Snag Decay Class Dynamics

This behavior controls transitions among snag decay classes and snag falls. There are two fall models, one that applies to trees that die in the current time step (and may fall without become a snag), and one that applies to pre-existing snags. Fall probability is a function of tree size, decay class, neighborhood basal area, and recent harvest activity. Trees and snags that do not fall are run through a snag decay class transition matrix.

Model forms are based on those in Vanderwel et al. 2006.

Parameters for this behavior

Parameter name	Description
Snag Decay Class Dynamics Snag Fall Alpha	Snag fall alpha parameter.
Snag Decay Class Dynamics Snag Fall Beta	Snag fall beta parameter.
Snag Decay Class Dynamics Snag Fall Eta	Snag fall eta parameter.
Snag Decay Class Dynamics Snag Fall Gamma 2	Snag fall gamma 2 parameter.
Snag Decay Class Dynamics Snag Fall Gamma 3	Snag fall gamma 3 parameter.
Snag Decay Class Dynamics Snag Fall Gamma 4	Snag fall gamma 4 parameter.
Snag Decay Class Dynamics Snag Fall Gamma 5	Snag fall gamma 5 parameter.
Snag Decay Class Dynamics Snag Fall Kappa	Snag fall kappa parameter.
Snag Decay Class Dynamics Snag Fall Zeta	Snag fall zeta parameter.
Snag Decay Class Dynamics Tree Fall Alpha	Tree fall alpha parameter.
Snag Decay Class Dynamics Tree Fall Beta	Tree fall beta parameter.
Snag Decay Class Dynamics Tree Fall Delta	Tree fall delta parameter.
Snag Decay Class Dynamics Tree Fall Iota	Tree fall iota parameter.
Snag Decay Class Dynamics Tree Fall Lambda	Tree fall lambda parameter.
Snag Decay Class Dynamics Tree Fall Theta	Tree fall theta parameter.
Snag Decay Class Dynamics Live To Class 1 Prob (0-1)	Probability of a snag that did not fall moving from live (killed this timestep) to decay class 1.
Snag Decay Class Dynamics Live To Class 2 Prob (0-1)	Probability of a snag that did not fall moving from live (killed this timestep) to decay class 2.
Snag Decay Class Dynamics Live To Class 3 Prob (0-1)	Probability of a snag that did not fall moving from live (killed this timestep) to decay class 3.
Snag Decay Class Dynamics Live To Class 4 Prob (0-1)	Probability of a snag that did not fall moving from live (killed this timestep) to decay class 4.
Snag Decay Class Dynamics Live To Class 5 Prob (0-1)	Probability of a snag that did not fall moving from live (killed this timestep) to decay class 5.
Snag Decay Class Dynamics Class 1 To Class 1 Prob (0-1)	Probability of a snag that did not fall remaining in decay class 1.
Snag Decay Class Dynamics Class 1 To Class 2 Prob (0-1)	Probability of a snag that did not fall moving from decay class 1 to decay class 2.
Snag Decay Class Dynamics Class 1 To Class 3 Prob (0-1)	Probability of a snag that did not fall moving from decay class 1 to decay class 3.
Snag Decay Class Dynamics Class 1 To Class 4 Prob (0-1)	Probability of a snag that did not fall moving from decay class 1 to decay class 4.
Snag Decay Class Dynamics Class 1 To Class 5 Prob (0-1)	Probability of a snag that did not fall moving from decay class 1 to decay class 5.
Snag Decay Class Dynamics Class 2 To Class 2 Prob (0-1)	Probability of a snag that did not fall remaining in decay class 2.
Snag Decay Class Dynamics Class 2 To Class 3 Prob (0-1)	Probability of a snag that did not fall moving from decay class 2 to decay class 3.
Snag Decay Class Dynamics Class 2 To Class 4 Prob (0-1)	Probability of a snag that did not fall moving from decay class 2 to decay class 4.
Snag Decay Class Dynamics Class 2 To Class 5 Prob (0-1)	Probability of a snag that did not fall moving from decay class 2 to decay class 5.
Snag Decay Class Dynamics Class 3 To Class 3 Prob (0-1)	Probability of a snag that did not fall remaining in decay class 3.
Snag Decay Class Dynamics Class 3 To Class 4 Prob (0-1)	Probability of a snag that did not fall moving from decay class 3 to decay class 4.
Snag Decay Class Dynamics Class 3 To Class 5 Prob (0-1)	Probability of a snag that did not fall moving from decay class 3 to decay class 5.
Snag Decay Class Dynamics Class 4 To Class 4 Prob (0-1)	Probability of a snag that did not fall remaining in decay class 4.
Snag Decay Class Dynamics Class 4 To Class 5 Prob (0-1)	Probability of a snag that did not fall moving from decay class 4 to decay class 5.
Snag Decay Class Dynamics Class 5 To Class 5 Prob (0-1)	Probability of a snag that did not fall remaining in decay class 5. This value is always 1.
Snag Decay Class Dynamics Maximum Snag Break Height	Maximum height at which snags break. The actual height is a random draw between this value and the minimum.
Snag Decay Class Dynamics Minimum Snag Break Height	Minimum height at which snags break. The actual height is a random draw between this value and the maximum.

