References for Pete

A Short Investigation of a (Very) Long Paleobotanic History

Andrea Baumgartel, Emily Burgess, Isaac Ferber

Paleobotany is the study of historic plant communities and species using information gathered from preserved geological records of past environments. It’s important because it tells us about what our environment used to be like, like how plant morphology changes over time, how old a species is, and how ecological communities undergo change over time. Paleobotany can also give us clues about what we can expect as our climate changes in the current day by telling us what the ecological communities of an area were like in the past, and what the environment was like back then. It’s common for communities of plants, including many of Iowa’s own, to have cyclical patterns of population and distribution decline, and recovery in response to similar shifts in climate, among other factors. Paleobotanic studies can be done using fossilized plants or pieces of plants that have escaped decomposition in peat bogs, and still remain relatively intact after thousands of years.

            Several studies have been completed in Iowa using paleobotany to reconstruct past environments. In 1996 Baker et al. did an analysis of the floodplain alluvium (material deposited by a river over years of flooding) of Roberts Creek in northeastern Iowa to determine how the plant community surrounding the floodplain had changed over the last 11,000 years. They found that there was a succession of boreal forest, followed by deciduous forest, prairie, and oak savanna, leading in the modern agricultural area. This is a good example of how paleobotany can inform us on what the environments we live in used to look like, in terms of plants at least.

            During the construction of the Robert’s Theatre on Grinnell College Campus, a layer of peat was uncovered during excavation, and a preliminary analysis of what was preserved in the peat was done by Ben Graham in 1962, who found some wood fragments and pollen. The rest he left for us, and in this year’s Evolution of the Iowa Flora class we set out to see what else we could find and identify.

Our investigation didn’t involve the actual unearthing (or rather, un-peating) of the identity of the unknown materials in the peat (which we’ve affectionately named “Pete”) itself, but rather the crafting and imaging of already-known specimens (dubbed “reference materials”) that could potentially be, or be related to, said unknowns. Once the ancient unknown organic matter (e.g., small-to-microscopic wood bits, leaves, stems, pollen, etc.) is extracted from the peat, the reference materials can be used comparatively in order to help determine the identification of those unknowns.

While there are certainly a fair amount of available reference images existing online, it’s always advantageous to make one’s own reference materials, because 1) it helps to have consistent imaging methods and settings (i.e., the prep & imaging processes will be the same for both references and unknowns) and 2) you can never 100% guarantee that the reference identifications you find online are actually what they say they are. Also, we’ve collected reference materials from the actual location (Grinnell Iowa) that had the potential to be found in the peat, and thus the reference materials we find will be more directly relevant to this location’s past.

The reference materials in question are wood samples from the following tree genera, all located throughout the central most parts of Grinnell College’s campus: Alnus glutinosa (alder), Betula nigra (birch), Carpinus caroliniana (hornbeam), Fraxinus americana (ash), Pinus strobus (pine), Populus deltoides (cottonwood), and Tsuga caroliniana  (hemlock). Our imaging began with pollen samples collected earlier by professor Vince Eckhart, which ended up not being used, as numerous attempts to detect pollen within the actual peat itself were unsuccessful. However, we remain optimistic that pollen evidence will be found from the peat sample in the future, as the seminal paper by Ben Graham included notes verifying that the peat did in fact contain pollen, despite our findings.

With no ancient pollen to identify, our focus moved from creating pollen references (of which there is a fantastic online database anyway), and we shifted our goals to collecting wood from around campus, processing them in the same way as the peat fragments, and imaging it so the rest of the class could use our slides to identify their mystery wood. The first task was to search campus to collect samples from the modern equivalent of trees that might be in the peat. After a day of collecting, we broke down our samples, which were branch fragments, into small bits that fit into small vials. These were soaked in 5% potassium hydroxide (KOH) to break down the wood in the same way the peat was processed. After a week in the solution at a constant 45°C, we were ready to begin to make our slides for imaging. Just putting chunks of wood on a glass plate wasn’t going to cut it, so we needed to dye and divide the samples into pieces as small as possible, ideally to expose individual cells.

We didn’t find definitive literature on how to process specifically macerated wood samples into something microscope friendly, so we spent a day working to create our own method. The system that yielded the best results involved two slide plates, one for staining the wood cells with safranin (aka basic red 2), and the other for cleaning excess stain and cutting the wood into the smallest pieces possible in deionized (distilled) water. The newly stained wood was cleaned in deionized water, and after waiting for the water to evaporate, the sample was sealed in melted gelatin under a cover slide. Our best samples were permanently enclosed with clear nail polish. We imaged cells we identified as containing visible, important structures, specifically searching for vessel elements in angiosperms, along with helical structures, and border pits in gymnosperms (Figures 1 & 2). These parts were the most likely to be identifiable in the ancient wood collected from the other groups.

Fig 1. Modern Alnus glutinosa sample with visible vessel elements and perforation plate.

Fig 2. Modern Carpinus caroliniana sample with visible helical structures within a vessel element.

References

Graham, B. F. Jr. 1962. A Post-Kansan Peat at Grinnell, Iowa: A Preliminary Report. Proceedings of the Iowa Academy of Science 69: 39-44.


Baker, R.G., E. A. Bettis III; D. P. Schwert; D. G. Horton; C. A. Chumbley; L. A. Gonzalez; M. K. Reagan. 1996. Holocene Paleoenvironments of Northeast Iowa. Ecological Monographs 66: 203-234.