Species richness patterns at the transition from the Tropics to the Subtropics: Studying environmental and historic effects

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The long known notable decline of biodiversity from the equator to the poles has been investigated intensively since its discovery but the causes are still controversially discussed. Especially fascinating is the apparent mirroring of this temperature-related gradient along larger tropical mountains, where comparable vegetation belts and plant adaptations can be found. During the last decades, it has not only been documented that the latitudinal gradient is not directly reflected in this elevational gradient (here the correlation is not linear but shows a hump-shape, with a clear diversity maximum at mid-elevations), it became also clear that the diversity patterns do not vary significantly within the tropical belt as long as orographic and edaphic parameter are kept comparable. Until now, comparable data from the transition zone between the tropics and sub-tropics where the latitudinal gradient must become apparent is not available. One reason is that in most areas in the world, tropical mountain rainforests do not border to comparable forests but are isolated by extensive lowlands and arid belts. Here we propose to establish a standardized elevational transect of the fern diversity in eastern Mexico, Veracruz state, where the northern-most tropical lowland rainforest eventually blends into a montane pine forest. The advantages of this approach are the already well-known biodiversity of the area and the availability of compara-ble data from past and current projects of our work group. We also regard phylogenetic position of the studied taxa in order to disentangle the relative roles of different drivers of species richness patterns.