Trees From Thin Air

Initial Publication Date: June 17, 2024

Summary

The exercise presented here is one of a suite of activities for learners of all ages that has been developed from the standard allometric equations for the above-ground biomass of trees (Chave et. al., 2005; Chojnacky et. al., 2014). When allometric equations are depicted as species-specific graphs of tree diameter and biomass, any student can measure the circumference of any tree and find its above-ground biomass, total biomass, and total carbon and CO2 sequestration. The concept of tree biomass originating as an invisible atmospheric gas is one of the most stubborn misconceptions for young science students. The activities provided here help students construct their own understanding of this concept and its relationship to the carbon cycle and to the greenhouse gasses that cause climate change.

The main activity involves identifying different types of trees, measuring their circumferences to determine the amount of CO2 stored in some of the local vegetation, and measuring temperatures in various locations to explore the concept of "urban deserts". This example presented here was completed at a small park in southern CA (Carbon Canyon Regional Park).

In the field, students use informational cards for tree identifications. One side has images and descriptions of tree characteristics, and the other side shows a graph plotting the amount of above ground biomass (y-axis) in a specific type of tree based on its diameter (x-axis) (see provided examples). Students measure the circumference of each tree using a flexible tape measure. With that data, students calculate the tree diameter, use the chart to find the above-ground biomass, add the biomass of the roots, from there they can calculate the mass of carbon, and then convert that to the mass of CO2. Students also use a temperature-gun to measure surface temperatures both in the shade of a tree as well as away from cover. While covering the field area, students have a blank map of the park and add to the map with schematic representations of the trees in their locations, creating a plant palette for the park (see provided PowerPoint for examples of plant palettes).

In the lab/classroom, students compile their data and fill out worksheets with questions and reflection prompts. Students also use a simple online app to determine how much carbon needs to be sequestered in trees and soils to lower atmospheric CO2.

Context

Audience

The version of this activity provided here was used in a lower level general education Earth Science course, "Geology of Parks". The majority of students are first- and second-year students and are non-STEM or undeclared majors. This class has short discussion sessions, a lab session, and several field trips.

Versions of this activity have also been used as short demos in eco-fair settings with children in K-12 grades, it has been expanded with additional scientific and quantitative content for high school students and lower division undergraduates, and has also been undertaken as a semester-length project by upper division undergraduate students (Moore, 2023).

Skills and concepts that students must have mastered

A high school level understanding of photosynthesis provides a basis for this activity; otherwise, students do not need to have any background about the carbon cycle, surface temperatures, or different tree types, as this activity is intended to be for building interest, understanding, and inquiry about the carbon cycle, and human-vegetation impacts.

How the activity is situated in the course

The version of the activity presented here is used at the beginning of a 3-week module on the carbon cycle, and is followed up with a sequence of exercises about the Carbon Cycle, sources and sinks of Carbon, global vegetation, forest management, and how the Carbon Cycle has changed from human influences.

Goals

Content/concepts goals for this activity

Trees are made up of carbon.
Different trees contain different amounts of carbon.
Tree size is one controlling factor for how much carbon is sequestered.
The role vegetation plays in urban heat "urban deserts".

Higher order thinking skills goals for this activity

Formulate and test hypotheses about how atmospheric CO2 and global temperature will change if changes are made to fossil fuel use, land use, ocean uptake and land uptake of CO2
Brainstorm ideas for mitigating urban desertification
Evaluate current models and plans for planting more trees and recording the impact

Other skills goals for this activity

Using information and making detailed observations to identify a variety of trees (given photos and written descriptions of key characteristics)

Description and Teaching Materials

This module has been created for a flipped approach to learning about the carbon cycle. Students take a field trip, make observations, and collect data to look for patterns and develop their own conclusions before learning about the carbon cycle on Earth and human contributions. The module presented here has been run at Carbon Canyon Regional Park in Brea, CA, a local park that can be reached in a 30-min drive, and thus the field trip and data collection portion of the module is completed during a 4-hour lab period. And the in-class and lab activities are completed the following week. Students are provided:

• A simple map of the park, which includes trails, roads, ponds, and parking lots
• Informational cards for tree identifications. One side has images and descriptions of tree characteristics, and the other side shows a plot of the amount of above ground biomass (y-axis) in that type of tree based on its diameter (x-axis)
• Flexible measuring tapes to measure the circumference of each tree
• An IR-thermometer gun to measure the temperature of the ground surface underneath the shade of the tree and ground surface areas not under a tree
• A trip guide that includes an example of a table showing how to record data and calculate the mass of CO2 in each tree using their circumference measurement and the provided graphs that give information on 'above ground biomass' for different tree types.

Prior to the trip, students do a pre-trip activity. They watch the "Photosynthesis MIT" video and then reflect on if they also had similar thoughts to these students, or other thoughts. Then they watch the "Trees From Thin Air" video and write a short summary about how trees 'come from thin air'. (Videos provided in online supplemental resources below).

