A team of scientists from Argentina and the United States has created a method that allows us to know the density of vegetation in the past through fossil records . At the moment, they have applied it in Patagonia to know the variations that this geographical region of South America suffered over 38 million years. The data they handle take into account the climatic changes of past times and the different shapes of the teeth of the herbivorous animals of the past.
Normally, biologists measure the density of vegetation through the leaf area index or LIA, which estimates the surface area occupied by plant leaves in relation to the area of land on which they settle. .
According to Regan Dunn, a paleobotanist at the University of Washington, leaves are the main photosynthetic organs on Earth, and photosynthesis determines the flow of energy and water. That is why the vegetation described according to the LAI takes into account many of the processes that occur on the planet, such as meteorological phenomena, the carbon cycle or erosion.
However, until now there was no system to analyze past vegetation and understand environmental changes , so these researchers from the American universities of Washington, Chicago and Boise State, and from the National University of La Plata (Argentina) assumed the challenge of measuring the leaf area index in the fossil record.
They first analyzed the current ecosystems of Costa Rica , where there is a great diversity of vegetation structures, from the dense jungle, which has a very high LAI, to savanna areas (low LAI). At the same time, they studied phytolith or fossilized plant remains in Argentine Patagonia , since when the leaves fall to the ground and decompose they release solid silica particles that can be preserved as fossils.
Phytoliths preserve the shape and size of the epidermis cells of the leaves, which provides a lot of information. For example, large, wavy cells are known to correspond to understory plant leaves that are in the shade.
The research carried out in Costa Rica served to confirm these observations and made it possible to establish a relationship between the fossil record and the leaf area index. Applied to the plant fossils of Patagonia, it was used to calculate the LAI at different times in the past between 49 and 12 million years and find very curious patterns of change.
The evolution that this work has revealed is related to the history of the terrestrial and marine climate that was already known . During the Eocene, 49 million years ago, the terrestrial climate was warmer and Patagonia was covered with dense forests that later opened up. 38 million years ago the area was very similar to the current one, with little vegetation, and in the following 20 million years it shows numerous fluctuations. Between 16 and 14 million years ago, Patagonia was once again a great forest that again opened in later times.
In sparsely vegetated areas there is more erosion and more wind-borne dust, which puts herbivores in contact with more abrasive minerals that wear down their teeth. Therefore, the researchers believe that when the leaf area index is lower, the number of herbivores with high crown teeth increases.
For its part, the sediments of the seabed are used to determine what the climate was like at a certain time . For example, when the sea temperature falls, less precipitation is recorded on land, and this causes a decrease in vegetation. In contrast, a warmer ocean causes the density of plants to rise.
The study authors believe that this method could be applied in many different contexts , on various time scales and in different geographies. It could even be used to build maps of the vegetation that existed on Earth in other times and thus understand the relationship between climate and vegetation and anticipate future climate changes.
