Latitudinal gradients in phylogenetic relatedness of angiosperm trees in North America
To test two predictions of the phylogenetic niche conservatism hypothesis for the latitudinal diversity gradient: (1) species in colder regions tend to be more phylogenetically related to each other (i.e. greater phylogenetic clustering) than those in warmer regions, and (2) clades are younger in colder regions.
Correlation analysis was used to relate richness, mean clade age and phylogenetic relatedness (measured as phylogenetic species variability and net relatedness index) of angiosperm trees in 1175 regional assemblages (each with 12100 km2) to latitude and minimum temperature. The analysis was conducted for North America north of Mexico as a whole as well as for each of the three longitudinal bands (eastern, central and western) of North America.
Species richness and mean clade age are negatively correlated with latitude and positively correlated with minimum temperature. Tree species in regional assemblages tend to be more phylogenetically related (clustered) in regions at higher latitudes with lower temperatures.
The results of this study support two of the major predictions of the phylogenetic niche conservatism hypothesis for the latitudinal diversity gradient: species tend to be more phylogenetically clustered and ages of clades tend to be younger in colder regions, compared with those in warmer regions.