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. If the temperature increases 1° Celsius in the next 100 years the southern border will move north 100 miles. That equates to one mile per year. The model asks you to enter degrees Fahrenheit. The formula "warming_per_100_years/1.8" converts from Celsius to Fahrenheit.
The second part of the model predicts how far the trees will be able to move north.
"Northern limit" starts with today's range. 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. Birds may carry the seeds north faster than this. The trees planted north of the maximum limit won't grow well because they are outside their preferred temperature range.
The second 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).
The model also calculates the range as it changes over the 100 years. The model takes the "Northern limit" and subtracts the "southern limit".
