Software for spatially-explicit simulation of forest dynamics

Episodic Mortality

Trees removed by this behavior will have a mortality reason code of "disease".

How it works

The Episodic Mortality behavior allows you to replicate tree-killing events with the same level of control you have when defining Harvest events. A planned mortality episode can simulate disease, an insect outbreak, fire, or the like. The main difference between Harvest and Episodic Mortality is that the Episodic Mortality behavior can create snags, or standing dead trees. A large snag proportion can significantly affect the light and substrate dynamics of a SORTIE run.

Defining a mortality episode is like defining a partial cut harvest. (Mortality episodes have no automatic impact on substrate dynamics like harvest events do, although the newly dead trees may be a source of harvest input.) You can define up to four size classes, and specify the amount of trees to kill in one of four ways: as a percentage of total basal area, as an absolute amount of basal area, as a percentage of total tree density, or as an absolute amount of tree density.

When the Episodic Mortality behavior is determining which trees to remove, it starts by finding the largest tree in the area of the plot affected by the mortality episode. It works its way through the trees from largest to smallest, assessing whether to kill each one until it either runs out of trees or reaches its cut target. This process preferentially removes the largest trees in each size range, unless the event is defined by a percentage of density, in which case all trees in the target size ranges have an equal probability of being killed. If Episodic Mortality is removing a percentage of basal area or an absolute amount of basal area, it will only kill a tree if its basal area will not cause the total to be more than the target. This means that, for basal-area-defined cuts, the behavior may skip some bigger trees and cut smaller ones in order to more exactly cut its target. Each species is cut separately. So, a request to remove 20% of three species will remove 20% of each of them, no matter what their relative proportions to each other.

Seedlings can also be killed as part of a planned mortality episode. You can specify the proportion of seedlings that are killed within the target area for each species. The seedlings of a species can be killed even if that species is not otherwise participating in the episode. Seedlings in the target area are randomly chosen to die based on the mortality rate for their species. They are given "disease" as a mortality reason.

What happens to dead trees depends on the rest of the run. If there are other behaviors in the run that deal directly with snags or create them, then the run is "snag-aware". In this case, all adult trees killed are turned into snags (saplings never become snags). If the run is not "snag-aware", then the trees are marked as dead. When/if the dead tree remover behavior runs, the dead trees will be removed at that time. These dead trees are available as input to Substrate.

The actual amount of trees killed may not be exactly what was specified, since the Episodic Mortality behavior can't remove part of a tree to get the numbers right. The behavior stores how much it actually cut each timestep in the Mortality Episode Results grid. To optimize the accuracy of the behavior, use larger kill ranges and high proportions of the plot area to make sure there is a big pool of trees to choose from.

How to apply it

Add the behavior to the run through the Model Flow window. You do not apply it to particular trees at that time. The behavior setup will allow you to design your mortality episodes.