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.