HandsOn 27 - Growing a Scattered-Seed Forest

Work in pairs. You need:

• a pencil,

• a ruler,

• a sheet of plain paper or graph paper,

• a green marker (any color will do if green is not available), and

• a red marker (a pencil or pen will do).

First create a random forest on a square grid, with 25 cells in the grid.

1. Draw a 5×5 square grid on a piece of paper, with each square approximately 2 centimeters on a side. Save time by using graph paper if it is available.


2. One student places a finger on the uppermost left square while the other student flips a coin.


3. If the coin is heads, use the green marker to make a green dot (representing a tree) on the square. If the coin is tails, leave that square blank.


4. Move the finger to the next square to the right.


5. Flip the coin. If it is heads, draw a green dot on the square; if it is tails, leave it blank.


6. Repeat this process until the coin has been flipped once for each square in the grid.

Q7.1: Now the model forest is grown. Does it look to you like a real forest? How is it different?

We are now ready to start a fire in our model forest, and see how far it spreads. The fire starts along the left edge of the forest and spreads from one tree to another if they are neighbors (up, down, right, left) on the grid. Here's how it works:

7. Place a red circle around each of the green dots (trees) that lie along the left edge of the grid (forest). A tree with a red circle is on fire.


8. Look at each tree that is on fire. Is there an unburned tree in the next square to the right? If so, draw a red circle around it, indicating that it has caught fire.


9. Now look at each burning tree. Is there an unburned tree in any neighboring square (up, down, right, left) ? If so, draw a red circle around it. This is how the fire spreads. (We are assuming that fire does not spread directly between two trees that are diagonally next to one another.)


10. Continue to "spread'' the fire from each burning tree to any unburned tree in a neighboring square (up, down, right, left) until it can go no further, so that there is no unburned tree left next to a burning tree.

Q7.2: Now you have burned your forest. Did the fire percolate? That is, did it spread all the way from the left edge to at least one tree on the right edge? Or did it stop part way across the grid?

11. Talk to the other teams in your class. In what percentage of the forests did the fire percolate?

Grow a new forest on the same size grid, but this time roll a die for each square, placing a tree if the die comes up 1, 2, 3 or 4 and leaving the square blank if the die shows 5 or 6. Now predict what you expect to happen when you start a fire along the left edge. Will more or fewer fires burn across to the right side of the forest than before? Try it!

Previous: 7.1 - Percolation in Nature