In the field, students begin by getting acquainted with the map and determining their location and other key features, then the entire group practices measuring, reading the graphs, and recording observations in their field notebooks. Once students are comfortable with the exercise, they are put into groups of 2-3 and let loose to collect data from many different trees.
The example, from Carbon Canyon Regional Park, presented here, includes informational cards for seven different tree types that were previously scouted and identified at this park (see supplemental materials for a PDF with the front and back of each card). Regional sets of Tree-ID/Biomass cards are available from "Climate Science Kit," on Etsy at https://www.etsy.com/listing/1669000113. Contact Dr. Alexandra Moore (afm113@gmail.com) for special requests of other trees in your area.

The activity includes students measuring tree circumference and surface temperatures (beneath trees and out in the open away from tree cover). Students are encouraged to do their calculations of CO2 as they go. However, this can be done later, either once they have finished and are waiting for others, or as a classroom activity after the trip. Students identify and measure 5 different trees of the same species. In this example, there were 7 different tree types, so each student group measured 35 different trees.

While measuring each tree, they should also use the temperature gun to measure the temperature of the surface beneath the shaded part of the tree, and then walk just outside the shaded part to obtain a temperature measurement in a nearby none-shaded part of the same type of surface (e.g., grass, dirt, sidewalk). Students may also be interested in finding an area nowhere near any trees, and taking a temperature measurement there as well, for comparison, or they may be interested in comparing different surfaces (e.g., the difference between a sidewalk and dirt under a tree vs sidewalk and dirt not under a tree). *Note, the differences may be small or non-existent if the day is cool or clouded, but this can lead to discussions and hypotheses about differences on hot and/or sunny days.

In addition to taking these measurements, students track where they are on their park maps, and each tree they measure should be added to the map. The tree should be drawn in to show the approximate area covered by the tree canopy. By the end of the trip, the students will each have a sort of rough draft "plant palette". It is helpful to show examples so that students see the goal of the plant palette (some are provided in the PowerPoint resource), and can be used to prompt discussion about the amount of shaded area at the park and how this may be similar or different to the areas the students live, and how that affects the heat they feel in their neighborhoods, for example.

The straight-forward nature of the activity makes it fun and accessible for students and makes it easy for the instructor to float between groups and check in periodically to ask questions and have students voice hypotheses and predictions based on their observations.

As a pre-lab activity, students watch the "Growing Hope" video (provided below).
Once students are back in the classroom, they can start to pick apart and synthesize the data. This can include looking at the amount of CO2 in the 5 trees from a single species, and comparing that to the others, or the amount of CO2 in all the trees they measured, the temperature differences depending on tree type, etc. (see worksheet resource used in this example).

The last part of the activity uses the Global Carbon Budget Applet to explore how the global carbon budget will change if current usage of land, oceans, and fossil fuels changes. Students design scenarios for fossil fuel usage, land use, ocean uptake, and land uptake from 2020-2100, and take note of the impact this would have on atmospheric CO2 and global temperatures. Students reflect on expected or surprising results, and then read and summarize an article (Harmon et al., 1990; provided in supporting materials) about whether it makes sense to cut old-growth trees (which take up carbon dioxide at a slow rate) in order to replace them with younger trees that take up CO2 at a faster rate.

This whole activity, including the field-trip and post-trip data analysis and reflection, prepares the students for the rest of the unit on the carbon cycle, and makes for engaging discussions with students referring back to their own collected data and observations as we move into carbon sources and sinks, forest management, human infrastructure, urban deserts, and current sequestration efforts.

Tree ID/Biomass Postcards (Acrobat (PDF) 295.7MB Jun5 24)
Activity Guide and Worksheet (Microsoft Word 2007 (.docx) 1.1MB Jun5 24)
PowerPoint Guide for pre-/post-trip activities (PowerPoint 2007 (.pptx) 949kB Jun5 24)
Paper about old vs. new trees (Acrobat (PDF) 1.2MB Jun5 24)
Paper about tree allometry (Acrobat (PDF) 407kB Jun5 24)

Chojnacky et. al., 2014_Forestry_IJFR.pdf (Acrobat (PDF) 731kB Jun5 24)

Teaching Notes and Tips

Before the field trip, we discuss the safety hazards and mitigation strategies. In the park used for this example, there are public restrooms and water fountains, some shade and seating areas, and trails that are both paved, unpaved, and graded. Main concerns were being careful of bicyclists, dogs (on and off leash), and park goers playing and relaxing in the park. We specifically talked about how to respond if/when people asked them what they were doing, and how to respectfully ask people hanging out near a tree if they would be OK with the students spending 5 minutes taking some measurements.

For the activity, it is important to get the units right. The biomass graphs are calibrated for a diameter input in centimeters, and give above ground biomass in kilograms. Make sure the tape measure is metric, or, if it is metric on one side and English on the other, make sure the students use the correct side.

Assessment

Formative, in the field as you check on them and ask questions
Formative in the lab as you answer questions and help them discuss data
Summative, lab worksheet with calculations, observations from the Applet, summary of science article
Summative, final plant palette