Most morphological and floristic studies of lycophytes and monilophytes have been conducted in regions with humid and sub-humid climates, leaving dryer regions of the earth virtually unexplored. Arid and semi-arid zones make up about 50% of Mexico's land area; hence, the objectives of this study were (1) to undertake an inventory of fern and lycophyte species present in a region of central Mexico covered mainly by xerophilous vegetation (Valle del Mezquital, Hidalgo); and (2) to analyze patterns of morphological variation of the species found with respect to environmental conditions. Monilophytes and lycophytes were collected in all vegetation types identified within the study area along an elevation gradient between 1200 and 2800 m. For each species, growth substrate, life form and foliar strategy were recorded, and ten morphological characteristics were evaluated in order to determine their variation and correlation with elevation, rainfall, temperature and vegetation type. The data obtained were analyzed using univariate and multivariate techniques. Ten families, 25 genera and 72 species of monilophytes; and one family, one genus and eight species of lycophytes were identified. Quercus forest had the highest species richness of lycophytes and monilophytes followed by arid tropical scrub. The most common life forms by substrate were epipetric and terrestrial, the latter represented by chamaephytic, cryptophytic and hemicryptophytic life forms. Most species showed a xeromorphic foliar strategy, as a consequence of prevailing dry conditions. An analysis of morphological leaf characteristics of the species revealed patterns of variation and covariation, primarily related to vegetation type, which are linked to differences in temperature and moisture conditions along the studied elevation gradient. Monilophytes and lycophytes, like other groups of plants, have developed a set of morphological adaptations, which may function together or in various subsets and at different degrees of efficiency to enable plant species to cope with the environmental conditions in their habitats.
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Vol. 105 • No. 3