The evolution of trees and seed plants 360 million to 380 million years ago dramatically altered the terrestrial landscape, providing new habitats where animals and other plants could flourish. But geologist Thomas Algeo of the University of Cincinnati thinks that trees might have played a more sinister role in the oceans—that of executioner. Algeo noticed that the rise of large land plants coincides with the late Devonian mass extinction, a devastating die-off during which at least 70 percent of Earth’s species went extinct. Marine organisms such as reef-dwelling corals suffered the most severe losses. Researchers have proposed that a severe global cooling might have been responsible. But Algeo suspects the deadly hand of “killer trees.”
Algeo’s idea is that as plants and trees grew larger, their roots plunged deeper into the ground, creating copious amounts of soil loaded with nutrients like phosphorus and nitrogen. Meanwhile, the spread of plants deeper onto land made for an even greater volume of soil around the planet. Much of that soil got washed into rivers and streams and ended up in the ocean, “where the nutrients liberated from the soil stimulated massive algal blooms,” Algeo says. “Excess nutrients are not inherently bad, but they almost always disrupt marine ecosystems, because most of those ecosystems are adapted to deal with low nutrient levels. When nutrients are suddenly washed in, only a few groups of organisms are capable of exploiting this bounty effectively, and their populations explode.” Meanwhile, other organisms suffer.
If such lethal overfertilization occurred during the Devonian, it would have left behind an obvious trace. When algae die and decay, they suck oxygen out of the water, which encourages the buildup of organic matter on the seafloor. Over millions of years, that material can turn into a type of sedimentary deposit called black shale, which is rich in organics. Vast deposits of black shale exist in late Devonian deposits but are rare in other periods. “Special conditions, such as land plant evolution, must have existed to facilitate their formation,” Algeo says.
“The evidence is mainly circumstantial,” he admits. To firm up the case, he and his colleagues are looking at organic deposits in the Appalachian basin to pin down exactly when large terrestrial plants first started dumping large amounts of debris into the oceans, and to prove that trees really were the villains in the great die-off of the Devonian era.