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AIMS: We tested whether plant species niche breadths decrease with increasing species richness due to competition on temperature, precipitation and disturbance gradients. We assumed that niche optima, niche breadth and niche volume are related to plant functional traits, indicating competitive ability and adaptation to environmental stress. Finally, we wanted to identify habitats dominated by species with small niche breadths most prone to environmental change. LOCATION: Southern slopes of Mount Kilimanjaro, Tanzania. METHODS: We calculated species distribution models for 1,492 plant species based on the presence/absence data on 969 plots using generalized linear models. We derived niche breadths, optima and volumes and investigated their relationship with plant functional traits with principal component regression to account for high trait correlations. RESULTS: Niche breadths of individual species increased with elevation, precipitation and disturbance. Averaged community elevation niche breadth and niche volume decreased with increasing species richness. Plant functional traits explained about 40% and 50% of the variation in niche optima and breadths, respectively. Size and growth traits were significant predictors of niche breadths on all gradients, whereas traits indicating reproductive strategy were not significant on the precipitation gradient. MAIN CONCLUSIONS: Our results support the notion of decreasing niche breadth with increasing temperature, possibly a result of competition due to increased diversification rates and hence species richness. However, the complexity of the niche breadth–species richness patterns on the other gradients shows that additional covariables shape species niche breadths apart from competition. Plant species with narrow niche breadths dominated natural savannas around Mount Kilimanjaro, indicating strong sensitivity to the ongoing conversion of savanna to fields and grasslands.