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Methods & Limitations
BC Old-Growth Run-Out map. How the numbers are built, what they mean, and where they are weak. Built from BC government primary data. Read this before citing a number.
1. What the map shows
For every Timber Supply Area (TSA) and Tree Farm Licence (TFL) in BC, the map estimates how many years of remaining old growth there are at the current pace of logging (and, for old growth, stand-replacing fire). It shows two clocks:
- Old-growth clock — years until the cuttable old growth of the chosen size class is gone. Three size classes by wood density: ≥300 m³/ha (big-tree), ≥200 (high-value), ≥100 (all merchantable old growth). The bigger the trees, the faster they run out, because logging takes them first.
- Primary-forest clock — years until every acre of never-logged forest, of any size, has been cut at least once and converted to managed second growth.
The fill colour is a continuous red-to-green gradient by years-to-gone. The harvest slider scales the logging rate to model higher provincial cut targets.
2. Data sources and how current they are
| Layer | Source | Currency |
|---|
| Forest inventory (age, volume, species) | VRI — VEG_COMP_LYR_R1_POLY 2025 | Photo-interpreted once, then grown forward each year by model (VDYP). Not re-inventoried. See limitation 2. |
| Logging (to subtract cut areas) | Consolidated Cutblocks (RESULTS + Landsat change detection) | Rebuilt quarterly. Lag from a few months to ~2 years. |
| Stand-replacing fire | Burn Severity (BARC), High severity only | Same-year, with a one-year-later rerun. Recent (2021–24) and longer (2015–24) baselines selectable. |
| Old-growth reference total | Old Growth Technical Advisory Panel (TAP) maps | 2021 provincial snapshot. |
| Protection / netdown | Parks, conservancies, legal OGMA, WHA/UWR no-harvest, riparian, slope (Copernicus DEM) | Current provincial layers. |
| Allowable cut + partitions | Chief Forester AAC Rationales | Determined at most every 10 years (extendable to 15). |
3. How the run-out is calculated
Plain arithmetic, no black box: run-out = cuttable stock ÷ annual loss rate.
Stock (the numerator)
From the VRI we take stands that are old (age ≥ 250 years on the coast and in the wet interior, ≥ 140 in the frequently-disturbed dry interior and boreal, matching the provincial ecosystem-based definition) and at or above the chosen volume class, on the Crown managed land base. We then net down to what is actually loggable: we remove parks, conservancies, legal old-growth reserves, no-harvest wildlife habitat, riparian buffers, and ground steeper than 60%. For TSAs we use each unit's published Timber Harvesting Land Base (THLB) fraction from its own timber-supply review (the gold standard). For TFLs, which have no published THLB, we use a slope-plus-legal-protection estimate, which is weaker (limitation 3).
Rate (the denominator)
The logging rate is each unit's own area of cutblocks over 2021 to mid-2025, divided by the years, measured against a fixed old-forest snapshot so that age-reset does not hide it. Stand-replacing fire (High severity only) is added to the old-growth clock. Fire is allocated across volume classes by their measured relative burn rate: a climate-controlled, area-correct study (see BURN_BY_STRUCTURE) found that within every region the largest old growth burns roughly half as often and at much lower severity, so the big-tree tier receives about 0.65x its area-proportional share. Fire is then scaled to the cuttable fraction of each tier, because fire burns the whole old-growth tier (parks and steep ground included) while only the loggable slice depletes the cuttable stock; without this, low-THLB units (boreal TSAs) read far too short. The primary-forest clock is logging only: a burned stand regrows as natural forest and is still never-logged, so fire does not convert primary forest into tree farm.
Legal caps
Where a determination sets an old-forest harvest partition (a legal m³/year ceiling on logging old stands), the scenario cut is capped at it. Where a determination sets a cedar cap (Haida Gwaii, TFL 41, Kamloops, Prince George), the cedar-rich old growth cannot be liquidated faster than that cap allows, shown as a separate cedar brake. A cap on paper is a ceiling, not a guarantee: the Haida Gwaii cedar cap is enforced on a five-year running average, so single-year overcuts are legal, and the earlier non-legal cedar target was documented as being exceeded before it was made law.
4. Validation
Three independent cross-checks, all passed:
- Total old growth by region lands within 3% of the government's Technical Advisory Panel comprehensive old-growth total (10.78M vs 11.13M ha).
