villaseñor et al.,

IntroductionTropical dry forests (TDF) are not only the most widespread tropical forest type but also the most threatened forest ecosystem worldwide. Yet, because their dynamics have been insufficiently studied, our knowledge about the factors responsible for the spatial and temporal variability in TDF dynamics remains very limited. In this study, we aimed (1) to assess the effects of two lithological substrates on TDF structure, diversity, and species composition, and (2) to determine whether and how these substrates affect temporal dynamics on TDF attributes.MethodsWe used information gathered through a 12-year long annual monitoring of 14 old-growth TDF permanent plots in southern Mexico, seven of which were established on limestone and seven on phyllite. Previous work shows that limestone-derived soils have higher humidity, conductivity, and phosphorous and calcium contents, but lower nitrogen content.ResultsTDF structure and diversity attributes were consistently higher on limestone TDF, while canopy height was higher on phyllite TDF. By contrast, temporal variation in TDF attributes, demographic rates (recruitment, growth, and mortality) and attribute residence times were indistinguishable between substrates. We also found a strong differentiation of species composition between substrates, yet substrates did not influence the temporal behavior of composition.DiscussionOur results suggest that limestone is a more favorable environment for TDF development (soil-effects mechanism), but the flora of the forest on phyllite seems to be better adapted to cope with harsher environmental conditions (composition-effects mechanism), which counterbalances the environmental advantage of limestone, ultimately resulting in spatially homogeneous TDF dynamics at the landscape level. Future studies should examine the role of functional attributes on old-growth TDF dynamics, as this will allow a better understanding of the impacts of extreme climatic events on forest attributes and their dynamics, as well as to foresee potential ecosystem state shifts and tipping points.

Fabaceae is a monophyletic family comprising more than 22 000 species worldwide. In Mexico, it is the second-most species rich family, inhabiting multiple ecosystems with a high diversity of life forms. Our aims were to (1) identify areas of endemism (AEs) by using endemicity analysis at different cell sizes (1°, 0.5° and 0.25°) for 488 Mexican legumes, (2) describe the climatic and topographic heterogeneity of AE, (3) determine whether there is a positive relationship between AEs and climatic and topographic heterogeneity and (4) examine the conservation status of legumes that defined the AEs. We found six general AEs, supported by 63 species, with endemicity index values ranging from 2.05 to 6.86. These general areas had zones where biogeographical provinces intersected and exhibited a higher environmental heterogeneity. Areas detected at 1° showed a positive relationship between endemicity index and environmental heterogeneity. Legume species currently lack legal protection in Mexico, 69% of the endemic species do not have an evaluation category according to the International Union for Conservation of Nature, and eight species are in some risk category. Future conservation efforts should focus on protecting endemism areas from accomplishing representative natural protected areas.