Snag dynamics behaviors
In this document:
Snag dynamics parameters
Snag Decay Class Dynamics behavior
Snag dynamics parameters
- Snag Decay Class Dynamics Snag Fall Alpha Snag fall alpha parameter. Used by the Snag Decay Class Dynamics behavior.
- Snag Decay Class Dynamics Snag Fall Beta Snag fall beta parameter. Used by the Snag Decay Class Dynamics behavior.
- Snag Decay Class Dynamics Snag Fall Eta Snag fall eta parameter. Used by the Snag Decay Class Dynamics behavior.
- Snag Decay Class Dynamics Snag Fall Gamma 2 Snag fall gamma 2 parameter. Used by the Snag Decay Class Dynamics behavior.
- Snag Decay Class Dynamics Snag Fall Gamma 3 Snag fall gamma 3 parameter. Used by the Snag Decay Class Dynamics behavior.
- Snag Decay Class Dynamics Snag Fall Gamma 4 Snag fall gamma 4 parameter. Used by the Snag Decay Class Dynamics behavior.
- Snag Decay Class Dynamics Snag Fall Gamma 5 Snag fall gamma 5 parameter. Used by the Snag Decay Class Dynamics behavior.
- Snag Decay Class Dynamics Snag Fall Kappa Snag fall kappa parameter. Used by the Snag Decay Class Dynamics behavior.
- Snag Decay Class Dynamics Snag Fall Zeta Snag fall zeta parameter. Used by the Snag Decay Class Dynamics behavior.
- Snag Decay Class Dynamics Tree Fall Alpha Tree fall alpha parameter. Used by the Snag Decay Class Dynamics behavior.
- Snag Decay Class Dynamics Tree Fall Beta Tree fall beta parameter. Used by the Snag Decay Class Dynamics behavior.
- Snag Decay Class Dynamics Tree Fall Delta Tree fall delta parameter. Used by the Snag Decay Class Dynamics behavior.
- Snag Decay Class Dynamics Tree Fall Iota Tree fall iota parameter. Used by the Snag Decay Class Dynamics behavior.
- Snag Decay Class Dynamics Tree Fall Lambda Tree fall lambda parameter. Used by the Snag Decay Class Dynamics behavior.
- Snag Decay Class Dynamics Tree Fall Theta Tree fall theta parameter. Used by the Snag Decay Class Dynamics behavior.
- 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. Used by the Snag Decay Class Dynamics behavior.
- 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. Used by the Snag Decay Class Dynamics behavior.
- 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. Used by the Snag Decay Class Dynamics behavior.
- 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. Used by the Snag Decay Class Dynamics behavior.
- 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. Used by the Snag Decay Class Dynamics behavior.
- 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. Used by the Snag Decay Class Dynamics behavior.
- 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. Used by the Snag Decay Class Dynamics behavior.
- 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. Used by the Snag Decay Class Dynamics behavior.
- 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. Used by the Snag Decay Class Dynamics behavior.
- 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. Used by the Snag Decay Class Dynamics behavior.
- 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. Used by the Snag Decay Class Dynamics behavior.
- 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. Used by the Snag Decay Class Dynamics behavior.
- 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. Used by the Snag Decay Class Dynamics behavior.
- 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. Used by the Snag Decay Class Dynamics behavior.
- 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. Used by the Snag Decay Class Dynamics behavior.
- 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. Used by the Snag Decay Class Dynamics behavior.
- 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. Used by the Snag Decay Class Dynamics behavior.
- 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. Used by the Snag Decay Class Dynamics behavior.
- 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. Used by the Snag Decay Class Dynamics behavior.
- 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. Used by the Snag Decay Class Dynamics behavior.
- 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. Used by the Snag Decay Class Dynamics behavior.
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. Used by the Snag Decay Class Dynamics behavior.
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.
How it works
For trees that died in the current timestep, the behavior represents the probability of falling before the end of the timestep as:

f(x) = α + βspp + 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
- βspp 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 + βspp + γdc + ζ ln(DBH) + η ln(DBH)2 + κ BAH
where:
- Pr(fall) is the probability of the snag falling
- α is the Snag Decay Class Dynamics Snag Fall Alpha parameter
- βspp is the Snag Decay Class Dynamics Snag Fall Beta parameter for the tree's species
- γdc 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
- BAH is the basal area (m2/ha) of harvested trees in the current snag's cell of the Snag Decay Class Dynamics Basal Area grid
Trees and snags that fall are removed completely from SORTIE and are not available for processes such as substrate.
If a tree or snag does not fall, it's 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 snags of any species. It should be executed after mortality and dead tree remover behaviors have been applied in each timestep.
Last updated: 16-Nov-2007 12:48 PM