ForSpace

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FORSPACE (Forest dynamics in spatially changing environments) is a model describing the principle processes that govern growth of plants and their dispersal, germination, establishment, and mortality. Herbivore birth and mortality and the dynamics body weight depend on consumption and costs.


General Concepts

FORSPACE is a spatial-explicit model that aims to give a realistic description of the processes that determine forest dynamics at a the scale of a landscape (up to 1000-ths of hectares). Gap-dynamics are an important aspect that determines the growth of individual trees and the succession of plant species in time. The forest dynamics as described in FORSPACE are also based on the dynamics of the vegetation on plots with a size of 20x20 m. However to be able to represent an entire landscape, FORSPACE does not track individual trees of shrubs but rather height cohorts of identical individuals per species. The vertical structure of a forest is represented by having different cohorts per species in the herb-, shrub- and tree layer. A cohort can shift to a higher layer (e.g. from shrub layer to tree layer) if it reaches the limit of its own layer and a cohort of individuals of the same species does not occur in a the higher layer. Thus, the cohort-structure has the consequence that height-growth may stagnate due to the presence of individuals of the same species in the higher layer. This is only realistic if the spatial resolution is limited to gaps that can contain maximally one or a few large adults trees. In such situations the tree layer may indeed physically block the height growth of the shrub layer. For a larger resolution, say 1 hectare, this assumption is not valid. For herbs and grasses, however, the concept of individuals cannot be maintained. Hence for these plant species the number of metre square that are occupied in the herb layer are simulated.

grazing

Grazing by herbivores is an important aspect of forest dynamics because they have a strong impact on the natural regeneration of seedlings from trees and shrubs. Moreover, some herbivore species graze heavily on saplings an thereby affect which tree species grow into the canopy, out of reach of large herbivores. On the other hand, the forest provides the food for the large herbivores, not only as foliage of the trees but also of the plants in the understory. The productivity of the herbs and grasses in the understory is driven by the amount of light available on the forest floor. Thus, there are strong interactions between the dynamics in the density of populations of herbivores and the dynamics in the density of herbs, grasses, shrubs and trees, and the succession of plant species within a plot. Therefore much attention has been paid to develop a mechanistic description of these interactions.

fire

As abiotic factor disturbing the autonomous development of the vegetation, a simple fire model was developed (Dam 1997, Van Goethem, 2000) based on fire models presented in the literature. Although fire is currently not an important factor in forests in the Netherlands, the forests may become more vulnerable for fire in the future due to global warming. Furthermore, small-scale prescribed burning may be used to create spatial heterogeneity and to avoid large wild fires.

dynamics

FORSPACE operates with a monthly time step. The static features of the model that do not change in time include properties of the herbivore species, the plant species, the number of patches forming the area of interest. The dynamic features of the model include the rates of change of the number and biomass of both juvenile and adult herbivores and the biomass of plants, and hence available metabolic energy for the herbivores. Output can be requested for all dynamic features of the model, for spatial variables as maps and but both as time series either as maptotals or mapaverage or as output for a single plot.

processes

The model consists of the description of continuous processes that are considered to be the most important for the dynamics of the populations herbivores and plants, and discrete events for both herbivores and plants. This results into interactions between plant species through competition for light, between herbivore species through competition for food, and between herbivores and plants through available metabolic energy for the herbivores and the alterations in biomass and structural features of the plants as a consequence of grazing. Abiotic factors such as fire disturb these interactions by the removal of plant biomass and by making space available for new individuals.

The continuous processes of the herbivores include: diet choice, digestion, respiration (including costs for transport), pregnancy, lactation and change of weight. The discrete events for the herbivores consider birth and mortality. It is assumed that the diet choice of the herbivores is based on the digestibility of the plants. Plots containing plants that are higher digestible have a better quality than plots containing plants with a lower digestibility. The amount of digestible plant material is not taken into consideration for the selection of a plot to avoid that plots with a large amount of food with a low digestibility are selected instead of plots containing a small amount of highly digestible food (cf. Van Wieren, 1996).

The continuous processes of the plants include: production and dispersion of seeds, interception of light, change of biomass of foliage, stalks or stems, branches and roots, change in crown radius, and height growth. The change in biomass and structural features are linked. The discrete events for the plant consider the conditions required for germination of seeds, as well as mortality. The rate of change in biomass of the plants directly depends on the amount of light absorbed. The light absorbed is converted into plant biomass, allowing an increase in plant height and crown radius. The processes are described for individual plants, which can be extrapolated to plot features by following the number of individual plants per plot. For herbs and grasses the concept of individuals is not relevant, so for these species their coverage, in m2 per plot, is considered. Increase of cover of herb- and grass-species is the consequence of either lateral expansion, based on growth and an optimal foliage density to increase in radius, or of germination and establishment of seedlings, if the conditions are favorable and seeds are available.

Further reading

  • Landscape forming processes and diversity of forested landscapes pdf
  • The interacting effects of ungulates and fire on forest dynamics pdf
  • Spatial interactions between ungulate herbivory and forest management pdf