Large-scale biogeographical patterns of species richness of trees

The spatial distributions of trees in North America are examined to determine which characteristics of the environment are most closely related to the species richness of different regions and can be accurately predicted from these North American patterns.

Abstract

Biologists have long recognized the striking geographical variability of species richness1. A primary goal of contemporary ecology is to identify the factors responsible for this variability2. We have examined the spatial distributions of trees in North America to determine which characteristics of the environment are most closely related to the species richness of different regions. Realized annual evapotranspiration, which is correlated with primary production and is therefore a measure of available energy, statistically explains 76% of the variation in species richness. Topography and proximity to the sea are significantly related to the residual variation, whereas seasonal climatic variability and glacial history are not. Tree richness in Great Britain and Ireland can be accurately predicted from these North American patterns. Our data are best explained by the hypothesis that contemporary available energy limits species richness3,4.