Contrasting relationships between clade age and temperature along latitudinal versus elevational gradients for woody angiosperms in forests of South America

Question: Is clade age positively correlated with minimum temperature in angiosperm woody assemblages along elevational thermal gradients, as predicted by the phylogenetic niche conservatism hypothesis and observed along latitudinal gradients? Location: South America. Methods: I assembled one latitudinal gradient and one elevational gradient of angiosperm woody plant assemblages in forest plots, both of which started at equatorial humid forests at low elevations. The latitudinal gradient comprised 37 plots and the elevational gradient included 34 forest plots. Each plot is 0.1 ha. Correlation analysis was used to determine relationships among taxonomic richness, mean clade age (measured as mean family age of each forest plot), latitude, elevation and minimum temperature (measured as mean temperature of the coldest month). Results: Woody plant species richness strongly decreases with increasing latitude and elevation on the two gradients. Mean clade age is positively correlated with minimum temperature for the latitudinal gradient but is negatively correlated with minimum temperature for the elevational gradient. Conclusion: The result of this study based on the latitudinal gradient supports the prediction of the phylogenetic niche conservatism hypothesis for the relationship between clade age and temperature, whereas the result based on the elevational gradient is contrary to the prediction of this hypothesis. The contrasting patterns found in the present study for the clade age-minimum temperature relationship along the latitudinal vs elevational gradients suggest that more studies are needed in order to determine how general is the positive relationship between clade age and temperature predicted by the PNC along both latitudinal and elevational gradients across the globe.