Explorations

For these questions, please select Half Hectare from the Scenarios menu. The Tiny Demo has so few trees that dumb luck is decisive. Watch Half Hectare run a couple times so you know what tends to happen.

These questions explore the ramifications of the basic model as described in Details. If you'd prefer to take the growth model on faith and instead explore by stressing the conditions (introducing landscape fragments or death sources, etc.), please consult the Options file instead.

  1. There are a lot of parameters in this simulator, mostly describing the growth habits of trees. Let's zero in on a decisive one: mortRandom. This is a tree's random chance of dying each year, before light competitions, etc. Since each species' parameters are on different popup menus, it's not obvious that mortRandom for Hemlock is one half that for Yellow Birch and Maple. Or, on average, Hemlock lives four times as long. Set it to match Yellow Birch and Maple (.01), then select Restart. What's different?
  2. Maybe that result had to do with Yellow Birch's quickness to reproduce. Try giving Hemlock an edge by setting its initNumber way higher than Birch, while keeping its mortRandom equal to that of Birch. Does it matter? Should it matter in the long run?
  3. Observation: The dominant reason a tree is here today, is because it was here yesterday. You can easily find wild raspberries next year by returning to where you found them this year. Seems trite, but it's predictable. Please consider: how do you trade off the really obvious with seemingly deep competitive issues?
  4. It has been asserted that Hemlock wins in forest succession because of its dark, dark shadow strangling younger trees. Please select Half Hectare from the Scenarios popup again to reset all parameters to their starting position, and press Pause to stop the simulation. Then get Hemlock's parameters and set its lightCoeff to exactly match Yellow Birch and Maple (lightCoeff=0.399). This makes Hemlock's shadow as light as the others'. Note that mortRandom is back to 0.005 for Hemlock. When does Hemlock outstrip Birch? Try it a couple times to get the range. Now go back to basic Half Hectare. When does Hemlock outstrip Birch? Is shadow darkness (lightCoeff) more or less decisive than lifespan (mortRandom) in this simulator?
  5. Make a prediction about what will happen if all random death is turned off (mortRandom=0 for all 3 species, Hemlock's shadow darkness is back to its default.) Test your theory.

Ginger Booth, December 14, 1999