Day 170Air 0 °CGPP 0Plant+fungi resp. 0Soil resp. 0NEE 0N supply 0 %FPS 0

Drag to orbit, wheel or pinch to zoom. Gold beads = sugar (photosynthate) running down the trunk and into the fungal network; blue beads = CO2 leaving the soil; white beads = CO2 entering the leaves. Units: g C per m² per day. Backdrop trees show the seasons in sequence (decor); the model runs at the day shown.

Forest Carbon Exchange

Tree, mycorrhiza and soil microbes trade sugar for nitrogen. Where does the carbon go over a year?

Time

Environment

Symbiosis

Live readouts (g C m⁻² d⁻¹)

Carbon pools (g C m⁻²)

Fluxes through the year

Cumulative NEE this year

Experiments

Each button runs ten model years headless for several values and lists the last year.

Independent numerical checks

Controls and missions

Drag / one finger: orbit; wheel / pinch: zoom; three camera presets.
Presets set season, drought, nitrogen and symbiosis together; every slider works live.
Missions: (1) find the fungal share φ that gives the highest GPP; (2) survive the drought preset with a still-negative NEE; (3) make the annual NEE of the forest close to zero; (4) switch the symbiosis off and read the N-supply drop.

The science

Light-use efficiency. Photosynthesis follows the Monteith light-use-efficiency idea: GPP = ε · APAR · f_T · f_W · f_N · f_CO2 with ε = 1.4 g C MJ⁻¹ (model value), absorbed light APAR = PAR (1 − e^−k·LAI) (Beer–Lambert, k = 0.5), LAI = 0.033 m² per g C of leaf, PAR = 0.48 × global radiation, f_T = 1 − ((T − 20)/18)², a linear water stress f_W and f_CO2 = 1 + 0.25 ln(CO2/400) [перевірити за джерелом].

Sugar for nitrogen. Roots and fungal hyphae together absorb mineral nitrogen with a saturating law U = C_eff / (K + C_eff), C_eff = C_roots + b C_fungi (b = 60: hyphae reach far more soil per gram of carbon). The plant pays φ · GPP of its sugar to the fungus (40 % of that is respired by the fungus, the rest becomes fungal biomass). Nitrogen supply sets the actual photosynthesis through f_N = min(1, supply / demand), with demand 0.4 GPP / (C:N = 35). Leaching removes 1 % of the mineral pool per day.

Respiration and NEE. Plant respiration R_a = R_maint(T) + 0.15 GPP with Q10 = 2; soil respiration is a first-order decay of the fast pool (k = 4·10⁻⁴ d⁻¹) times the Q10 and moisture factors; 10 % of it is humified into a slow pool. NEE = R_a + R_f + R_h − GPP; negative NEE means the forest takes up carbon.

Simplifications. One stand, one layer, daily Euler steps; no fire, no disturbance, no explicit microbial biomass, no nitrogen mass balance in the plant, a synthetic temperate climate (mean 8 °C, amplitude 12 K), and fixed leaf phenology (flush days 110–140, fall days 280–310). Whether fungi really move carbon and nutrients between trees is still debated; this model only treats the plant–fungus trade. All coefficients are illustrative model values. Assets built with Blender; scene by Three.js.