Image 1: Peat Bog from Sumava National Park in the Czech Republic. Kuttelvaserova Stuchelova/Shutterstock. Found in an article by Angela Nelson: Why You Should Care About Peat Bogs.
Peat bogs are often considered time capsules of the ecological history of an environment. Dead bugs and plant material freeze in time, partially decomposed, in the acidic layers of bogs. This is because as new peat accumulates, the older material underneath begins to rot. This rotting releases humic acid which preserves the organic material: the time machines that show us a vegetative past. Peat bogs originate from the incomplete decomposition of plant remains and other organic material growing in waterlogged conditions (usually in standing water of lakes, slow moving rivers, or areas of high precipitation).
In 1960 a layer of peat deposit was discovered while excavating the foundations of the new Arts building at Grinnell College (Graham 1962). Radiocarbon dating of three pieces of wood found in the peat suggests that this section of Iowa was once a peat bog approximately 27,000 years ago. By identifying the organic material in the peat, we can construct a vegetative history. This vegetative history gives insight into the composition of the landscape, including the trees, plants and other organisms that used to live here.
In an attempt to continue the study of the peat deposit, a group of Grinnell students in 2018, under the firm direction of Professor Vince Eckhart, took up the task. They found possible evidence of conifers (types of gymnosperms or seed producing plant) such as spruce (Picea), pine (Pinus), and larch (Larix), as well as a possible deciduous tree (a type of angiosperm or flowering plant). This corresponds with the current thoughts on the vegetative history of Iowa 27,000 years ago: mostly pine and spruce forest (Baker et al. 1989, 2009). This is consistent with what we know of the flora of bogs: lots of evergreen trees that can handle the acidic environment.
The work done in 2018 was a start, but as Grinnellians we had to ask the hard question: what else can we find out? This year we are continuing this vegetative endeavor by dividing into teams studying the macrofossils, pollen and wood samples. By studying the wood samples we collected from the peat we hoped to identify tree species comparable to what is currently known about the vegetative history of Iowa, and about the vegetative composition of ancient peat bogs.
To begin we did exactly what anyone would, we played with some peat. With geology hammers, tweezers, assorted dentistry tools, and microscopes in hand we searched and searched for anything resembling wood. After hours of tedious work, we had eight pieces to work with. We also collected twigs from ten different tree species currently on Grinnell’s campus to compare to our peat wood.
With our wood samples all together, we quickly found out that our work wasn’t over, as the reference wood we collected outside looked much different than the wood from 27,000 years ago. Our beautifully cut reference twigs made our dirt-covered peat fragments look alien. After trying to compare the wood by smell, we settled on a more comprehensive plan shown in Figure 1.
Figure 1. Wood slide preparation methods, including collecting, softening, maceration, and preparing slides. Methods adapted from Larter et al. (2017).
After getting our precious wood samples onto our slides, we realized our identification was going to be trickier than we thought. Our peat wood looked 27,000 years old. Some slides had mangled cells, others had unrecognizable globs of mush, and two weren’t wood at all. Even our best peat wood samples looked very different from the orderly cells of our reference wood. But the key was in the pits.
Gymnosperm cells have rows of little pits, or holes that allow fluid exchange, along the tracheids. Angiosperms, on the other hand, have fibers and vessel elements. We realized none of our peat wood had vessel elements, so we knew we only had gymnosperms.
We also realized many of our peat wood samples have helical thickenings. Helical thickenings are the extra layer of the cell wall that occur more often in softwoods and usually when the wood is under stress or compression, often at the joint of a branch. We wondered if these helical thickenings could have added structural integrity to the cells, allowing them to stick around for 27,000 years.
Based on the arrangement of the pits and the size of the tracheids, we think our samples are most likely Pinus or Picea. We agree with the wood group from 2018 in that Pinus has bigger pits than Picea, but our measurements for pit and tracheid diameter are a lot smaller. Despite the difference in measurements, our identification of our wood as Picea and Pinus is consistent with the pollen identification by Graham (1962) and with the pollen group’s analysis this year (A Hitchhiker (Pollen)’s Guide to the Universe (Iowa Flora)). We hope that this work will continue as there is much to consider, especially when we are limited by what we can mount on a slide.
Figure 2. Pictures of magnified (400x) wood cells. Left column: Picea references collected from Grinnell College Campus by 2022 Wood Team. Middle column: Peat samples, from PP2A and PP2B, prepared by Wood Team, and PP5C, prepared by Professor Vince Eckhart. Right column: Pinus references collected from Grinnell College Campus by 2022 Wood Team.
