Three layers, one light budget
The canopy (about 8-18 m) catches most sunlight, the understory (2-8 m: hazel, saplings) takes a quarter of what is left, and the floor (ferns, moss, fungi, logs) gets a few percent. Each layer has its own animals and its own microclimate.
Beer-Lambert light extinction
I = I0 * exp(-k * C * A / sin(b)) where I0 is light above the canopy, A the leaf area above that point (leaf area index, m2 of leaf per m2 of ground), b the sun's elevation, k = 0.5 for randomly tilted leaves and C = 0.8 for clumping. Low sun means a longer path through the leaves, so the floor goes dark long before sunset. Sky (diffuse) light is blended in with an average angle, so overcast days are brighter on the floor than the formula for a direct beam alone suggests.
Leaf area changes with the date: deciduous oak and birch leaf out around day 125 and drop leaves around day 300, spruce stays green. Bare winter woods let several times more light through. Typical measured values: 1-5 % of open-sky light reaches the floor of a closed summer forest.
Sun position
Solar declination d = 23.44 deg * sin(2 pi (day - 81) / 365), then sin(b) = sin(lat) sin(d) + cos(lat) cos(d) cos(h) at latitude 52 N. That gives short winter days (about 8 h) and long summer ones (about 16.5 h).
Day and night shifts
Each species has an activity curve driven by the sun's elevation, not the clock: roe deer peak at dawn and dusk, foxes, boar and owls are night animals, squirrels and woodpeckers need daylight, bats emerge at sunset and need roughly 6 degrees C or more. Because the curve follows the sun, nocturnal species start earlier in winter and later in a summer evening. Each animal has its own wake-up threshold, so the group does not switch on in lockstep.
Decay and mushrooms
Leaf litter L (g m-2) obeys dL/dt = input - k L with k = 0.5 per year * 2^((T-10)/10) * moisture (a Q10 of 2: ten degrees warmer, twice as fast). Mushrooms (fungal fruiting bodies) appear when moisture is high, temperature is near 11 degrees C and there is litter to feed on: that is why autumn rain is the main season.
Gaps
A fallen tree or a burn removes part of the leaf area above a patch. Light on the floor there jumps, ferns grow faster, and the gap shows up as the dashed curve on the light graph.
What is simplified
- Leaf area is spread evenly in two layers; real forests are patchy and clumped.
- The model changes leaf area and light, but trees and animals do not grow, breed, or eat each other.
- Temperature and moisture are smooth idealised seasons for a temperate European lowland, not weather data.
- Small animals are drawn larger than life (squirrel 1.5x, woodpecker 1.8x, bat 2.4x) so you can see them.
- Time-lapse speeds skip the walk to bed: animals change state on the spot.