The effect of elevation on species richness in tropical forests depends on the considered lifeform: results from an East African mountain forest

Elevation gradients in tropical forests have been studied but the analysis of patterns displayed by species richness and elevation have received little attention. We examined whether the effect of elevation on species richness varies according to forest lifeforms and the main plant families in the Kahuzi-Biega National Park, within the Albertine Rift. We established 20 1-ha plots from 810 to 2760 m asl. Inside each plot, species inventories were carried out within three nested sub-plots: the tree lifeform (i.e. species with a dbh ≥ 10 cm), the shrub lifeform (dbh < 10 cm) and the herbaceous lifeform. For trees and shrubs (woody lifeforms) abundance data (i.e. number of individuals per species) were taken into account whereas the herbaceous lifeform was surveyed using presence–absence data. We plotted species counts vs elevation for each of the ten richest families per forest lifeform and resorted to Poisson regression models to assess the statistical meanings of the displayed results. Hurdle models (truncated Poisson regression) were used to account for overdispersion in the data. For woody lifeforms, we observed a monotonic decrease of species richness, while species richness appeared to be increasing with elevation for the herbaceous lifeform. Woody lifeforms displayed various vegetation patterns according to the considered families, therefore, contrasting with the general pattern observed in the herbaceous lifeform. These findings suggest the existence of specific eco-physiological properties pertaining to each forest lifeform and the existence of family-specific elevation patterns of species richness.