In order to better understand the evolutionary history of ecosystems and make more accurate climate models, a team of paleobotonists have compared structures made by plants 50 million years ago to those that are made today. This provides information about how vegetation density and tree cover have changed over the eons. Regan Dunn of the University of Washington in Seattle was lead author of the paper, which was published in Science.
Dunn’s research focused on phytoliths taken from various soil sites in Patagonia, Argentina. Phytoliths are glass-like structures created by plants after they have taken up silica from the soil. After the plant dies and begins to decay on the ground, the phytoliths remain behind and can become fossilized. It isn’t known why plants create phytoliths, but they are very useful in understanding what ancient ecosystems were like. Tree cover affects the amount of precipitation that reaches the ground and vegetation prevents erosion, so understanding the plant density will help explain environmental factors that could have influenced life.
"Knowing an area's vegetation structure and the arrangement of leaves on the Earth's surface is key for understanding the terrestrial ecosystem. It's the context in which all land-based organisms live, but we didn't have a way to measure it until now," Dunn said in a press release.
The recent Patagonian phytolith samples were compared to some that were formed 49-11 million years ago, painting a picture of how those ecosystems have changed over time. Radiometric dating was used to verify the timeline. While previous studies of fossilized pollen grains and prints of leaves can be used to reveal which specific plant species were around at any given time, studying phytoliths can supplement those findings by showing what the ecosystem looked like in general.
"The significance of this work cannot be overstated," added co-author Caroline Strömberg, also from the University of Washington. "Vegetation structure links all aspects of modern ecosystems, from soil moisture to primary productivity to global climate. Using this method, we can finally quantify in detail how Earth's plant and animal communities have responded to climate change over millions of years, which is vital for forecasting how ecosystems will change under predicted future climate scenarios.”
The shape and size of the phytolith are determined by the plant and specific structure it was formed in, which can be influenced by other factors such as the amount of sunlight and precipitation it receives. This allows the phytoliths to provide a snapshot of the vegetation density and tree cover experienced by the plant during its life. The extent of the vegetation’s canopy is described as a leaf area index, and the phytoliths can help resolve the index throughout history.
"Leaf area index is a well-known variable for ecologists, climate scientists and modelers, but no one's ever been able to imagine how you could reconstruct tree coverage in the past—and now we can," concluded co-author Richard Madden from the University of Chicago. "We should be able to reconstruct leaf area index by using all kinds of fossil plant preservation, not just phytoliths. Once that is demonstrated, then the places in the world where we can reconstruct this will increase.”