F r

Figure 10.10 Results of the spatial simulation of succession, Alp Stabelchod. First row: temporal model only. Second row: spatial diffusion only. Third row: spatial and temporal processes combined. The sequence of white towards dark grey accords with succession states: Aconitum (white pixels) - Trisetum - Deschampsia - Festuca -Carex - Pinus (dark grey shading).

Figure 10.10 Results of the spatial simulation of succession, Alp Stabelchod. First row: temporal model only. Second row: spatial diffusion only. Third row: spatial and temporal processes combined. The sequence of white towards dark grey accords with succession states: Aconitum (white pixels) - Trisetum - Deschampsia - Festuca -Carex - Pinus (dark grey shading).

The persistence of the meadow through subsequent successional guilds and finally Pinus forests lasts about 500 years in simulations using the temporal model. Because of the lack of spatial interactions, vegetation boundaries do not move and only vegetation composition changes. As a result, the pattern formed by the edges remains unchanged over the entire simulation time. This can be seen in all states of the simulation run shown in the first row of Figure 10.10. Vegetation boundaries finally only vanish because all plots reach the final state of succession.

The effect of spatial exchange among pixels can be simulated in isolation. Assuming an extremely low rate of exchange of d = 0.001, the state of the system after 320 years is shown in the middle row in Figure 10.10. Overall composition is almost the same as at the beginning, but the compositional pattern of the maps has become more homogeneous compared to the initial state. Along the forest edges, Pinus mugo has invaded the first row of cells. Other types have spread as well. The Aconitum stage (white cells) has increased in surface. The spatial process changes the state of the meadow very slowly and does not explain the results from the permanent plot survey.

Finally, the spatial and temporal processes are run simultaneously. This accelerates the simulation of succession considerably and the meadow is almost covered by Pinus mugo after 320 years (Figure 10.10, bottom series). It can now be seen that vegetation boundaries have moved and differ from their initial state. The diffusion process causes Pinus to invade the meadow from the edges towards the centre.

The lesson to be learned from this simulation exercise is that neither the temporal model alone nor the spatial is suited to explain succession. In order to understand the major processes invasion has to be allowed, suggesting a spatial component be added to the temporal model. While simple models may be easy to handle and understand, extra complexity is often indispensable.

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