Forest demography
How trees grow, die and are born in ForestStudio
A forest in ForestStudio is not an average or a curve. It is every single tree, each with its own species, size and neighbours, living through one simulated year after another.
Three things can happen to a tree population: trees grow, trees die, and new trees arrive. We call this forest demography. Everything else you see in a simulation, from basal area to timber revenue, is the sum of these three processes playing out tree by tree.
Growth
How much thicker this year?
A species’ potential, held back by crowding from its neighbours.
Mortality
Does the tree survive?
A yearly risk that rises with competition, poor growth and drought.
Regeneration
Who fills the gaps?
Seeds travel from mature trees and establish where climate and light allow.
A forest made of individual trees
The simulation starts from the forest as it stands today. Each tree enters the model with what a forester would record in the field, and the model works out the rest:
What each tree carries
Its species, age, diameter, a vitality score from 1 (very weak) to 5 (very vigorous), and where it stands.
What the model derives
Its height, stem volume and basal area, and each year how much its diameter has grown.
Where it lives
In a small (~\(75m^2\)) hexagonal cell of the landscape.
One year at a time. The forest is updated once per year. Behind that yearly step, climate and soil water are followed month by month, and some effects are remembered across years: a tree that has been through several dry summers does not start the next one as if nothing had happened.
Growth
Every year, each adult tree adds to its diameter. How much depends on two questions, asked in turn:
Species and size
What could this tree do?
Each species has a best-case growth rate. Trees keep it up while they are young and mid-sized, then slow down steadily as they approach the largest diameter their species can reach, where growth stops.
Neighbours
How crowded is it?
The model adds up the trees in the tree’s own cell and in two rings of surrounding cells, 19 cells in all. Close neighbours count more than distant ones. The more crowded it is, the less the tree grows, down to nothing in a very dense stand.
So a beech in a gap races ahead, while the same beech squeezed between large neighbours barely moves. Nobody tells the model which trees dominate: that emerges from the neighbourhood. It is also why thinning works in the model the way it does in the forest: remove a neighbour, and the trees around it respond.
Mortality
Every year, each tree faces a risk of dying. The model builds that risk from two sources and then lets chance decide.
Always present
Background mortality
The everyday risk in any forest. It depends on the tree’s species and size, on how well it has been growing, on how dense the stand is, and on the local climate. A tree that has almost stopped growing under heavy competition is much more likely to die than a vigorous one.
In dry periods
Drought stress
An extra risk on top. It builds up in two ways: slowly, through dry months and consecutive dry years that wear a tree down, and suddenly, through an exceptionally dry year combined with dry soil in the previous autumn or in spring.
Three ideas make the drought part realistic:
- Species differ. Each species has its own drought tolerance, so the same summer is a hard one for spruce and an ordinary one for oak.
- Drought is judged against the local climate. A dry late summer is normal in a Mediterranean forest and alarming in a moist mountain one. The model compares each year with what is usual for that place.
- Trees have a memory. Vitality carries the history of a tree. A weakened tree copes worse with the next stress, so deaths can follow a drought by several years, as they do in real forests.
Why chance? The model does not decide that “3% of the trees die”. It gives every tree its own probability and draws a random outcome for each one. Two identical trees side by side can meet different fates, which is what happens in a real stand, and it is why two runs of the same scenario are never tree-for-tree identical.
Regeneration
New trees do not appear from nowhere. A species can only turn up in a cell if its seeds get there, and then only if the place suits it. Regeneration follows the path of a seed:
Seed production
Only trees past their species’ age of maturity produce seed.
Seed dispersal
Most seeds land close to the parent, a few travel far. Many seed trees nearby add up.
Climate check
Winters must not be too cold for the species, and the growing season must be warm enough.
Browsing
Deer and other browsers can wipe out the seedlings of a cell in a given year.
Establishment
The more light reaches the ground, the more young trees make it.
The result is a forest that renews itself in a believable way. Species spread outwards from where they already grow instead of appearing everywhere at once. A cold site filters out species that cannot stand its winters. A closed, dense stand lets few young trees through, while a gap left by a dead or harvested tree fills quickly. And where browsing pressure is high, which you can set for the landscape, regeneration is patchy.
Young trees join the simulated population once they reach 7 cm in diameter. From that moment they grow, compete and face mortality like every other tree.
Watch it happen
The three processes are simple on their own. The interesting part is what they produce together: crowding, self-thinning, gaps, and new trees filling them.
Try it: a small forest, year by year
Each hexagon holds one tree, drawn larger as it grows. Press play, then switch on dry years and see who suffers and who moves in.
- Broadleaf: slower, shade-tolerant, copes with drought
- Conifer: faster, needs light, sensitive to drought
- A tree that died this year
Where to go next
Forest management
How thinning, clear-cuts and habitat trees act on this living population.
Tree species
The species ForestStudio can simulate.
Assessment
How harvested trees become volumes, prices and revenue, and what the forest provides beyond timber.