EVOLUTION OF FLOWERING PLANT PHYSIOLOGY TRIGGERED EXPANSION OF TROPICAL RAINFOREST

The veins that irrigate leaves during photosynthesis have been demonstrated to be strikingly more abundant in flowering plants than in any other vascular plant lineage. Angiosperm vein densities average 8 mm of vein per mm² of leaf area and can reach 25 mm mm², whereas such high densities are absent from all other plants, living or extinct (Boyce et al. 2009, Proc. Roy. Soc. B). Leaves of non-angiosperms have consistently averaged close to 2 mm mm² throughout 380 million years of evolution despite a complex history that has involved four or more independent origins of laminate leaves with many veins and dramatic changes in climate and atmospheric composition. The high leaf vein densities unique to the angiosperms were further demonstrated to enable unparalleled transpiration rates, extending previous work indicating a strong correlation between vein density and assimilation rates. Because vein density is directly measurable in fossils, these correlations provide new access to the physiology of extinct plants and how they may have impacted their environments. First, the high assimilation rates currently confined to the angiosperms among living plants are likely to have been unique throughout evolutionary history. Second, the transpiration-driven recycling of water that is important for bolstering precipitation in modern tropical rainforests would have been significantly less in a world before the angiosperms. Here we now show through climate modeling that the replacement of angiosperm with non-angiosperm vegetation would result in a hotter, drier, and more seasonal tropics, particularly in the Amazon basin where dry season length increases by 80 days over the eastern Amazon and overall area of everwet conditions decreases by a factor of five. Evolution of angiosperm physiology has uniquely facilitated spread of warm everwet forests and their enormous biodiversity, perhaps including their early Cenozoic expansion to extra-tropical latitudes. Divergent responses may be expected to general climate parameters and discrete environmental perturbations before and after evolution of angiosperm dominated ecosystems.