Weibull snag mortality behavior

This behavior controls snag fall. Snags are standing dead trees. Obviously, they can't die again, so the word "mortality" is a bit of a misnomer. We call the behavior a mortality behavior because it functionally fits in this behavior class.

Snags that are "killed" by this behavior are considered to have fallen over. If Substrate behaviors are enabled, then these fallen trees are available to become new fresh log substrate. Any that are not picked up by substrate will be dealt with by the Dead tree remover behavior.

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

Parameters for this behavior

Parameter name	Description
Weibull Annual "a" Parameter for Snag Size Class 1 Mortality	Weibull annual "a" parameter for those trees whose DBH is less than or equal to the value in "Upper DBH of snag size class 1".
Weibull Annual "a" Parameter for Snag Size Class 2 Mortality	Weibull annual "a" parameter for those trees whose DBH is greater than the value in "Upper DBH of snag size class 1" but less than or equal to the value in "Upper DBH of snag size class 2".
Weibull Annual "a" Parameter for Snag Size Class 3 Mortality	Weibull annual "a" parameter for those trees whose DBH is greater than the value in "Upper DBH of snag size class 2".
Weibull Annual "b" Parameter for Snag Size Class 1 Mortality	Weibull annual "b" parameter for those trees whose DBH is less than or equal to the value in "Upper DBH of snag size class 1".
Weibull Annual "b" Parameter for Snag Size Class 2 Mortality	Weibull annual "b" parameter for those trees whose DBH is greater than the value in "Upper DBH of snag size class 1" but less than or equal to the value in "Upper DBH of snag size class 2".
Weibull Annual "b" Parameter for Snag Size Class 3 Mortality	Weibull annual "b" parameter for those trees whose DBH is greater than the value in "Upper DBH of snag size class 2".
Weibull Upper DBH of Snag Size Class 1	The upper DBH value of trees in size class 1.
Weibull Upper DBH of Snag Size Class 2	The upper DBH value of trees in size class 2. Trees with a value greater than this are considered to be in size class 3.

How it works

The behavior uses a Weibull function to determine the number of snags of a certain age left standing at a given time. The equation is:

where:

S is proportion of snags still standing, between 0 and 1
a and b are Weibull parameters (Weibull Annual "a" Parameter for Snag Size Class X Mortality parameter and Weibull Annual "b" Parameter for Snag Size Class X Mortality parameter)
T is the snag age in years

Different sizes of snags fall at different rates. This behavior allows you to define three snag size classes and enter different "a" and "b" parameters for each.

A random number is used against this equation for a given tree to determine if it falls in the current timestep.

How to apply it

This behavior can be applied to snags of any species.