Trees and Forests Inquiry Grade 6

Does the Weaselhead Natural Area produce enough oxygen to sustain Calgarians?

Greg Neil~ Grade 6 Math and Science

Our grade 6 students investigated this question in their Math/Science classes this year. The question was developed as part of a brainstorming session with students. This inquiry required the use of numerous mathematical skills, introduced students to the idea of assumptions and bias in Science, reinforced the need for accurate data and provided an authentic investigation into an important natural region close to the school. A current proposal to build a ring road through this natural area, made this project even more relevant and engaging for students.

After introducing the question, students started the investigation by gathering important background information. Sources such as Environment Canada, Statistics Canada and NASA were useful in establishing some basic facts about Oxygen production, current population and average values of Oxygen consumption by humans.


Armed with this information, students set about the first task, which was to calculate the area of the Weaselhead. We needed to define the space that would be investigated. The challenge was for students to develop a strategy that would work for an irregular shape. To help facilitate this work, students were first asked to determine the area of a leaf. This introduced the idea of calculating the area of both quadrilaterals and triangles. This scaffolding resulted in an abundance of effective strategies including the use of manipulatives, string, transparencies, grids drawn by ruler, along with the use of measurement tools in Google Earth. This process helped students build a more concrete foundation and paved the way to more accurate calculations when we focused on the area of the Weaselhead. Once students arrived at their final answers for the area of the Weaselhead, we came together to discuss the various responses, to eliminate outliers and to average the reasonable answers to produce a final value that we could use as the basis for the next steps. 

With an established area of two square kilometers, the next step was for students to determine a way to accurately estimate the total number of trees in the Weaselhead. Students were quick to recognize that large trees were likely to produce more Oxygen than small saplings, (a fact supported by our research), so we left them to consider ways of counting and classifying the trees. Students settled on weighting the different sizes of trees, so that an XL tree (more than six times the height of a student) would be multiplied by the value provided by Environment Canada, which was 118 kgO2/year. Students decided that a Large tree, (3-6 times their height) would on average produce 3⁄4 as much O2 as an XL tree, so they multiplied 118 kgO2 by 0.75. Medium trees, (up to three times their height) were considered 1⁄2 of an XL tree and Small trees, (less than eye level) were considered 1⁄4 of an XL tree.

Students also knew from numerous trips to the Weaselhead that the forest had three distinct areas where the forest make-up was unique. We defined these three areas as: Meadow, Succession and Climax Forest. Through discussion and debate, students decided that we should divide all one hundred Grade 6’s into groups of four, resulting in 25 groups that could spread out across the forest to collect the necessary data. Each group was assigned one of the three forest types to have a good sample and students agreed that a 10m x 10m quadrant would be the most effective sample size. The day we counted trees, students spread out across the forest using their area and perimeter skills to create a 100m2 quadrant and then began counting and classifying trees using a data sheet that they had developed in class.

The data was brought back to the classroom and plugged into a Google Spreadsheet in order to calculate average values for each of the three forest types. Once the averages were determined, students could plug these numbers into the formula we created based on tree size, in order to calculate the average amount of Oxygen produced by a representative area of each forest type.  

The next step was for students to estimate the percentage of Weaselhead that comprised each of these three distinct forest components. Using similar techniques to those developed when they calculated the area, they were able to come up with values that were debated during a group discussion. We included all of the results in a table, students averaged the results and we came to a consensus about the make up of the area, which was 40% Meadow, 30% Succession and 30% Climax forest.

Students were then asked to determine the total area of each forest type by converting these percentages into square meters. This introduced many students to the concept of multiplying by decimals in order to determine percentage equivalents. Once they knew the area of each forest type, they divided this value by 100m2 (Sample Quadrant Size) to know how many of our samples would fit into the total area. Multiplying the resulting value by the total amount of Oxygen produced by the Sample Quadrant allowed us to arrive at the total Oxygen production for each forest type. Adding the three forest types together produced the total Oxygen production for all the trees in the Weaselhead. At this point, a discussion ensued regarding assumptions and the fact that we were ignoring all of the broad leaf plants and shrubs in our calculations, so we knew that the real values would likely be higher than those calculated. We also talked about the potential for error when using averages and sample areas, but students agreed that counting every single tree was unreasonable and that our methods likely produced accurate results.

The final mathematical step was for students to divide the total value for Oxygen production, with the value provide by NASA for average human O2 consumption, which was 237 kgO2/year. This gave us the total number of Calgarians that would be supported by the trees in the Weaselhead, which turned out to be a little less than 400,000.

Once students had completed all of their calculations and had arrived at their final solutions, they were asked to compile all of their work into a digital document that explained their understanding of this inquiry process. Many students chose to create iMovie’s, but some created slideshows or even Prezzie’s in order to showcase their work. The goal was for students to prove their understanding of this work, by explaining their calculations along with the various strategies and methods used throughout the process.

Students were clearly engaged throughout this investigation and many commented on how interesting and exciting it was to finally arrive at an answer after weeks of hard work. They also commented on how proud they were because they knew that all of their careful planning and thorough research had produced answers that were both accurate and reliable and could be replicated by professional scientists. One of the great things about this inquiry, is that it could be reproduced by any teacher using any park, forest or natural area as the starting point. In its simplest form, students could calculate the oxygen production from trees on their family’s property or on the school grounds.

We would love to hear your comments and questions. If you try this inquiry, or one similar, with your students we would be really interested in hearing how you adapted the question and what your results are.