August 3, 2008
Plants As a Force of Nature
By McElwain, Jennifer
PALEOBIOLOGY Plants as a Force of Nature THE EMERALD PLANET: How Plants Changed Earth's History. David Beerling. xvi + 288 pp. Oxford University Press, 2007. $30. Plants, according to paleoclimatologist David Beerling, have shaped the atmosphere (and thus the climate) of the Earth to an amazing extent. In The Emerald Planet, Beerling guides readers through geological time from the earliest record of life through the greening of the planet to the present day, describing a series of major evolutionary events in the plant kingdom that have influenced global environmental conditions over the eons. His account interweaves hard scientific facts with rich anecdotes about the scientists who have pieced together that evolutionary record over time. The result is a book that is fascinating and exciting to read. (In the interest of full disclosure, I should note that when I was a postdoctoral researcher at the University of Sheffield, where Beerling is a professor, we coauthored several papers.)After briefly introducing the book's major themes, Beerling delves straight into early plant evolution, demonstrating that the environment was a critically important influence and that plant evolution likewise shaped the environment. His explanation of why leaves did not evolve until some 40 million years after vascular land plants first originated is based on discoveries in molecular developmental genetics (such as the knotted homeobox gene), modeling of plant physiology and examination of a wealth of evidence from fossil plants.
He makes a compelling case that environmental conditions held back leaf evolution (a position he initially staked out in a letter in the March 15, 2001, issue of Nature): An atmosphere rich in carbon dioxide, like the one in place 450 million years ago, would have interfered with the development of stomata (microscopic pores) on the surface of leaves, he says, and without a fairly high density of stomata for ventilation, leaves would have overheated. Also, the Earth's transformation from a largely plant-free planet to a fully vegetated one coincided with the 90 percent drop in carbon dioxide levels that occurred between 450 and 350 million years ago. Since that time, vegetation has served as a "thermostat," regulating climate by controlling atmospheric carbon dioxide levels through its influence on the global carbon cycle.
Beerling portrays the Carboniferous period (from 350 to 290 million years ago) as a "lost world of giants." In that time of exceptionally high levels of atmospheric oxygen, gargantuan dragonflies with wingspans of two-thirds of a meter took to the skies among 40-meter-tall pole-like trees (lepidodendrons), whose nearest living relatives today, the modern lycopsids, reach maximum heights of less than half a meter.
Vast tropical and subtropical swamplands covered much of the area now known as North America and Europe. Beerling characterizes these watersodden forests of the Carboniferous as "the lungs of the Earth on a geological timescale," because they exhaled oxygen as a by- product of photosynthesis. Simultaneously, they lowered atmospheric levels of carbon dioxide by trapping vast quantities of plant biomass in the swamps-biomass that eventually became the peat and coal that now fuels much of the world. Thus immense amounts of carbon that took plants millions of years to store are currently being released into the atmosphere as carbon dioxide, warming our planet in mere decades.
My only criticism of Beerling's coverage of the Carboniferous is a minor one: He gives greater attention to the effect of varying levels of atmospheric oxygen on the physiology of animals than he does to their effect on plants.
Next the book turns to ozone. Beerling briefly recounts its discovery, explains how and where it forms, and provides an entertaining account of how scientists of the British Antarctic Survey documented the existence of an Antarctic ozone hole in the early 1980s. Then he considers the various lines of evidence that support a provocative hypothesis: that mass ozone depletion occurred 248 million years ago, as the Permian period drew to a close. Such an "ancient ozone catastrophe" would have greatly increased the amount of harmful ultraviolet-B radiation reaching the Earth's surface. The fossil record of microscopic spores shows elevated rates of mutated forms at that time, making such radiation a plausible cause of the end-Permian extinction.
As the Triassic ended and the Jurassic began, around 200 million years ago, another mass extinction took place. Beerling examines its causes and consequences, guiding the reader from cliffs off the Yorkshire coast of England to a fjord in eastern Greenland as he unravels the rich fossil history that marks this event. He concludes that the release of huge quantities of greenhouse gases into the atmosphere by massive volcanism triggered natural global warming, which then intensified when heated surface waters eventually circulated to the seafloor, thawing and releasing methane that had been frozen into sediments there. As a result, most plant species- 90 percent of them-went extinct.
Beerling uses this fact as a grim reminder of what may become of Earth's current biodiversity in a world warmed by anthropogenic increases in greenhouse gases. Some of the ideas presented here are a little oversimplified, but readers who want more detail can consult the book's comprehensive bibliography.
Next Beerling discusses the fact that, for more than 80 percent of the last 500 million years, forests were present well within both the Arctic and Antarctic polar circles. The unusual ancient environment at the poles included hot summers, mild winters and up to six months of perpetual darkness. Deciduous trees predominated, but evergreens were also present. The two types of plants would have needed to employ different strategies to survive; to investigate those, scientists have used a variety of approaches: creating computer models of plant physiology (virtual forests), reading the growth rings of fossil wood, and attempting to cultivate trees under conditions that simulate the past polar environment. More research on the ecology of ancient polar forests is urgently needed, Beerling says, to help us understand how polar vegetation may change in response to global warming.
To provide another dramatic example of what a warmer world may look like, Beerling describes a series of localities in southern England where fossil plants have been found that are about 45 million years old. These fossils indicate that subtropical to tropical vegetation and mangrove swamps once fringed the southern reaches of the British Isles. The implication is that whole vegetation biomes may migrate northward as global climate warms. Beerling explains that the "hothouse" conditions tropical vegetation would have required cannot be attributed solely to higher levels of the greenhouse gas carbon dioxide; nitrous oxide and methane may also have contributed, and both of these can be modulated by feedbacks between climate and vegetation. Here Beerling once again reinforces his central thesis that plants play an undeniably important role in shaping the trajectory of global climate.
Continuing his chronological account, Beerling arrives at what he calls "nature's green Revolution": the evolution 32 to 12.5 million years ago of plants with a novel form of photosynthesis, called C^sub 4^ because it converts carbon dioxide into organic acids with a backbone of four carbons rather than the usual three. Subsequently there was a dramatic expansion of vegetation, dominated by C^sub 4^ plants, including certain grasses. Beerling ponders the influence that climate change, wildfires and low levels of atmospheric carbon dioxide may have had in spurring the evolution of the C^sub 4^ photosynthetic pathway. Understanding how C^sub 3^ plants evolved C^sub 4^ capacities could be useful in coming up with ways to meet rising world food demand, he says, noting that the creation of genetically modified rice capable of C^sub 4^ photosynthesis (rice is normally a C^sub 3^ plant) may be "the 'next frontier' in crop science."
The book really should have ended here, because its final chapter is a little preachy and rather repetitive. For the most part, however, The Emerald Planet is beautifully written, fresh and provocative. Beerling is a good teacher, using imaginative analogies to explain complex material that might otherwise seem dry. His book will appeal to amateurs and professionals alike-everyone interested in how plants have changed and will continue to change our world.
These mutated fossil plant spores date to 251 million years ago. Individual spores are stuck together in unseparated clumps, which is unusual. The scale bar at top left equals 50 micrometers. From The Emerald Planet.
Jennifer C. McElwain is a lecturer in plant paleobiology and paleoecology at University College Dublin in Ireland and an adjunct professor in the Department of Earth and Planetary Sciences at Northwestern University in Evanston, Illinois. She is coauthor with Kathy Willis of The Evolution of Plants (Oxford University Press, 2002).
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