This behavior assesses tree survival as a function of climate and larger neighbor trees. A tree has a maximum potential annual probability of survival that is reduced due to several possible factors.
Trees killed by this behavior will have a mortality reason code of "natural".
Parameter name | Description |
---|---|
Weibull Climate Survival - Competition Effect "C" | The C parameter for the competition effect. |
Weibull Climate Survival - Competition Effect "D" | The D parameter for the competition effect. |
Weibull Climate Survival - Competition Gamma | The gamma parameter for the competition effect. This controls the response of a target tree to competition as a function of its size. |
Weibull Climate Survival - Max Survival Prob (0-1) | The maximum possible annual probability of survival for a target tree, expressed as a probability between 0 and 1. |
Weibull Climate Survival - Precip Effect "A" | The A parameter for the precipitation effect. Units of precipitation are millimeters per year. |
Weibull Climate Survival - Precip Effect "B" | The B parameter for the precipitation effect. Units of precipitation are millimeters per year. |
Weibull Climate Survival - Precip Effect "C" | The C parameter for the precipitation effect. Units of precipitation are millimeters per year. |
Weibull Climate Survival - Size Effect X0 | The mode of the size effect curve. |
Weibull Climate Survival - Size Effect Xb | The variance of the size effect curve. |
Weibull Climate Survival - Temp Effect "A" | The A parameter for the temperature effect. The effect is based on mean annual temperature in degrees Celsius. |
Weibull Climate Survival - Temp Effect "B" | The B parameter for the temperature effect. The effect is based on mean annual temperature in degrees Celsius. |
Weibull Climate Survival - Temp Effect "C" | The C parameter for the temperature effect. The effect is based on mean annual temperature in degrees Celsius. |
Weibull Climate Survival - Max Neighbor Search Radius (m) | The maximum distance, in m, at which a neighboring tree has competitive effects on a target tree. |
Weibull Climate Survival - Minimum Neighbor DBH (cm) | The minimum DBH for trees of that species to compete as neighbors. Used for all species, not just those using Weibull Climate growth. |
Weibull Climate Survival - Size Effect Minimum DBH | The minimum possible DBH for size effect. Trees with a DBH less than this value will use this value in the size effect calculation instead. |
For a tree, the annual probability of survival is calculated as:
Max Survival Probability is the maximum possible annual survival probability, entered in the Weibull Climate Survival - Max Survival Prob (0-1) parameter. Size Effect, Precipitation Effect, Crowding Effect, and Temperature Effect are all factors which act to reduce the maximum survival probability and will vary depending on the conditions a tree is in. Each of these effects is a value between 0 and 1.
Size Effect is calculated with a lognormal function, as follows:
where:
You can set a minimum DBH for the size effect in the Weibull Climate Survival - Size Effect Minimum DBH parameter. Any target tree whose DBH is less than this value will get a size effect based on the minimum DBH instead. This allows you to avoid problems with very small trees that can occur because of the shape of the lognormal function.
Precipitation Effect is calculated as:
where:
Temperature Effect is calculated as:
where:
Crowding Effect is calculated as:
where:
The ND value is a count of all larger neighbors with a DBH at least that of the Weibull Climate Survival - Minimum Neighbor DBH, in cm parameter, out to a maximum distance set in the Weibull Climate Survival - Max Neighbor Search Radius (m) parameter. The value is a straight count - it is not scaled or relativized in any way. Seedlings never compete.
The probability of survival is for a single year. For multi-year timesteps, the timestep survival probability is the annual probability raised to the power of the number of years per timestep. Once the probability has been calculated, a random number is used to determine whether a tree lives or dies.
This behavior can be applied to saplings and adults of any species. It cannot be applied to seedlings.