Maggie Loery, Siri Bruhn, and Vishva Nalamalapu
Pop quiz! What is peat?… Not to worry, we didn’t know either until our professor brought a box of it to class, which his colleague had found in the basement of our college's science building. He explained that while peat may look like dirt, it’s actually organic material that has been poorly decomposed due to acidic or anaerobic conditions. It turns out that poor decomposition is good news for us, or for anyone else investigating life at the time the peat was deposited. Since our class, Evolution of the Iowa Flora, is all about understanding how plants in Iowa have changed throughout time, our professor explained that this peat could be a rich source for furthering this understanding.
The box of peat didn’t come with a whole lot of information, but our professor explained that Ben Graham, a former professor at our college, originally found the peat sixty years ago at a construction site on campus (Fig. 1). He published a preliminary report on the peat (Graham 1962) where he mentioned that the peat contained many wood fragments preserved in "cellulosic form," and identified some wood fragments as Tamarack (Larix larcina). He estimated the peat was deposited between 12,000 and 130,000 years ago, corresponding to the time between the early Wisconsin and Illinoian glacial stages.
Figure 1. Note in the box of peat
Once we selected a 24x16 cm chunk of peat to study, we fondly named it “Peter,” and gently began disaggregating it. We uncovered fragments of wood, as well as fragments of leaves and insects. Though we hoped to find pollen as Ben Graham did, we were sadly not able to. Thus, we split the investigation into three groups: Team Wood, Team Macrofossils, and Team Reference. While we, as Team Wood, tackled the identification of wood fragments, Team Macrofossils did the same for other plant fragments. Team Reference set out on a scavenger hunt to collect reference wood fragments from trees on campus. Our hope was if they visualized their reference samples with the same techniques as we used on our samples, we could compare the two and better identify the historic wood fragments.
We began by separating cells. Our professor explained that we could create a maceration fluid as described by Huang and Yeung (2015), which would separate the cells, allowing us to search for particular cell types and features that distinguish between trees. We had to macerate the samples for longer than expected as the components binding the cells were not adequately digested. And that was not the only complication we encountered. Preparing microscope slides involved carefully transferring small quantities of cells onto a block of gel on the slide, melting the gel, and then placing a coverslip on top (Fig. 2)… Easier said than done. The cells were often either not adequately digested or over-digested, and the slides often had bubbles that made the cells difficult to visualize. We should probably admit that we never did perfect this slide-preparation technique, but we did manage to make slides for seven of our samples that allowed us to view the features we were hoping to see.
Figure 2. Microscope slide preparation setup
Our identification relied on three cellular characteristics: bordered pits, helical thickenings, and the presence of vessel cells (Fig. 3). Bordered pits, which resemble donuts, but are far too small to eat, are abundant in gymnosperm tracheid cells. Helical thickenings are spiral ridges in tracheid and vessel cells. Angiosperms have small bordered pits compared to gymnosperms. Using the findings of Baker et al. on the composition of the flora of Iowa during the Holocene, we used the size of bordered pits to distinguish between three trees that grew in Iowa around the time the peat was likely deposited: larch, pine, and spruce (Baker et al. 1990). Spruce cells have bordered pits in a single row, pine cells have larger bordered pits in a single row, and larch cells have bordered pits in pairs. Vessel cells, which comprise long, water-conducting structures, are only present in angiosperms.
Figure 3. Our key to identify wood fragments
Keeping an eye out for these features, we visualized our microscope slides (Fig. 4). The wood fragments seemed to include larch, pine, spruce, and an angiosperm! We observed single rows of ~20 µm diameter bordered pits in the tracheids of three samples, leading us to believe these wood fragments were from spruce trees. Single rows of ~30 µm bordered pits of another sample led us to believe this wood fragment was from a pine tree. Pairs of bordered pits in tracheids of another sample indicated this wood fragment was from a larch tree (Fig. 5). We only found helical thickenings in one sample. They appeared to be on vessel cells and were dotted by bordered pits (~50 µm diameter), which indicated this wood fragment was from an angiosperm (Fig. 6).
