AFFINITY AMONG MOUNTAIN-RANGES IN MEGAMEXICO - A PHYTOGEOGRAPHICAL SCENARIO

The concept of Megamexico as a phytogeographic unit has been introduce d by Rzedowski (1991) on the basis of distribution patterns of genera. Until now precise information on the resemblance between plant commun ities in the proposed Megamexico was scarce. This phytosociological st udy documents the affinities between plant communities within Megamexi co, based on studies from mountain ranges in Mexico (Sierra Chichinaut zin, Sierra Nevada) and Guatemalan mountain ranges (Sierra de los Cuch umatanes, Cadena Volcanica). Data, collected in the style of Braun-Bla nquet during extensive fieldwork, were pooled into a single data set, and analyzed by using (1) TWINSPAN, for plant community classification , (2) ordination (DCA), (3) alpha log series biodiversity-index to mea sure intergroup diversity, and (4) information on the altitudinal dist ribution of the vegetation belts. Classification showed that four comm unities were common to the two groups of mountain ranges, namely, alpi ne bunchgrassland, pine forest, fir forest and mixed forest. Along the altitudinal gradient Mexican communities are distributed 200 m higher than their Guatemalan equivalents. This appears to be an illustration of the 'Massenerhebungseffekt'. DCA showed that the first axis repres ents a set of minor differences of closely related ecological factors (e.g., temperature, precipitation) and the second a humidity gradient. The biodiversity index showed that the Mexican mixed forest was signi ficantly more diverse than the Guatemalan mixed forest. From the prese nt results, ecological conditions among mountain ranges in Megamexico differed significantly. To conclude, phytogeographical units in Megame xico can better be defined on basis of both historical and ecological characteristics of the communities.