Evolution of the angiosperms and hydrologic cycle

Terrestrial vegetation especially forest plays a critical role in the terrestrial hydrologic cycle. Plant transpiration transfers water from soil to atmosphere and the recycling of water through transpiration is one of the most important sources of terrestrial precipitation. However, the transpiration capacity of different groups of plants in the geological past would have been distinct, mainly because of differences in the vein density of their leaves, and in the structure of their water-conducting tissues. Angiosperms, which are dominant in most modern terrestrial ecosystems, have transpiration rate much higher than other extant and extinct plants. Fossil evidence shows that the earliest forests appeared in the Middle Devonian. During the Late Carboniferous and Permian, wetlands forests, known as Paleozoic tropical rain forests and dominated mainly by arborescent lycopsids, tree ferns, seed ferns, Cordaitales and Gigantopteridales, were well developed in large areas of the palaeotropics. During the Triassic, Jurassic and Early Cretaceous, the Earth's forests were dominated mainly by ferns, conifers, ginkgophytes, cycads and Bennettitales. The earliest angiosperm fossils are from the early Early Cretaceous, but angiosperms had not become ecologically dominant until the Late Cretaceous. The origin and evolution of angiosperms involved many innovations in both reproductive and vegetative structures, and their rise to ecological dominance in the Late Cretaceous ecosystems has been considered to be the result of the competitive advantages of these evolutionary innovations. The key innovations of angiosperms that relate to transpiration capacity and water transport are the xylem vessels which transport water from roots to leaves, and the broad leaves with reticulate, open venation, freely ending veinlets and high vein density. The long, multicellular xylem vessels of angiosperms are thought to be evolved from the short, unicellular tracheids of non-angiosperms. However, comparative physiological studies show that the early diverging angiosperms bearing vessels with scalariform perforation plates do not have competitive advantages in water transport capacity compared to gymnosperms and ferns. The probable later evolution of vessels with simple perforation plates in angiosperms would have conferred a critical competitive advantage in water transport capacity. Leaf vein density is also critical for transpiration capacity, because the mesophyll tissue offers major resistance for water transport within the leaf, and more veins can shorten distance between water-conducting tissues and stomata, the sites of evaporation in the leaf. A previous study reveals that the average leaf vein density of angiosperms is about four times greater than that of other extant or extinct plants. However, similar to the situation of xylem vessels, fossil data show that the vein density of early angiosperm fossil leaves from the early Early Cretaceous is more or less similar to that of contemporary gymnosperms and ferns. Fossil evidence also indicates that the transpiration rate of angiosperms had increased significantly in midCretaceous and in Cretaceous-Paleocene transition respectively, as inferred from the increased vein densities of fossil angiosperm leaves in these two phases. The evolution of vessels with simple perforation plates as well as the significant increase in leaf vein density in angiosperms, greatly enhanced their hydraulic conductivity, transpiration rate, and photosynthetic capacity, which further triggered the radiation of angiosperms in the Late Cretaceous, and probably resulted in a more vigorous global and regional hydrologic cycle. Climate model simulations show that if angiosperms were replaced by non-angiosperms in modern vegetation, the tropics would become hotter, drier and more seasonal. On the other hand, the moisture conditions themselves are also the key climatic factor in determining the plant diversity, systematic components and leaf physiognomy of terrestrial vegetation. Thus the paleoclimatic variables related to hydrological cycle, including the precipitation and humidity, can be estimated based on angiosperm fossils. Previous paleoclimatic studies based on plant fossils suggest that during the globally warm Late Cretaceous, Paleocene and Eocene, Arctic regions had a temperate and humid climate and lush temperate forests. It is also suggested that a warm Arctic Ocean and well-developed temperate forests in Arctic regions resulted in a vigorous hydrological cycle and year-round high humidity, which further maintained a warmer climate in the regions.