An international team of scientists has discovered a way to determine the tree cover and density of trees, shrubs and bushes in locations over time based on clues in the cells of plant fossils preserved in rocks and soil. Tree density directly affects precipitation, erosion, animal behavior and a host of other factors in the natural world. Quantifying vegetation structure throughout time could shed light on how the Earth’s ecosystems changed over millions of years.
The findings are published Jan. 16 in the journal Science.
“It may now be possible to track historical change in vegetation structure in a quantitative way,” said study co-author Richard Madden, PhD, Research Associate in the lab of Callum Ross, PhD, in the Department of Organismal Biology and Anatomy at the University of Chicago. “If this can be reconstructed for times in the past, it may provide empirical tests and eventually improve our confidence in predicting change.”
The team, led by Regan Dunn, PhD, a paleontologist at the University of Washington’s Burke Museum of Natural History and Culture, focused its fieldwork on several sites in Patagonia, Argentina, which have some of the best-preserved fossils in the world and together represent 38 million years of ecosystem history (49-11 million years ago).
“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,” said Dunn.
Work by other scientists has shown that the cells found in a plant’s outermost layer, called the epidermis, change in size and shape depending on how much sun the plant is exposed to while its leaves develop. For example, the cells of a leaf that grow in deeper shade will be larger and curvier than the cells of leaves that develop in less covered areas.
Dunn and collaborators found that these cell patterns, indicating growth in shade or sun, similarly show up in some plant fossils. When a plant’s leaves fall to the ground and decompose, tiny silica particles inside the plants called phytoliths remain as part of the soil layer. The phytoliths were found to perfectly mimic the cell shapes and sizes that indicate whether or not the plant grew in a shady or open area.
The researchers checked if fossilized cells could tell a more complete story of vegetation structure by taking soil samples from sites in Costa Rica that varied from covered rainforests to grassy savannahs to woody shrub lands, along with photos looking directly up at the tree canopy (or lack thereof) at each site to note the total vegetation coverage.
Back in the lab, they extracted the phytoliths from each soil sample and measured them under the microscope. When compared with tree coverage estimated from the corresponding photos, Dunn and co-authors found that the curves and sizes of the cells directly related to the amount of shade in their environments. The researchers characterized the amount of shade as “leaf area index,” which is a standard way of measuring vegetation over a specific area.
Testing this relationship between leaf area index and plant cell structures in modern environments allowed the team to develop an equation that can be used to predict vegetation openness at any time in the past, provided there are preserved plant fossils.
“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,” Madden said. “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.”
When Dunn and co-authors applied their method to 40-million-year-old phytoliths from Patagonia, they found something surprising — habitats lost dense tree cover and opened up much earlier than previously thought based on other paleobotanic studies. This is significant because the decline in vegetation cover occurred during the same period as cooling ocean temperatures and the evolution of animals with the type of teeth that feed in open, dusty habitats.
The research team plans to test the relationship between vegetation coverage and plant cell structure in other regions around the world. They also hope to find other types of plant fossils that hold the same information at the cellular level as do phytoliths.
Additional co-authors include Caroline Strómberg at the University of Washington, Matthew Kohn of Boise State University and Alfredo Carlini of Universidad Nacional de La Plata in Argentina.
This story was adapted from a release from the University of Washington.