Microhabitat partitioning promotes plant diversity in a tropical montane forest

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Aim To assess the degree of fine-scale environmental determinism in fern community composition in a diverse, mid-montane tropical forest, and to test whether species local microhabitat distributions are linked to their elevational ranges regionally. Location Tunquini biological station, eastern Bolivian Andes. Methods We recorded fern species composition and environmental descriptors in two montane forest transects on opposite valley slopes at c. 1650 m elevation. Redundancy analysis and variation partitioning were run to examine how much floristic variation was explained by spatial and environmental (soil, forest structural, topoclimatic) variables. We then linked species local environmental distributions, as evidenced in these analyses, to their elevational ranges in Bolivia using herbarium data. Results There was a striking floristic difference between the transects, attributable to differences in exposure, temperature and humidity. Environmental and floristic variation were also strongly linked within transects, with soil variation being most important. The overlap between spatially and environmentally explained variation was high, but there was congruence in the main environmental predictors selected in both transects, suggesting considerable spatial structure arose from species responses to a patchy environment. Species local microclimatic distributions were significantly linked to their regional elevational ranges. Species associated with the drier north-easterly transect had average range mid-points and maxima significantly higher (by 211 m and 402 m, respectively) than those associated with the more humid south-westerly transect. Within transects, species associated with more acidic soils and warmer microsites had higher average range minima, mid-points or maxima (by > 200 m in all cases) than those associated with less acidic and cooler microsites. Main conclusions Strong environmental determinism of community composition was apparent in relation to both local topoclimatic and soil gradients. Moreover, our results suggest that fine-scale variation in environmental conditions enhances the mixing of lowland and highland floras in mid-montane forests, thereby contributing to mid-elevation diversity.