Presets
Playback
Environment
Nitrogen supply and recycling.
Competition
Light parameters that decide who wins under shade.
Fire
Repeated disturbance resets the tall, slow trees.
Layers
Model checks
The science
Resource competition for nitrogen
Three functional types (pioneers: lichen, moss and grasses; shrubs; trees) take up soil mineral nitrogen N at a Monod rate and lose biomass at mortality mi. Everything is measured in g N/m2 so nitrogen is conserved exactly: dBi/dt = Vi N/(Ki + N) fi Bi − mi Bi + seedi, dLit/dt = Σ mi Bi − d Lit, dN/dt = dep + fix + d Lit − Σ uptake − leach N.
The R* rule (Tilman)
Alone in full light a type draws mineral N down to N* = K m/(V − m). When types compete for one resource, the one with the lowest N* wins in the end: here the trees, which is why the sequence ends in forest.
Light and facilitation
Taller types shade shorter ones (Beer-Lambert): light at the pioneers is exp(−apB0 − asB1 − atB2), and each light factor is f = I (1 + KL)/(KL + I). Pioneers fix atmospheric nitrogen, down-regulated by soil N, which builds litter and mineral N and enables shrubs and trees (facilitation). A fire kills a fraction of living biomass; 40 % of its nitrogen is lost to the air and the rest becomes litter.
Honest simplifications
- Three functional types, one nutrient, no water, constant seed rain, no individual trees (the drawn plants are a picture of the three biomass pools).
- Parameters are illustrative, in the range of textbook succession models, not fitted to a site. Successional timing depends on them.
What to try
- Run the baseline: pioneers peak first, shrubs next, trees dominate after about 150 years.
- Press "Fire every 25 years" and watch the fire-regime graph: frequent fire keeps trees out.
- Press "Shade-intolerant trees": shrubs hold the site much longer.
- Use the disturb button on a mature forest and compare with the clear-cut preset.
- Watch the mineral-N curve sink towards the lowest R* line as the winner takes over.