The forest as a production system

A forest is a slow solar collector with a maintenance bill: photosynthesis brings carbon in, respiration and decomposition carry it out, and the difference is the wood. Every number on a forest manager’s desk follows from the shape of that balance over the life of a stand.

The growth curve and rotation

Annual volume increment follows a sigmoid. Young stands accumulate slowly while the canopy builds; after closure, increment runs near its maximum for years to decades; in old age it declines, because the respiration cost of maintaining an ever larger mass of living wood — heartwood included — eats into photosynthetic gain, while the hydraulic cost of lifting water through taller stems constrains the leaves. Plot the running average of increment against the current year’s increment and they cross exactly once. The crossing is the biologically defensible harvest point: cut earlier and you forgo the fastest growth, cut later and the average is already falling. Rotation lengths encode where that crossing falls: under a decade for tropical eucalyptus pulp plantations, several decades for temperate pines, approaching a century for boreal spruce.

Site productivity sets the ceiling

The same species on two sites can differ several-fold in growth, because photosynthesis is bounded by light, water, nutrients and length of growing season. Foresters grade sites by height at a reference age — a proxy that integrates all of these factors — and the site grade caps everything downstream. Genetics, thinning and fertiliser can move a stand toward its ceiling, not past it, and only part of annual production, often around a third, ends up in harvestable stem wood at all; the rest builds leaves, fine roots and root exudates, most of it returned to the soil within the year. The classical rule of sustained yield — harvest no faster than the forest regrows — is not an ethic but an accounting identity, and harvesting above it shows up as falling standing stock.

Where the carbon sits

Gross photosynthesis minus plant respiration is net primary production; subtract the decomposers and you have the ecosystem balance; subtract fires and harvests and you have what actually accumulates. That carbon lives in four pools — living biomass, deadwood, litter and soil — and the split matters: in boreal forest the soil holds more carbon than the trees, so the fate of residues and organic layers can outweigh the fate of the trunk. A harvest moves carbon from biomass into the atmosphere (decomposing residues), the soil and long-lived products; the replanted stand runs a carbon debt against the uncut baseline until regrowth repays it — decades — and whether the ledger ultimately balances depends on what the wood displaces, steel and concrete above all. How quickly old forests stop accumulating is genuinely contested: the long-standing assumption of near-zero net uptake has been challenged by measurements of continued storage in old stands.

Thinning and self-thinning

A closed stand kills its own competitors: as average tree size grows, stem number falls along a fixed power law, and the dead, suppressed trees rot on the floor. Thinning merely schedules that mortality for the mill — taking the volume before decomposition takes the carbon back, and reallocating site resources to the stems that remain.