- Big-tree old growth (site index ≥ 20 m) reproduces at 412,000 ha versus the Price/Holt/Daust published 415,000 ha.
- Netted ≥300 m³/ha stock (2.25M ha) matches the government's own "available and economical" old growth (2.31M ha).
5. Limitations (read this)
Every model has assumptions. Here are the ones that matter, stated plainly. None of them is hidden in the numbers.
High-grading by volume was tested, and is not visible in today's cut. We compared the volume of old growth inside approved-but-uncut cutblocks to all standing old growth. The ratio is about 1.0 (queued 306 versus standing 308 m³/ha, province-wide). So the current cut is not biased toward the densest old growth. The likely reasons: the easy high-volume stands were largely taken decades ago, and what high-grading remains now operates by species and road access rather than raw volume. The two clocks differ because the big-tree subset is a smaller stock, not because the model assumes the saw targets it. Historical high-grading is real, but it is already baked into how little big-tree old growth is left.
Cedar caps are shown as a brake, now grounded in billing data. A cedar harvest cap slows cedar-rich old growth. Turning it into a run-out needs cedar's share of the actual cut, which lives in the Harvest Billing System, not a map layer. We pulled it: cedar is 9% of the provincial cut and 19% of the coastal cut (billing-confirmed, and it matches the cutblock-footprint estimate to about one point). Haida Gwaii is harvesting roughly 138,000 m³/yr of cedar against its 250,000 m³ combined cap, currently under it. Per-TSA cedar is extractable from billing but not pre-tabulated, so each cedar cap is shown as a separate floor on its unit rather than blended into the headline until a full per-unit cedar-billing layer is built.
Age is a proxy, weakest where it matters. "Old growth" is defined by stand age because age is the only attribute the inventory carries everywhere. The government's own panel notes inventory age accuracy drops past 200 years and ancient forest is under-counted. The threshold is ecosystem-keyed, and this model matches it: ≥250 years on the coast and in the wet interior (CWH, CDF, MH, ICH, ESSF, IDF, PP), ≥140 in the frequently-disturbed dry interior and boreal (SBS, MS, BWBS and similar). A 200-year coastal stand is not counted as old growth, which is correct. The residual weakness is inventory age accuracy for the oldest stands, not the threshold.
The inventory under-reports fresh cutting on its own. Raw VRI keeps a stand's pre-logging age and volume until the cut is reported. We correct for this by subtracting the consolidated cutblock layer (quarterly, satellite-assisted), so recent logging does drop out, but very fresh or unreported cutting can still linger for months.
TFL stock is an estimate, not a published THLB. TSAs use real published loggable-land fractions. TFLs use a slope-and-protection estimate because no per-TFL THLB is published, so TFL run-outs are rougher than TSA run-outs.
The cut rate is a recent five-year snapshot. 2021 to 2025 spans the worst fire years on record and a current harvest slump. Rates change, and declining accessible supply over time (falldown) is not modelled.
Primary forest is slightly over-counted, and its clock ignores fire. "Never logged" is inferred from a cutblock record that thins out before about 1960, so some old logging is invisible and the base is an upper bound. The primary clock counts logging only, not fire.
The harvest slider is uniform. It scales every unit's cut by the same factor. A real higher provincial target would fall unevenly across the province.
Boundary edge cases. A few TSA and TFL polygons overlap (for example Arrow TSA and TFL 23 near Upper Arrow Lake), and dispersed BC Timber Sales units (Cascadia, Pacific, Arrowsmith) leave visual gaps because their parcels are scattered.
6. What this is not
This is not a government product, and it is not a precise prediction. It is a transparent estimate, built from public data, meant to make the per-region picture legible and to be argued with. The firmest numbers are the logging-only figures for logging-driven TSAs. Fire-driven numbers carry wider uncertainty and should be read with the baseline caveat. Do not cite a single year as a hard fact; cite the method and the range.
7. Sources and reproducibility
All inputs are BC government open data: the Vegetation Resources Inventory, Consolidated Cutblocks, Burn Severity, the Old Growth Technical Advisory Panel maps, RESULTS, FTEN, the FADM tenure layers, Chief Forester AAC Rationale documents, and the Copernicus GLO-90 elevation model. The processing scripts and a full methodology write-up are available on request for technical review. Independent forester review is welcome; the limitations above are the starting point for it.
Last updated June 2026. This page is part of the tool and is revised as the method changes.
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