Decomposition: Leaf & Soil Collection

Engage:

Assign the reading and accompanying questions the night before the scheduled activity.  A formal pre-assessment is provided as part of this activity, which provides insight into what aspects of the carbon cycle students are aware of, and what components they need more experience with.  Write the following statement on the board, or project, as a “do now” or “warm up” activity the day you are planning on collecting the leaves:

Why do some trees lose their leaves during cold, heat, or drought?

Spend the first 5 - 10 minutes of the class period discussing the question posted, and/or the pre-assessment.  Hopefully, students will discuss some of the following:

• Photosynthesis: probing question– do all plants photosynthesize at the same rate?  Why or why not?
• Soil:  probing question – how does the soil become nutrient rich?  Can the soil become depleted?
• Water: probing question – why can some plants (such as desert plants) survive with minimal water and others (such as deciduous plants) require more?

Explore:

Day 1: Place students into lab groups and assign each group a different species of tree, using the trees in your study plot.  Ideally, compare deciduous and coniferous trees.  Try to choose trees that are relatively close together so that the instructor has a view of all groups working and has the ability to assist each group as necessary.  Clearly establish rules for outside work and articulate your expectations of students, and remember to use safe field procedures and point out hazards such as poison ivy. 

Have students lay down a hula hoop/quadrat at a designated distance away from the base of the tree, using the transect tape as a guide.  Students should collect and record the number of leaves in the hula hoop until there are no more leaves to collect.  Once leaf collection has been completed, students should use a trowel to collect enough soil to fill a one gallon Ziploc bag or fill a large Tupperware container approximately ¾ full ( a space about 8"x12", using the first 2" of soil).  If time permits, repeat. 

Explain:

Day 2: students will complete the laboratory by graphing the shared class data (teacher may wish to extend this by working with a graphical computer program, such as Excel).  Optional components of the investigation include weighing each sample of leaves and conducting a soil nutrient test on each soil sample.  Regardless, students should graph the amount of leaves collected by tree species.  Students may see outliers in their data; have them discuss why this may be happening based on the structure of the tree or other factors.

During the last 10 minutes of the second day, students should discuss why they think the relationship between species and amount of litter exists.  While groups are presenting, each student should be taking notes in the graphic organizer which will help them with that night’s homework assignment (at the end of the worksheet).

Extend:

As a follow up homework assignment, ask the students if they believe that the different tree species have different amounts of nutrients stored in their leaves.  Have them create a hypothesis and give supporting statements.

Evaluate:

Use students' responses to the laboratory questions, graph and follow up homework assignment to evaluate how well students understand the stated objectives.  Specifically, focus on students' explanations of why the amount of leaf litter might vary between species (question #2 on the worksheet).  Students may give reasons such as wind or animals moving the leaves, but they may also begin thinking about decomposition.