Energy, water, and species richness.
(a)	Species richness of North American trees increases most predictably with actual evapotranspiration, which is determined by the amount of solar radiation, temperature, and water availability.	(b)	Vertebrate species richness in North America increases most predictably with potential evapotranspiration, a measure of energy availability but not water availability, and is highest in regions of high solar radiation and temperature.