Figure 4. Maggie and Siri visualizing microscope slides.
Although we originally intended to compare our images to Team Reference’s, many of their images showed structures we did not observe (i.e. bordered pit fields in vessel elements; Fig. 7). We were, however, able to see that the bordered pits on the pine tree reference (~50 µm diameter) were slightly larger than those on ours (~30 µm diameter), which could indicate mis-identification, or could simply represent an actual range of sizes.
Figure 7. Team Reference’s bordered pit field in a vessel element
Soon, we’ll have better information about the age of our peat too. In addition to our wood identification, we prepared and submitted three wood fragments to Beta Analytic for radiocarbon dating. We’ll let you know once we have a better idea of when these larch, pine, spruce, and angiosperm trees were inhabiting what is now Grinnell College!
So, in our two short weeks of analyzing “Peter’s” wood fragments, we found evidence that the Grinnell flora included larch, pine, spruce, and angiosperm trees when the peat was deposited. If you want to see what Iowa was like back then, drive a few hours up to northern Minnesota and imagine you’re instead in Iowa long ago. Compare the landscape you see to Iowa today, and consider the evolution from conifer woods to prairie to the acres on acres of corn and soybeans you see today. Studies like ours and that of Baker and colleagues allow us to understand how and when these changes took place. Our work is just the beginning; there is plenty of other work to be done, and there’s plenty of peat left to do it!
The box of peat didn’t come with a whole lot of information, but our professor explained that Ben Graham, a former professor at our college, originally found the peat sixty years ago at a construction site on campus (Fig. 1). He published a preliminary report on the peat (Graham 1962) where he mentioned that the peat contained many wood fragments preserved in "cellulosic form," and identified some wood fragments as Tamarack (Larix larcina). He estimated the peat was deposited between 12,000 and 130,000 years ago, corresponding to the time between the early Wisconsin and Illinoian glacial stages.
Figure 1. Note in the box of peat
Once we selected a 24x16 cm chunk of peat to study, we fondly named it “Peter,” and gently began disaggregating it. We uncovered fragments of wood, as well as fragments of leaves and insects. Though we hoped to find pollen as Ben Graham did, we were sadly not able to. Thus, we split the investigation into three groups: Team Wood, Team Macrofossils, and Team Reference. While we, as Team Wood, tackled the identification of wood fragments, Team Macrofossils did the same for other plant fragments. Team Reference set out on a scavenger hunt to collect reference wood fragments from trees on campus. Our hope was if they visualized their reference samples with the same techniques as we used on our samples, we could compare the two and better identify the historic wood fragments.
We began by separating cells. Our professor explained that we could create a maceration fluid as described by Huang and Yeung (2015), which would separate the cells, allowing us to search for particular cell types and features that distinguish between trees. We had to macerate the samples for longer than expected as the components binding the cells were not adequately digested. And that was not the only complication we encountered. Preparing microscope slides involved carefully transferring small quantities of cells onto a block of gel on the slide, melting the gel, and then placing a coverslip on top (Fig. 2)… Easier said than done. The cells were often either not adequately digested or over-digested, and the slides often had bubbles that made the cells difficult to visualize. We should probably admit that we never did perfect this slide-preparation technique, but we did manage to make slides for seven of our samples that allowed us to view the features we were hoping to see.
Figure 2. Microscope slide preparation setup
Our identification relied on three cellular characteristics: bordered pits, helical thickenings, and the presence of vessel cells (Fig. 3). Bordered pits, which resemble donuts, but are far too small to eat, are abundant in gymnosperm tracheid cells. Helical thickenings are spiral ridges in tracheid and vessel cells. Angiosperms have small bordered pits compared to gymnosperms. Using the findings of Baker et al. on the composition of the flora of Iowa during the Holocene, we used the size of bordered pits to distinguish between three trees that grew in Iowa around the time the peat was likely deposited: larch, pine, and spruce (Baker et al. 1990). Spruce cells have bordered pits in a single row, pine cells have larger bordered pits in a single row, and larch cells have bordered pits in pairs. Vessel cells, which comprise long, water-conducting structures, are only present in angiosperms.