How it works

Trees that died in the current timestep have a certain probability of falling before the end of the current timestep. (Note that trees that die in a mortality episode do NOT count as trees that died in the current timestep, but as existing snags.) The behavior represents the probability of falling before the end of the timestep as:

f(x) = α + β _spp + δ DBH + θ BA + ι H + λ (DBH * H)

where:

Pr(fall) is the probability of the tree falling
α is the Snag Decay Class Dynamics Tree Fall Alpha parameter
β _spp is the Snag Decay Class Dynamics Tree Fall Beta parameter for the tree's species
DBH is the DBH in cm
δ is the Snag Decay Class Dynamics Tree Fall Delta parameter
θ is the Snag Decay Class Dynamics Tree Fall Theta parameter
BA is the basal area (m2/ha) of live trees in the current tree's cell of the Snag Decay Class Dynamics Basal Area grid
ι is the Snag Decay Class Dynamics Tree Fall Iota parameter
H is whether (1) or not (0) a harvest occurred this timestep
λ is the Snag Decay Class Dynamics Tree Fall Lambda parameter

For snags that were created in a previous timestep, the behavior represents the probability of falling during the timestep as:

f(x) = α + β _spp + γ _dc + ζ ln(DBH) + η ln(DBH)² + κ BA_H

where:

Pr(fall) is the probability of the snag falling
α is the Snag Decay Class Dynamics Snag Fall Alpha parameter
β _spp is the Snag Decay Class Dynamics Snag Fall Beta parameter for the tree's species
γ _dc is the Snag Decay Class Dynamics Snag Fall Gamma X parameter, where X is the snag's decay class
DBH is the DBH in cm
ζ is the Snag Decay Class Dynamics Snag Fall Zeta parameter
η is the Snag Decay Class Dynamics Snag Fall Eta parameter
κ is the Snag Decay Class Dynamics Snag Fall Kappa parameter
BA_H is the basal area (m2/ha) of harvested trees in the current snag's cell of the Snag Decay Class Dynamics Basal Area grid

If a tree or snag does not fall, its condition at the end of the timestep will be represented by a snag decay class. Decay class 1 is the least decayed condition and decay class 5 is the most decayed. Parameters should be entered to specify the probability of going from a live tree or one of five decay classes to each of the higher decay classes over a five year timestep, given that the snag is still standing. The transition probabilities for each initial condition must sum to 1.

For all models, parameter values should correspond to the probability of fall for a five-year timestep. The behavior will re-scale the probability of fall for other timestep lengths if necessary.

Basal area amounts are tracked in the Snag Decay Class Dynamics Basal Area grid. Each timestep, the amount of basal area, in square meters per hectare, is totaled across all species for live trees (live adults and adults that died in the current timestep) and harvested trees. The size of the grid cells is approximately 400 square meters. A tree's neighborhood basal area is the value in the grid cell where the tree is located.

How to apply it

This behavior can be applied to adults and snags of any species. It should be executed after mortality and dead tree remover behaviors have been applied in each timestep.