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

# Growth and Resource-Based Mortality

This behavior calculates probability of survival as a function of growth and some second resource. The identity of the second resource is unimportant and could be anything, from exchangeable calcium levels to soil moisture.

Trees killed by this behavior will have a mortality reason code of "natural".

### Parameters for this behavior

Parameter nameDescription
Growth/Resource - Scaling Factor (rho)Scaling factor to reduce survival at the mode of the survival probability function.
Growth/Resource - Function Mode (mu)Determines the mode of the function along a gradient of the resource R (the mode is the optimal niche of a species).
Growth/Resource - Survival Increase with Growth (delta)Specifies the increase in survival caused by amount of growth.
Growth/Resource - Low-Growth Survival Parameter (sigma)Affects the shape of the survival probability distribution in low-growth conditions.

### How it works

The probability of survival for a tree is calculated with the following equation:

where:
• Prob is the annual probability of survival, as a value between 0 and 1
• R is the amount of the second resource
• G is the amount of radial growth, in mm/yr
• ρ is the Growth/Resource - Scaling Factor (rho) parameter, which is a scaling factor to reduce survival at the mode of the survival probability function
• μ is the Growth/Resource - Function Mode (mu) parameter, which determines the mode of the function along a gradient of the resource R (this corresponds to the optimal niche of a species, meaning where it is the top competitor, the absolute winner of competition)
• δ is the Growth/Resource - Survival Increase with Growth (delta) parameter, which specifies the increase in survival caused by amount of growth
• σ is the Growth/Resource - Low-Growth Survival Parameter (sigma) parameter, which affects the shape of the survival probability distribution in low-growth conditions

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.

The survival probability as calculated above is an annual probability. For multi-year timesteps, the timestep probability is APX, where AP is the annual probability and X is the number of years per timestep. Once a tree's timestep survival probability has been calculated, it is compared to a random number to determine whether the tree lives or dies.

### 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 growth behavior applied. You must also enter a map of second resource values into the Resource grid.