Figure 3. Our key to identify wood fragments
Keeping an eye out for these features, we visualized our microscope slides (Fig. 4). The wood fragments seemed to include larch, pine, spruce, and an angiosperm! We observed single rows of ~20 µm diameter bordered pits in the tracheids of three samples, leading us to believe these wood fragments were from spruce trees. Single rows of ~30 µm bordered pits of another sample led us to believe this wood fragment was from a pine tree. Pairs of bordered pits in tracheids of another sample indicated this wood fragment was from a larch tree (Fig. 5). We only found helical thickenings in one sample. They appeared to be on vessel cells and were dotted by bordered pits (~50 µm diameter), which indicated this wood fragment was from an angiosperm (Fig. 6).
Figure 4. Maggie and Siri visualizing microscope slides.
Although we originally intended to compare our images to Team Reference’s, many of their images showed structures we did not observe (i.e. bordered pit fields in vessel elements; Fig. 7). We were, however, able to see that the bordered pits on the pine tree reference (~50 µm diameter) were slightly larger than those on ours (~30 µm diameter), which could indicate mis-identification, or could simply represent an actual range of sizes.
Figure 7. Team Reference’s bordered pit field in a vessel element
Soon, we’ll have better information about the age of our peat too. In addition to our wood identification, we prepared and submitted three wood fragments to Beta Analytic for radiocarbon dating. We’ll let you know once we have a better idea of when these larch, pine, spruce, and angiosperm trees were inhabiting what is now Grinnell College!
So, in our two short weeks of analyzing “Peter’s” wood fragments, we found evidence that the Grinnell flora included larch, pine, spruce, and angiosperm trees when the peat was deposited. If you want to see what Iowa was like back then, drive a few hours up to northern Minnesota and imagine you’re instead in Iowa long ago. Compare the landscape you see to Iowa today, and consider the evolution from conifer woods to prairie to the acres on acres of corn and soybeans you see today. Studies like ours and that of Baker and colleagues allow us to understand how and when these changes took place. Our work is just the beginning; there is plenty of other work to be done, and there’s plenty of peat left to do it!
Works Cited
Baker, R. G., Chumbley, C. A., Witinok, P.M., & Kim, H. K. (1990). Holocene Vegetational Changes
in Eastern Iowa. Journal of the Iowa Academy of Science, 97, 167-177.
Hoadley, R. Bruce. (1990). Identifying Wood: Accurate results with simple tools. Newtown, CT: Taunton Press.
Graham, B. F. (1962). A Post-Kansan Peat at Grinnell, Iowa: A Preliminary Report. Proceedings of the Iowa Academy of Science, 69, 39-44.
Yeung, E. C. T., Stasolla, C., Sumner, M. J., & Huang, B. Q. (2015). Plant microtechniques and protocols. Springer International Publishing.
Hoadley, R. Bruce. (1990). Identifying Wood: Accurate results with simple tools. Newtown, CT: Taunton Press.
Graham, B. F. (1962). A Post-Kansan Peat at Grinnell, Iowa: A Preliminary Report. Proceedings of the Iowa Academy of Science, 69, 39-44.
Yeung, E. C. T., Stasolla, C., Sumner, M. J., & Huang, B. Q. (2015). Plant microtechniques and protocols. Springer International Publishing.
1) Why did you use the Baker et al. (1990) paper on the Holocene to guide what trees you expected to see at a time much longer ago?
ReplyDelete2) It's weird to see the note reporting an age of (about) 29,000 radiocarbon years right after you reported Graham's wide age estimate. What's the story here? Where did the more precise estimate come from?
3) Not all angiosperms have vessel elements with helical thickenings. Of the angiosperm tree species that might have been there, do you know which of them have?
4) Team Macrofossil thinks they may have found epidermis of the "water plantain" (family Alisimitaceae) Scheuchzeria in the peat. Do you know whether that genus grows in "northern" spruce bogs today?
Fig. 3 looks fuzzy. You might want to edit your post, replacing the fuzzy image with a clearer one.
Delete