An exceptional role for flowering plant physiology in the expansion of tropical rainforests and biodiversity

Abstract

Movement of water from soil to atmosphere by plant transpiration can feed precipitation, but is limited by the hydraulic capacities of plants, which have not been uniform through time. The flowering plants that dominate modern vegetation possess transpiration capacities that are dramatically higher than any other plants, living or extinct. Transpiration operates at the level of the leaf, however, and how the impact of this physiological revolution scales up to the landscape and larger environment remains unclear. Here, climate modelling demonstrates that angiosperms help ensure aseasonally high levels of precipitation in the modern tropics. Most strikingly, replacement of angiosperm with non-angiosperm vegetation would result in a hotter, drier and more seasonal Amazon basin, decreasing the overall area of ever-wet rainforest by 80 per cent. Thus, flowering plant ecological dominance has strongly altered climate and the global hydrological cycle. Because tropical biodiversity is closely tied to precipitation and rainforest area, angiosperm climate modification may have promoted diversification of the angiosperms themselves, as well as radiations of diverse vertebrate and invertebrate animal lineages and of epiphytic plants. Their exceptional potential for environmental modification may have contributed to divergent responses to similar climates and global perturbations, like mass extinctions, before and after angiosperm evolution.

Figure 1.

The differences in (a) mean gross primary productivity (µmol m⁻² s⁻¹), (b) annual evapotranspiration (mm yr⁻¹), (c) mean air temperature (°C) 2 m above the surface, and (d) annual precipitation (mm yr⁻¹) between simulations with present day vegetation and with vegetation lacking the high transpiration of angiosperms. Precipitation increases with angiosperms can be larger than local transpiration increases owing to changes induced in atmospheric convection and the redistribution of moisture by prevailing winds.

Figure 2.

The per cent differences in annual precipitation ((((P_+A−– P_−A)/P_+A) 100%), where P_+A and P_−A are simulated precipitation with and without angiosperms). Precipitation is higher with angiosperms over most forested regions. The proportional changes in dry regions are exaggerated because there is very little precipitation.

Figure 3.

(a) The differences in annual precipitation (mm yr⁻¹) and (b) September–October–November temperature (°C) for tropical South America between simulations with the present day angiosperm-dominated vegetation and with vegetation lacking the high transpiration of angiosperms.

Figure 4.

(a) Monthly mean precipitation and (b) temperature over the eastern part of the Amazon basin (figure 3, inset) from observations (solid black line) and in simulations with either present day vegetation (blue dotted line) or vegetation lacking the high transpiration of angiosperms (red dotted line). Shaded areas show one standard deviation. 100 mm month⁻¹ represents minimum precipitation for rainy season (Morley 2000).

Figure 5.

(a) Differences in number of rainy days per year between simulations with present day vegetation and with vegetation lacking the high transpiration of angiosperms. (b) Histogram of the area in tropical/subtropical South America (30° S–10° N) for each 20-day bin of the number of rainy days per year for simulations with present day vegetation (blue, back crosshatch) or with vegetation lacking the high transpiration of angiosperms (red, forward crosshatch). A wet day is defined as one with more than 3 mm of rain. Number of rainy days greater than 240 days (dashed black line) roughly coincides with ever-wet rainforest.