SORTIE-ND
Software for spatially-explicit simulation of forest dynamics

# Double resource relative growth

This behavior uses a double Michaelis-Menton function to calculate relative growth based on two resources: light and a second resource. The identity of the second resource is unimportant and could be anything, from exchangeable calcium levels to soil moisture.

### Parameters for this behavior

Parameter nameDescription
Asymptotic Diameter Growth (A)Asymptote of the Michaelis-Menton growth function at high light - function term A below.
Slope of Growth Response (S)Slope of the Michaelis-Menton growth function at zero light - function term S below.
Double resource - Influence of Resource (C)The parameter governing the influence of the second resource on the double-resource Michaelis-Menton equation.

### How it works

Relative growth is calculated with the equation: where:

• Y is the amount of annual relative growth
• A is the Asymptotic Diameter Growth (A) parameter
• S is the Slope of Growth Response (S) parameter
• C is the Double resource - Influence of Resource (C) parameter, in units appropriate to the value of R
• R is the amount of the second resource, in units appropriate to the value of C
• GLI is the global light index, calculated by a light behavior

Growth is compounded over multiple timesteps with the equation:

G = ((Y + 1)T - 1) * diam

where:

• G is the amount of diameter growth for the timestep, in cm
• diam is the diameter of the tree in cm (at 10 cm height if seedling or sapling, or DBH if adult)
• T is the number of years per timestep

Note that setting the C parameter in the equation above to 0 eliminates the second resource and makes this equivalent to the "Non-limited relative growth" behavior.

The amount of the second resource is captured in a grid object called Resource. Currently it is up to you to enter a map of the values for this resource grid; for instructions on how to do this, see the Grid Setup Window topic. This behavior does not in any way alter the values in this grid.

### How to apply it

This behavior can be applied to seedlings, saplings, and adults of any species. Any tree species/type combination to which it is applied must also have a light behavior applied. You must also enter a map of second resource values into the Resource grid. You can use either the diam with auto height or diam only version.