Design an investigation

Plant recovery after wildfire

Units 2 and 5
10 points
About 23 minutes

You can use a calculator on this question, just like on exam day.

You read about a real-world environmental situation with a described figure or model, then pick out the parts of a scientific investigation (hypothesis, variables, controls), explain the science behind it, and propose solutions to the problems it raises. On the exam: Question 1 of 3 in the 70-minute free-response section (Section II is 40% of the exam score).

The question and its sources

Wildfires burned different parts of a mountain pine forest in different years. The soil remained after each fire. Ecologists counted plant species in plots in each burned area and in a nearby area that has not burned in about 120 years.

Table 1: Plant species in forest plots of different ages

Years since fire	Mean number of plant species per plot	Most common plants
1	12	Fireweed and grasses
5	31	Grasses, wildflowers and young shrubs
15	26	Shrubs and aspen saplings
40	18	Aspen trees and young pines
120 (unburned)	14	Tall, mature pines

Source: Hypothetical data

Planned greenhouse investigation

Fireweed is one of the first plants to grow after a fire. To find out whether ash helps fireweed seedlings grow, the ecologists will fill 30 identical pots with soil collected from an unburned area. They will mix 0 g of ash into 10 pots, 15 g into 10 pots and 30 g into 10 pots, and plant one fireweed seed in each pot.

All pots will be kept in the same greenhouse and given the same amount of water. After six weeks, the ecologists will measure the dry mass of each seedling.

Source: Hypothetical study design

Suggested time: 23 minutes

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Part (a)

1 point

Identify the type of ecological succession occurring after the fires, and justify your answer.

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Part (b)

1 point

Using Table 1, describe how the number of plant species changes with time after a fire.

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Part (c)

1 point

Explain why the number of plant species is lower in the 120-year-old forest than in the 5-year-old plots.

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Part (d)

1 point

Describe one characteristic of fireweed that makes it a successful pioneer species after a fire.

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Part (e)

1 point

Identify the independent variable in the greenhouse investigation.

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Part (f)

1 point

Identify the dependent variable in the greenhouse investigation.

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Part (g)

1 point

Identify the control group in the greenhouse investigation.

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Part (h)

1 point

Identify one variable, other than the amount of water, that the ecologists should keep the same for all 30 pots.

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Part (i)

1 point

In many pine forests, small wildfires occur naturally every few decades. Describe one effect that putting out every fire for 100 years could have on the forest.

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Part (j)

1 point

Propose one forest-management practice that could reduce the risk of a very severe wildfire in this forest.

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