Our findings match what is currently understood about the vegetative composition of Iowa 27,000 years ago and what we would expect to find in a bog: lots of spruce and pine. We found no further evidence of gymnosperms, and no indication of angiosperms. By learning what species were present 27,000 years ago, we can understand the ecological changes that Iowa experienced, which can help us predict how our environments will change in the future. Iowa 27,000 years ago is what Minnesota and Canada look like today, and as warmer temperatures reach latitudes further North, those environments and vegetation will begin to change. Knowing how species composition has changed will help us prepare for our changing future.
Authors:
Elinor Arneson, Maria Eure, Elena Friedman, Noah Guyton, and Cicely Krutzsch
Acknowledgements:
We would like to thank Vince Eckhart for his guidance and assistance, the BIO 305 class from 2018 for starting this journey for us, and the Professors Ben Graham and Andrew Graham for respectively collecting the peat in 1962 and then finding the box of peat in basement of the science building in 2018.
References:
Baker, R. G., Bettis III, E. A., Schwert, D. P., Horton, D. G., Chumbley, C. A., Gonzalez, L. A., & Reagan, M. K. (1996). Holocene Paleoenvironments of Northeast Iowa. Ecological Monographs. 66(2); 203--234.
Baker, R. G., Bettis III, E. A., Mandel, R. D., Dorale, J. A., & Fredlund, G. G. (2009). Mid-Wisconsinan environments on the eastern Great Plains. Quaternary Science Reviews. 28; 873-889. doi:10.1016/j.quascirev.2008.12.021
Graham Jr., B. F. (1962). A Post-Kansan Peat at Grinnell, Iowa: A Preliminary Report. Proceedings of the Iowa Academy of Science. 69(1); Article 7.
Larter, M., Pfautsch, S., Domec, J., Trueba, S., Nagalingum, N., & Delzon, S. (2017). Aridity drove the evolution of extreme embolism resistance and the radiation of conifer genus Callitris. New Phytologist. 215; 97–112. doi: 10.1111/nph.14545.
Links:
https://www.treehugger.com/why-you-should-care-about-peat-bogs-4863716
https://nph.onlinelibrary.wiley.com/doi/10.1111/j.1469-8137.2007.02317.x
How do your tracheid measurements compare to those in published references, as opposed to those from the 2018 students?
ReplyDeleteHow do vessel elements and fibres differ from tracheids?
Our peat wood and reference wood tracheid diameter measurements were around 9-15um whereas "Identifying Wood" shows Picea to be 25-55um and Pinus 20-60um.
DeleteVessel elements are spacious cells that allow sap and water conduction. While tracheids also allow fluid conduction through pits, they are less efficient than vessel elements. Fibers are long, skinny cells that taper at the end and have thicker walls than tracheids. Fibers also have a greater structural purpose than tracheids.
Regarding the sizes, it sounds like the scale was off by a factor of ~4. Maybe your instructor's (i.e., my) calibrations were off by a factor of 4.
DeleteThanks for explaining the cell types. "Spacious" is an interesting word to apply to things that are so small (compared to us), but it's true that vessel elements are generally much larger in diameter (but shorter) than tracheids. Plus, they're stacked end to end, with big openings (perforation plates) between adjacent cells.
https://www.researchgate.net/publication/254146291_Evolutionary_Voyage_of_Angiosperm_Vessel_Structure-Function_and_Its_Significance_for_Early_Angiosperm_Success/figures?lo=1
Can you give readers an idea of what to look for in your figure of micrographs? Also, what do the designations (e.g., "PP2A") mean?
ReplyDeleteUnfortunately, our pictures uploaded slightly blurry, but in Figure 2 it is important to look at the sizes and arrangements the pits (the little holes along the middle of the wood cells). Looking at if they are in a line, how spread out they are, and if there appears to be two layers, a "pit inside a pit," so to speak.
DeleteAdditionally, our designations were more for our purpose in categorizing each wood piece we collected, especially important if other researchers want to refer to our samples for future study. We collected 8 pieces total from the peat, identified as PP1, PP2, PP3...Then, we made multiple slides for each peat sample, designated by A, B, and C, resulting in PP1A, PP1B, PP1C, PP2A, PP2B, PP2C, etc.
Got it. Thank you.
ReplyDelete