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Forest regeneration after human-induced or natural disturbances results in a sequential replacement of species, starting with colonizing of pioneer species that modifies the physical conditions of the environment, which later allows the establishmentof mid- and late-successional species. Whether the forest succession implies a reduction of population size of tree speciesmight be expected a less genetic diversity in populations inhabiting oldest succession stages. We aimed to study whether genetic diversity of Macuilillo, Oreopanax xalapensis (a understory tree), decreases in advanced age-classes and in late successional of montane cloud forest at Huitepec Ecological Reserve (Chiapas, Mexico). We counted individuals from three age classes: seedlings, saplings, and young reproductive individuals in nine plots along three successional stages (early-, middle-, and late-successional forests). Seven enzyme systems (SOD, AP, FUM, G6PDH, GOT, GPI, and G3PDH) were used to obtain common genetic diversity; Ф-statistics and a Bayesian model were used to infer population structure. Atthe early-successional stage, O. xalapensishad a bigger population size and significantly high numbers of seedlings. In general, among the successional stages, the genetic diversity of O. xalapensis was moderate to high (He=0.25-0.46); the saplings presented the highest expected genetic diversity (He). Populations were genetically structured by age-classes but not by successional stages. The successional stages affected the demographic dynamics with a mild impact on the genetic structure across age-classes of O. xalapensis. The scenic and recreational conservation strategies implemented in this forest have effectively allowed for population development of O. xalapensis and provide some explanation of genetic diversity at early-succession stage. Inglés

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