|Summary||What is sustainability?||
|General description||Examples for model output||
Running the model.
Running the SUFOR modell will yield several graphs and tables. Each number must be interpreted as answers to the question of what happens if a forest with characteristics (area, species etc.) provided by the user will be managed according to the scenario that is also set by the user.
In order to demonstrate the possibilities and limitation of the model, how it works and what outputs it produces,
below are some examples concerning the scenarios that can be set in the section of
Defining scenarios. One can analyse what happens in a
natural forest; in a forest managed according to the principles of the
sustained yield concept; but also what happens in a forest managed with
overshoot , which is of course unsustainable in the long term.
Before starting the analyses, one has to set the structure of the forest to be modelled , which can be of different type. One is when the area of all age classes is the same; another when young forests predominate; and yet another when old forests predominate at the beginning of the simulation. (A subtype has the distribution of the forest area of the Hungarian forests in the age classes. This, just like most of the model, is for demonstration only as the model is only able to simulate processes for one tree species only, which does not reflect the diversity of 150 taxa that are grown in the Hungarian forests. However, this does not have a major effect on the possible conclusions with regard to whether a system is sustainable or not.)
The result of the simulation, at least during the period analysed, does really depend on the age class distribution.
Nevertheless, the results are worth analyising in two figures. One shows the status of forest characteristics (the amount of standing volume and dead wood) in each year of the simulation, the other annual changes (wood increment, harvest and mortality).
This can easily be answered by simulating a "management" of zero harvest in the forest.
When there is any type of active management in the forest, the simulation must begin with setting up a "natural" forest that is not managed. This is done so that we can compare the characteristics of the managed forest to that of an unmanaged one. This setting up the natural forest ("running up") does happen automatically.
When stocks and changes of natural forest are compared to those of managed forest we see that they are time-independent in case of an even area distribution, but are time-dependent in case the area is covered by predominantly young or old forests:
Below the characteristics of the area with even distribution is analysed further, because the effects of the various management options are more expressed.
Harvest levels of a larger forest area under the sustained yield concept are smaller than the (mean) annual increment. If the model is run with setting such levels, then - after the run-up period of 240 years that is required to start the simulation - the following graph is produced:
The graph is easy to interpret: when a forest is managed under the sustained yield concept, neither volume stocks nor increments are reduced. However, mortality (the amount of deadwood that is produced) declines, and because of this, the amount of deadwood also declines. This is because trees that would die and become dead wood in lack of thinning are removed during thinning. (Note that it is not modelled here what the long-term effects of the reduction of the amount of deadwood on the productivity of the forest as there is little knowledge about that. We know, however, that removing deadwood from the forest reduces biodiversity which raises sustainability issues in a broad sense.)
This can e.g. be modelled by using the harvest levels that are incorporated in the model, and which represents the same overshooting, or unsustainable ecological footprint, which the Earth has had to endure recently. In the model, this unsustainable harves level is brought back to the maximum sustainable level in 30 years, but other periods can also be set. The effects of such unsustainable levels are demonstrated in the next graph. (The harvest "footprint", i.e. the level of harvests relative to current annual increment in this simulation is assumed to be the same for thinnings and final harvests.):
In order to compare the characteristics of natural forests, as well as forests managed in a sustainable and an unsustainable way, the following two sets of graphs can be considered (stocks are shown in the upper set, wehreas changes are shown in the lower set):
It is obvious that whereas neither stocks nor increments are reduced in the sustainable management (it is sustainable just because of this), unsustainable management substantially reduces both stocks as well as increments. The extent of the reduction and its pattern over time, as well as the possible recovery of the system depends on the extent of the overshoot over time. However, recovery happens only after a long delay. The amount of deadwood is also worth attention as it is also considerably reduced under an unsustainable management.
This webpage was last modified by Zoltan Somogyi 18 June 2012.