Altitudinal gradients in tropical forest composition, structure, and diversity in the Sierra de Manantalan

1. Data on the composition, structure and diversity of plant communities were gathered along a 1000-m altitudinal transect from tropical seasonal dry forest to cloud forest on calcareous Cerro Grande in Jalisco, Mexico. 2. A total of 470 species, 292 genera and 103 families of vascular plants occurred in 43 samples of 0.1 ha, stratified at 100-m elevational intervals between 1500 and 2500 m a.s.l. There were 97 tree species, 76 shrubs, 70 vines, 181 terrestrial herbs, 39 epiphytes, 3 hemiparasites, 3 succulent rosette shrubs and 1 saprophyte. 3. Forest composition varied continuously with altitude, based on the Shipley & Keddy (1987) test, ordination via reciprocal averaging, and elevational trends in the Sorenson similarities of samples at adjacent altitudes, supporting the individualistic hypothesis of plant community organization. 4. Understorey herbs, shrubs and vines showed the greatest decline in species number with increasing altitude. This pattern is hypothesized to result from the more open, more frequently disturbed, and more completely deciduous canopies at lower, drier elevations. The proportion of evergreen woody plants was greater at higher altitudes, reflecting less seasonal aridity and greater soil leaching. The proportion of endozoochorous species increased with altitude, while the proportion of pterochorous and ectozoochorous species decreased, reflecting trends in the hypothesized efficacy of these mechanisms of seed dispersal. 5. Total basal area of woody plants > 2.5 cm d.b.h. and basal area per tree both increased roughly fourfold between 1500 and 2500 m. 6. Species richness decreased sharply with altitude, due mainly to decreases in terrestrial herbs, and (to a lesser extent) shrubs and vines. The average number of species per 0.1 ha declined from 134 at 1500 m to 43 at 2500 m. The numbers of species, genera and families per sample declined linearly with elevation. Species composition of samples within an altitudinal band showed greater horizontal turnover (beta diversity) at lower elevations, showing that low-elevation forests are not only locally more diverse, but spatially more patchy. Community composition varies roughly six times as rapidly with elevation as with the same distance horizontally. 7. A cascading series of effects of elevation on soil fertility, anti-herbivore defences, and the level of density-dependent mortality may account for the observed drop in diversity with elevation, and would be consistent with lower beta diversity and greater basal area at higher elevations.