You could to this by defining separate Transition Types that only apply to certain states. For example you could have Transition Types “Firewood Harvest: Late Seral” and “Firewood Harvest: Mid Seral” and set Transition Targets separately for each Type. This would allow you to set different targets for each state class – however the targets for each would be independent of each other. You could also put the two Types in a single Transition Group (say called “Firewood Harvest”) and then set a single target for this group – this would be a more typical way of doing things, as generally people harvesting firewood don’t set targets independently for different seral stages. If the Transition Probabilities for the two Transition Types are the same then the model will distribute this target with equal probability to all cells on the landscape, and thus will harvest firewood from different seral stages in proportion to their relative abundances at any point in time. This is often a very reasonable assumption for many systems. However you can also use these Transition Probabilities to favor one Type over the other – for example, setting the Probability for “Firewood Harvest: Late Seral” to be twice that of “Firewood Harvest: Mid Seral” (in conjunction with setting a Transition Target for the “Firewood Harvest” Transition Group) will have the effect of assigning twice the probability of harvest to Late Seral cells as those that are Mid Seral. The challenge here is deciding on these relative probabilities, as rarely are they known.
A more common way to direct timber harvest across a landscape is to use Transition Attribute Targets. These allow you to express targets in terms of volume (or biomass) of wood harvested, rather than area harvested. With this approach you provide volume curves over age for each seral stage and the model uses these to calculate the harvest. As cells in the Late Seral stage will typically have greater volume than those in the Mid Seral stage, these cells will contribute more to the total harvest. In most forested systems this is much more realistic approach to modeling timber harvest, as a stakeholder harvesting firewood would generally target a certain number of bush cords of wood (i.e. volume), rather than an area each year. Note that this feature is covered (with a timber harvest example) in Exercise 6 of the self-directed online ST-Sim course.
- This reply was modified 1 year, 3 months ago by Colin Daniel.