Timber harvests: reconcile biological growth and removals
Standing timber is a biological stock that changes through growth, mortality, harvests and changes in the land base. A larger closing inventory does not by itself prove that harvest levels are sustainable or that immediately merchantable wood increased. Reconcile the physical components before connecting timberland disclosures to a business thesis.
Preserve the inventory definition
The US Forest Service’s Iowa Forests 2018 summary, published in 2022, explains growth, mortality and removals within its inventory framework. It is a historical methodological source, not current evidence about a listed timber owner. Its net-growth concept deducts mortality from gross growth, so mortality should not be subtracted again from an already net figure.
Confirm species, minimum tree size, land classification and volume unit. Live-tree volume, growing stock and sawtimber are different populations. Cubic metres, tons and board feet require appropriate conversion conventions and cannot be combined as if they were identical counts. A national inventory estimate also need not match a company’s merchantability rules.
A hypothetical timber-stock bridge
Assume a fictional owner opens the year with ten million cubic metres of compatible standing inventory. Gross biological growth adds 0.6 million, mortality removes 0.1 million, harvests remove 0.4 million and acquired land adds 0.2 million. Assume no other land or measurement changes.
| Hypothetical movement | Million cubic metres |
|---|---|
| Opening inventory | 10.0 |
| Gross biological growth | +0.6 |
| Mortality | −0.1 |
| Harvest removals | −0.4 |
| Acquired standing timber | +0.2 |
| Closing inventory | 10.3 |
Closing inventory rises 3%, but 0.2 million of the increase was purchased with land rather than grown organically. Net biological growth is 0.5 million. Harvests equal 80% of that net growth in this simplified year. The ratio is informative, yet it is not sufficient evidence of long-term ecological or commercial sustainability.
Check age and merchantable composition
Suppose only three million of the closing stock currently meets the owner’s assumed harvest specification. Dividing total inventory of 10.3 million by annual harvests would overstate immediately available cutting stock. Even dividing the mature stock by harvest gives only a static ratio; younger stands can enter the merchantable population and restrictions can remove other stands.
Map age classes, species, quality and access. A younger fast-growing plantation can add biological volume while producing little near-term sawtimber. Mature inventory can be concentrated in areas with poor roads, seasonal access restrictions or protected buffers. Physical volume is not the same as volume economically accessible for the next sale.
Keep disturbance and harvest responses separate
Fire or disease can increase mortality and prompt salvage harvesting. Avoid counting the same lost trees once as mortality and again as ordinary harvest without inspecting the reporting convention. Salvage wood can have different quality, timing and realized prices, and a temporary surge in harvesting may not describe normal future operations.
Review when removals exceed net growth for several comparable periods, when mature stands decline despite rising total volume, or when mortality assumptions change sharply. The first condition requires land-base and management checks; the second calls for an age-class bridge; the third requires disturbance and measurement evidence.
A company may sell land or change its inventory model. Preserve those changes separately from biological growth. An accounting fair-value increase can reflect discount rates or timber prices without adding a single cubic metre. The physical bridge should therefore remain independent from a valuation roll-forward.
Define a bounded output
Create a parcel or region panel containing opening stock, gross or net growth convention, mortality, harvests, land transactions, age classes and access restrictions. Public data rarely reproduce every stand’s growth model. State the missing cohorts and report ranges rather than a precise future harvest schedule.
FAQ
Does harvesting less than net growth prove sustainability?
No. Species, age structure, habitat, access and the measured land base also matter. A single annual volume ratio cannot settle all those questions.
Does higher timber fair value prove more trees grew?
No. Prices and valuation assumptions can change carrying values. Reconcile physical inventory separately from the accounting valuation.