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Leaf Bags Under the Oaks: A Year of Matter and Energy

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Leaf Bags Under the Oaks: A Year of Matter and Energy

Use a park food web, litter bag data and soil carbon dioxide readings to model how matter cycles.

The oak grove model

Read the passage, then answer the questions.

What Happens to the Leaves?

1Fairmount Commons is a city park with a grove of old oak trees. For years, park workers raked up the fallen oak leaves every fall, bagged them and hauled them away to a landfill outside the city. This year the park's naturalist, Lorenzo Basile, asked the Green Team, an after-school club of seventh graders, to study what happens to leaves that are left on the ground.

2Mr. Basile helped the club build a model of the grove. The oak trees capture energy from sunlight and use it to make sugars from carbon dioxide in the air and water from the soil. Their roots also take up minerals, such as nitrogen, dissolved in soil water. In spring, caterpillars eat the young oak leaves, and chickadees catch caterpillars to feed their chicks. A hawk sometimes catches a chickadee. Earthworms, millipedes, fungi and bacteria feed on the fallen leaves, animal droppings and dead bodies on the ground, breaking them down.

3To measure how fast leaves break down, club member Yuki Morimoto and the others filled 16 mesh bags with 20.0 g of dried oak leaves each in November. Eight bags had coarse mesh with holes about 5 mm wide, big enough for earthworms and millipedes to crawl in. The other eight had fine mesh that lets in fungi and bacteria but keeps out soil animals. The bags were tucked under the leaf layer in the grove. Every three months the club collected two bags of each kind, rinsed, dried and weighed the leaves left inside, and found the average. A buried thermometer logged the soil temperature.

4Once each season, the club also measured the carbon dioxide coming out of the soil. They pushed an open-bottomed can a little way into the ground, placed a carbon dioxide sensor inside, and measured how fast the gas built up. They took each reading at midday on a dry day, at the same three spots in the grove.

1.

Use paragraph 2 to complete this part of the grove model.

Oak leaves fall → break them down → minerals go into the → oak roots take up the minerals → new grow in spring

2.

Which statement best describes how energy moves in the grove model?

A. Energy enters as sunlight, passes along in food and is given off as heat.B. Energy is made by the chickadees as they eat and passed down to the oaks.C. Energy starts in the soil, where oak roots absorb it along with minerals.D. Energy cycles: decomposers return it to the soil, and the oaks take it up again.

3.

Which path could one nitrogen atom follow in the grove model?

A. Sunlight → oak leaf → caterpillar → chickadee → hawk → sunlightB. Soil → earthworm → oak root → oak leaf → sunlightC. Soil → oak leaf → caterpillar → chickadee droppings → fungi → soilD. Hawk → chickadee → caterpillar → oak leaf → soil → air

Litter bags through the seasons

Use paragraphs 3 and 4, Table 1 and the graph.

Table 1. Average Leaf Mass Left in the Bags
Month collected	Soil temp. (°C)	Coarse mesh (g)	Fine mesh (g)
November (start)	—	20.0	20.0
February	2	18.8	19.4
May	11	16.0	18.0
August	21	11.0	15.4
November	12	8.0	13.8

Soil temperature is the average for the three months before each collection.

4.

How much leaf mass did the coarse mesh bags lose over the whole year? g

5.

What percent of their starting leaf mass did the fine mesh bags lose over the year? %

6.

During which three months did the coarse mesh bags lose the most mass, and what is the best explanation?

A. August to November, because new leaves fell into the bagsB. February to May, because melting snow washed the leaves awayC. November to February, because fresh leaves are the easiest to break downD. May to August, because decomposers are most active in warm soil

7.

What does comparing the coarse mesh and fine mesh bags show?

A. Soil animals such as earthworms sped up decay along with fungi and bacteria.B. Earthworms and millipedes slowed decay by eating the fungi on the leaves.C. The fine mesh bags lost more mass, because fungi work best without earthworms.D. The coarse mesh let leaves blow away, which is why those bags lost more mass.

8.

Use the graph. The soil released how many times as much carbon dioxide per day in summer as in winter? times

9.

The leaves lost mass as decomposers fed on them. Circle the two items that are matter from the leaves moving on to another part of the ecosystem.

10.

Suppose next winter is much milder, and the soil stays near 8 °C from November to February. Which result is most likely for the coarse mesh bags collected in February?

A. They gain mass, because decomposers add their own bodies to the leaves.B. They lose more than 1.2 g, and the soil releases more carbon dioxide.C. They lose less than 1.2 g, because warm soil holds leaves together.D. They lose the same 1.2 g, because the leaves were the same kind.

11.

A park worker says, “Dead leaves are just trash. Hauling them away does not change anything for the oak trees.” Do you agree? Write a claim, use evidence from Table 1 or the graph, and use the grove model to explain how matter moves when leaves are left or hauled away.