Weibull climate growth

This behavior calculates tree growth as a function of climate and larger neighbor trees. A tree has a maximum potential growth rate that is reduced due to several possible factors.

Parameters for this behavior

Parameter nameDescription
Weibull Climate Growth - Competition Effect "C" The C parameter for the competition effect.
Weibull Climate Growth - Competition Effect "D" The D parameter for the competition effect.
Weibull Climate Growth - 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 Growth - Precip Effect "A" The A parameter for the precipitation effect. Units of precipitation are millimeters per year.
Weibull Climate Growth - Precip Effect "B" The B parameter for the precipitation effect. Units of precipitation are millimeters per year.
Weibull Climate Growth - Precip Effect "C" The C parameter for the precipitation effect. Units of precipitation are millimeters per year.
Weibull Climate Growth - Temp Effect "A" The A parameter for the temperature effect. The effect is based on mean annual temperature in degrees Celsius.
Weibull Climate Growth - Temp Effect "B" The B parameter for the temperature effect. The effect is based on mean annual temperature in degrees Celsius.
Weibull Climate Growth - Temp Effect "C" The C parameter for the temperature effect. The effect is based on mean annual temperature in degrees Celsius.
Weibull Climate Growth - Max Neighbor Search Radius (m) The maximum distance, in m, at which a neighboring tree has competitive effects on a target tree.
Weibull Climate Growth - Max Potential Growth (cm/yr) Maximum potential diameter growth for a tree, in cm/yr.
Weibull Climate Growth - 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 Growth - Size Effect X0 The mode of the size effect curve.
Weibull Climate Growth - Size Effect Xb The variance of the size effect curve.
Weibull Climate Growth - 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.

How it works

For a tree, the amount of diameter growth per year is calculated as:


Growth = Max Growth * Size Effect * Precipitation Effect * Crowding Effect * Temperature Effect

Max Growth is the maximum diameter growth the tree can attain, in cm/yr, entered in the Weibull Climate Growth - Max Potential Growth (cm/yr) parameter. Size Effect, Precipitation Effect, Crowding Effect, and Temperature Effect are all factors which act to reduce the maximum growth rate 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:

NCI size effect equation

where:

You can set a minimum DBH for the size effect in the Weibull Climate Growth - 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:

PE = Weibull climate growth precipitation effect equation

where:

Temperature Effect is calculated as:

TE = Weibull climate growth temperature effect equation

where:

Crowding Effect is calculated as:

Weibull climate growth crowding effect equation

where:

The ND value is a count of all larger neighbors with a DBH at least that of the Weibull Climate Growth - Minimum Neighbor DBH, in cm parameter, out to a maximum distance set in the Weibull Climate Growth - 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 amount of growth is in cm/year. For multi-year timesteps, the behavior will calculate total growth with a loop. Each loop iteration will increment DBH for one year. For each year, any portion of the growth equation with DBH as a term is recalculated with the previous year's updated DBH value.

How to apply it

This behavior can be applied to saplings and adults of any species. It cannot be applied to seedlings. You can use either the diam with auto height or diam only version.