ANGIOSPERMS HELPED PUT THE RAIN IN THE RAINFORESTS: THE IMPACT OF PLANT PHYSIOLOGICAL EVOLUTION ON TROPICAL BIODIVERSITY

The recycling of transpired water is well known to be an important source of rainfall, particularly in the tropics, and angiosperms have transpiration capacities higher than any other plants throughout evolutionary history. Thus, the evolution and rise to ecological dominance of flowering plants are proposed to have strongly altered climate. Transpiration capacity is closely correlated with leaf vein density, and the average vein density of angiosperm leaves is four times greater than that of all other plants, living or extinct. A rapid transition to high vein densities occurred separately in three or more flowering plant lineages about 100 million years ago. Climate modeling of the impact of this physiological revolution indicates that the tropics would be hotter, drier, and more seasonal in the absence of the angiosperms, and the overall area of tropical rainforest would decline substantially. Because angiosperm diversity is influenced by rainforest area and by precipitation abundance and evenness, the high diversity of angiosperms is partially a product of a positive feedback loop with the climate modifications initiated by the angiosperms themselves. Lineage diversifications among vertebrate and invertebrate animals and nonangiospermous plants in the wake of the angiosperm radiation may be tied to the unprecedented impact of angiosperms on climate.