Patterns of species richness and conservation in the Caatinga along elevational gradients in a semiarid ecosystem

• The number of plant species increases with altitude in the Brazilian semiarid.
• The mountains of the Brazilian semiarid region are refuges of native vegetation.
• Humans have altered the pattern of plant richness in the Caatinga.

Altitudinal gradients provide a useful model for studying plant community composition along environmental gradients. Altitudinal gradients also vary in their degree of human exploitation, as human activity is generally greater at lower elevations than at higher elevations. We examined the effect of altitude on plant community composition along an altitudinal gradient ranging from 400 to 600 m a.s.l. by sampling 75 plots in a Brazilian savanna steppe characterized by a diversity of plant community types. Plant species richness increased with elevation, and the lowest species richness was associated with lower elevation areas dominated by human disturbance. Our results suggest that increased human pressure at lower altitudes, together with enhanced microclimates and reduced access at higher altitudes, determine the plant species richness patterns in dry tropical forests.