Will Global Warming Push Trees to Extinction?

You will use a STELLA model to answer the question, "Which trees and plants will be threatened with extinction by global warming?" The model calculates the change in the tree's geographic range. The model predicts how far the northern and southern boundary of the range will move.

First look at how the model calculates the movement of the southern boundary. The stock "southern limit" starts the simulation at 0. Each year the flow, "shrink north" adds a mile or two to the southern limit.

Double clicking on "shrink north" shows its algroithm: warming_per_100_years/1.8. The slider "warming per 100 years" lets you enter the degrees Fahrenheit that you think the temperature will rise. "Shrink north" divides degrees Fahrenheit by 1.8 to convert it to Celsius.

If the temperature increases 1 degree Celsius in the next 100 years the southern border will move north 100 miles. That equates to one mile per year. The flow "Shrink north" increases the stock "southern limit" by one mile for each degree Celsius. If teh temperature rises 3.5 degrees Celsius, then Southern limit moves morth 3.5 miles each year.

The second part of the model predicts how far the trees will be able to move north.

Before you run the model you need to input the range for a tree species. Type the tree's range into the "initial northern range" Input Device. Click outside of the device to activate the new range.

Before the start of the run, input a number indicating the range north of the "southern limit". now the stock "Northern limit" starts with today's range. The flow "expand north", adds miles to teh "Northern limit" each year.

There are 2 parts to the calculation in the flow "expand north'. First "warming_per_100_years"/1.8 sets the maximum that the trees can move north. A faorable temperature will move the possible range this number of miles north. Birds may carry the seeds north faster than this.

The second part of the calculation is how fast the trees can move north. The trees take a step north every time a tree seed is planted further north. The next step has to wait until the seed grows into a tree that produces seeds.

The 2 parts of the calculation are how many years between steps and how far is each step. The converter, "years to reseed", holds the number of years between steps. The converter "seed propagation" holds how far each step is.

The flow "expands north" calculates the 2 values and picks the smaller of the 2. This smaller value is added to "Northern limit". The formula inside the flow "expands north" is: if(seed_propagation/years_to_reseed>warming_per_100_years/1.8) then(warming_per_100_years/1.8)else(seed_propagation/years_to_reseed).

Why does the algorithm need to pick the smaller of the two values?

The model also calculates the range as it changes over the 100 years. The model takes the "Northern limit" and subtracts the "southern limit".

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