Phylogenetic structure of liverwort assemblages along an elevational gradient in the tropical Andes: geographic patterns and climatic drivers

Liverworts are an ancient plant lineage that occurs worldwide with the highest species richness in cool and humid habitats such as tropical montane and temperate rain forests. It has been proposed that liverworts originated under such temperate climatic conditions and have later expanded into more tropical conditions, but how this is reflected in their phylogenetic diversity along the strong climatic gradients associated with elevation remains unexplored. We studied the phylogenetic diversity of regional liverwort floras along the elevational gradient in the tropical Andes, comparing indices that emphasize deeper and shallower phylogenetic relationships, and relating these to temperature- and precipitation-related variables, as well as to climatic extremes and seasonality. We found that whereas liverwort species richness peaks at around 2000 m a.s.l., richness-corrected phylogenetic diversity increases with elevation, and the standardized effect of size of phylogenetic diversity is highest at 2500-4000 m a.s.l. This is in accordance with an origin of liverworts under cool conditions, followed by more recent diversification in warmer climates at lower elevations. We further found temperature-related climatic parameters to be stronger predictors of phylogenetic diversity of liverworts than precipitation-related variables, and climatic extremes to have a stronger influence than climatic seasonality. We interpret these patterns as reflecting the physiological challenges of adapting to low temperatures as well as rare occurrences of extreme climatic events. All this reveals a strong signal of the evolutionary dynamics of this ancient plant lineage linked with its physiological adaptations to climatic conditions. The age of this group and its poikilohydric nature, i.e. its inability to regulate water loss, lead to patterns that contrast with those of vascular plants, allowing for discerning evolutionary generalities that are independent of physiology and lineage age.