Pollen and the allergies it triggers are often signs of a new season. But pollen can also be a sign of the past. By identifying plant species from fossilized pollen samples, scientists can reconstruct historical changes in vegetation and climate.
Pollen doesn’t always fossilize well, but new research is finding a better way to search for preserved pollen from the past—and finding it in a wider range of places than typically studied.
“There’s been no large-scale statistical analysis of what types of soil properties are most related to having pollen,” said Matthew Ricker, a soil scientist at North Carolina State University and lead author on the new study. “This study is one of the first that actually uses a lot of heavy statistics, with hundreds of [soil layers], to really look at the statistically significant variables that result in pollen preservation.”
Beyond the Bog
Traditionally, scientists have turned to bogs to gather pollen samples. These areas, with their low-oxygen environments, are considered the best preservers of pollen. However, this approach limits analysis in drier regions that don’t have many natural bogs or lakes. In lieu of these natural bodies of standing water, scientists have turned to floodplains. But these areas can be hit or miss—typically, only 60% of samples have preserved pollen—so most studies haven’t focused on them.
“You can have stratified layers, one on top of another, and one can have 50,000 grains of pollen per gram, and the one below it can have zero,” Ricker said. “If you’re not cognizant of the processes that lead to preservation of pollen in these systems…you’re going to spend a lot of time and energy on [samples] with a very low probability of actually containing any pollen information.”
Ricker and his collaborators gathered samples of soil layers, known as horizons, from over 100 different floodplain areas around Connecticut, Rhode Island, and Massachusetts. Studying the chemical and physiological makeup of the samples, they were able to present a method of predicting which soils had the best likelihood of harboring preserved pollen.
The researchers looked specifically at five properties of soil that are known to affect pollen preservation: concentration of organic matter, depth of sample, silt content, carbon-to-nitrogen ratio, and total iron content.
Their results were published in the Soil Science Society of America Journal.
“I think it’s a very significant step forward in terms of being able to quantify time effects on riparian landscapes,” said Patrick Drohan, a soil scientist at Pennsylvania State University who was not involved with the new study. “It really extends that multiproxy toolbox and gives us now a pretty sweet little technique that’s applicable to looking at species shifts, which is a real question a lot of us have now about how species are responding to humans in their evolution.”
Hydric Soils
“The best soils to look for are the soils in the floodplain environment that are essentially what we call hydric soils or wetland soils,” Ricker said. “These wet areas or landscapes within the floodplain environment are better because they have less oxygen to break down the pollen.”
Researchers also found soils with more silts or clays had greater pollen preservation. These finer soils have less space between individual grains, reducing the amount of oxygen that can infiltrate the soil and degrade the pollen.
“[The study] is interesting. It’s sort of a statistical way to show what palynologists [who study plant pollen and spores] already knew in terms of what kind of sediments are likely to have pollen preserved in them,” said Debra Willard, a palynologist with the U.S. Geological Survey who was not involved with the study. “It is pretty similar to what we’ve seen just empirically.”
For those without the instrument capabilities in-house, processing soils can cost $60 per sample and take a week to complete, so understanding what soils are best for pollen preservation is important for saving time and money. Increasing the availability of preserved pollen samples for study will also help researchers better understand the changes in vegetation and climate in new regions. Pollen samples from floodplains have been shown to keep records back 10,000 years.
“Rivers and floodplains…are present across much of the landscape, and they have the potential to fill in data gaps in the Holocene record,” Willard said. “Work such as this will be important to generate long-term records of wetland environments and their susceptibility to changing land use and climate.”
—Mara Johnson-Groh (marakjg@gmail.com ), Science Writer