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Tree Rings as Proxy Data

Tree rings can be used as proxy data to determine the past climate in a local area. Tree rings show evidence of droughts, wet seasons, forest fires, and even earthquakes. Tree rings can not be used alone as evidence for past climate. Students need to understand that no one object holds the key to the debate. It is the correlation of all these key pieces of evidence that builds our understanding of past climate events. This has been an evolving lesson I have been developing for my 9th grade integrated science class which has taken 3 years to come to fruition, or rather come to the point I am content with the level of depth of understanding students are leaving with.

I contacted my local tree company, Asplundh, to get some tree cookies. They came through and then some! I received a call from the admin assistant in my school that I had a "wood delivery". Imagine my surprise when my students delivered all my tree cookies! Students were fascinated, trying to count the rings, and smelling the fresh-cut wood. So much excitement and engagement as they arrived. However, this also led to a scramble to get them prepared for analysis!

If your classroom is like mine, these will dry out very quickly. I had some students who were working on some community service hours, take my cookies down to the industrial arts room and they sanded and sanded and sanded until they were smooth. This makes the rings so much easier to observe. They then sealed the cookies with beeswax and mineral oil. The cookies will crack, it is inevitable, but I do not think it detracts from the student's experience and the crack is one more opportunity for students to make more observations.

To start, I have students observe and diagram their tree cookie sample. I have my students diagram and label every chance I get. Students draw and label what they see, then describe the sample in words. I am focusing on building some of my student's observation skills. Their scientific sketches need to have pictures, words, and numbers. Diagramming and describing in words are key observation skills. Since we know when the tree was harvested, I then have the students count the rings to determine the year the tree was planted or sprouted from seed as well as identify and diagram low growth (drought) and high growth (wet) years.

Students create a graph of the tree ring width vs the year of growth and annotate the graph to identify drought and wet years. They compare their graph to historical precipitation data for the area and see if the thin rings correlate to low precipitation years and wider rings to wet years. If they do not correlate, then we brainstorm other causes of climate disruptors that affect how a tree grows. This leads into my next unit on the Year without Summer. This task focuses on practicing accuracy and precision in measurements, SEP-4 Analyzing and Interpreting Data, and SEP-5 Using Mathematics and Computational Thinking.

My honors students additionally calculate the total biomass of the tree and carbon sequestered by the tree and make connections to their individual carbon footprint and calculate how many trees it would take to offset their impact. This gets some good discussion going and opens up the opportunity for me to talk about deforestation in the United States between 1600 and 1900, and how New Hampshire was 80% deforested at the turn of the 20th century. This is also a connection to our 9th- grade History curriculum and the impact of economic development on the environment. I like to also make mention of the American Chestnut Blight with my honors students.

Now it is time to head out into the woods, so students can complete a local tree study and collect a tree core sample. We have one increment borer. [They are "spendy", and I am looking to get a second one next year. It doesn't always fit in my budget as we have been adding AP courses which have drained my budget the last few years.]

I demonstrate on a tree how to collect the tree core and how to properly use an increment borer. There are a couple of good videos out there that you can use. It is important to roll the auger bit in beeswax after it is extracted from the tree to keep the metal lubricated and reduce the friction the next time the borer is used. I also use a long test tube brush to keep the inside of the borer clean. Resins and sap can gum up the borer and make extraction of the tree core much more difficult. One of the most difficult tasks I find students have is to not drop their tree sample as they are extracting it. I make sure they have a partner hold their hands under the extractor to catch and piece of the sample that might drop. Students are notoriously impatient and pull out the sample too quickly.

I also found some old microtomes in my reorganization of the science closet, so I now have students use them to create a thin slide of their tree core to view under the microscope. For my lower level students or my disciplinary issue studnets, I make them for them. This allows us to observe and compare a variety of different tree species and their cell structures under microscopes. We can usually find some insect boreholes and insect casings that are not visible to the naked eye. If you don't have microtomes or slides that contain wood cells, you can create your own wet mount from a core or a tree branch using an Exacto knife or box cutter. Just try and get the thinnest shaving. You'll likely get several layers of cells but if it is thin enough the microscope light will penetrate the cells. If none of those options work, search for some digital photos to use or email me directly. I have a folder of photos in your Google Drive of photos I have taken under the microscopes of our tree cores and wood cells.

I have students, then mount their cores just like a dendrochronologist does in the lab. My husband and I ran a few 1"x1" pieces of wood through the table saw to create a grove for the tree core to sit in. Students then use wood glue to secure their tree core to the mounting block with some small rubber bands.

Once the mounted core is dry, they can remove the rubber bands and use sandpaper to sand the core flat.

This exposes the individual growth rings in the cores they collected and then they can use the dissection microscopes one more time.

One of my first structured lectures is really to define and identify our vocabulary for the unit. We talk about the anatomy of a tree, a tree's lifecycle, the tools of a dendrochronologist, and the importance of a tree in the climate system. After the lecture and discussion, we use our new vocabulary to label our scientific diagrams in our notebooks.

In the following class to break up all the laboratory work, we complete an Article Roundtable. I am looking for some additional podcast sources so I can change it up in the future to do a Podcast Walk but I have only found one I really like, so I assign that for homework as an additional primary source in my Honors class. Each group reads a different article and creates a poster about that article. We then present our posters to share out the article information and share out so we can all take notes on the information.

During remote learning, I used the above template on a Google Jamboard and students put their evidence as sticky notes under the article they were assigned. Now that we are back in person we have been working on close reading strategies as text analysis is a skill our students have been struggling

with. I change up my reading strategies depending on if I have them read independently the same

article or several articles as groups. I want them to work with the text as much as possible. So, this year for this part of the unit, I had them create a four-corner poster (Big Ideas, Supporting Ideas, Questions/Ideas/Opinions, and Text Evidence ) for the article they read. Students then completed a gallery walk to gather evidence for their tree climate story.

Students complete their climate study by compiling all their information on a scientific poster or infographic on the climate history of their town.

An extension activity or additional homework assignment I assign if there is time is the article "The Ladder Link" by Donna Christensen. The ladder link is about how xylotomist Arthur Koehler used tree structure and tree ring analysis to help solve the Lindberg baby kidnapping in 1932-33. This is a great connection between forensic science and mystery. My students love when I talk about different historic crimes (I also teach forensic science) and this is a great article to connect the science that we are learning in class to a real-life case.

You can find my lesson plan here. Stay tuned for more climate science lesson plans.

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