Plant fossils can be used to determine prehistoric tree density

Chuck Bednar for – Your Universe Online
Clues contained in the cells of plant fossils could be used to determine the density of trees and other forms of vegetation some 50 million years ago, according to new research published in this week’s edition of the peer-reviewed journal Science.
Since tree density directly affects precipitation, erosion, animal behavior and several other ecological factors, determining vegetation structure throughout the years could help scientists discover how the planet’s ecosystems have changed with time.
“Knowing an area’s vegetation structure and the arrangement of leaves on the Earth’s surface is key to understanding the terrestrial ecosystem,” said Regan Dunn, a paleontologist at the University of Washington’s Burke Museum of Natural History and Culture. “It’s the context in which all land-based organisms live, but we didn’t have a way to measure it until now.”
The research, which was funded by the National Science Foundation (NSF), involved fieldwork at multiple locations in Patagonia, home to some of the most well-preserved fossils on Earth. Paleontologists have long used radiometric dating in this area to determine their age, and this new research builds on those efforts.
“The new methodology provides a high-resolution lens for viewing the structure of ecosystems over the deep history of our planet,” explained Alan Tessier, acting director of the NSF Division of Environmental Biology. “This capability will advance the field of paleoecology and greatly improve our understanding of how future climate change will reshape ecosystems.”
Quantifying vegetation openness (and no, this has nothing to do with trees trying to pick up chicks at a bar)
While records of fossilized pollen and leaves have given scientists working in Patagonia and elsewhere some notion of what types of plants were alive at various periods throughout history, there had not previously been a way to precisely quantify vegetation openness, as opposed to closed or tree-covered habitats, the NSF explained.
“These researchers have developed a new method for reconstructing paleo-vegetation structure in open versus dense forests using plant biosilica, likely to be widely found in the fossil record,” says Chris Liu, program director in NSF’s Division of Earth Sciences.
Dunn noted that the technique gives scientists a tool to examine key intervals in history where they are unsure what happened to the structure of vegetation, including the period just after the mass extinction event that resulted in the extinction of the dinosaurs.
“Vegetation structure links all aspects of modern ecosystems, from soil moisture to primary productivity to global climate,” said Caroline Stromberg, co-author of the study and a curator of paleobotany at the Burke Museum. “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, vital for forecasting how ecosystems will change under predicted future climate scenarios.”
The key is in phytoliths
Previous research had demonstrated that the epidermis, the outermost layer of a plant, changes in size and shape based on the amount of sun exposure it receives during leaf development. A plant that grows in deeper shade will have larger, curved cells compared to those that develop in areas that have less coverage.
These cell patterns, the researchers explained, found that cell patterns which indicate growth in sunny or shaded conditions can also be found in some plant fossils. When a leaf falls and starts to decomose, tiny silica particles inside the plants known as phytoliths remain as part of the soil layer. These phytoliths can indicate if the plant grew in shade or sun.
They tested this hypothesis in modern Costa Rica, obtaining soil samples from sites there that varied from covered rainforests to open savannas to woody shrublands, and also taking photos from the tree canopies at each site to note total vegetation coverage.
Phytoliths from each soil sample were then extracted and measured using microscopes. When compared with estimated tree coverage based on the corresponding photos, the study authors discovered the sizes and curves of the cells related directly to the amount of shade or sun exposure they received while forming leaves.
They characterized the amount of shade received by plants as “leaf area index,” a standard way of measuring vegetation over a specific area, and tested the relationship between it and plant cell structures in modern environments. This allowed the researchers to develop an equation that can predict vegetation openness at any point in history, if preserved fossils are available.
“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,” said co-author Richard Madden of the University of Chicago.
“We should be able to reconstruct leaf area index by using all kinds of fossil plant preservation, not just phytoliths,” he added. “Once that is demonstrated, then the places in the world where we can reconstruct this will increase.”
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