Stand Basal Area per Hectare

Sum the basal areas of every stem on a plot, divide by the plot's area, and you have the single number forestry uses to describe how heavily a site is stocked. It is reported as square metres of stem cross-section per hectare of ground — 25 to 40 m²/ha in a fully stocked managed conifer stand, under 10 in a savanna or a fresh shelterwood, over 45 in unmanaged old growth, and above 300 in the coast redwood stands that hold the record.

A note on units before anything else, because this page has an honest wrinkle. Basal area per hectare is an area over an area, so strictly it has no dimensions at all — 32 m²/ha is the pure number 0.0032. Nobody quotes it that way. It is quoted as m²/ha in metric countries and ft²/ac in the United States, and those two conventions differ by a factor of exactly 4.356, because both the cross-section unit and the land-area unit change. The units engine behind this site has no area-per-area type to convert between them, so this page always reports m²/ha and always prints the ft²/ac equivalent beside it. Read the unit, not just the number.

The reason forestry settled on this measure rather than, say, stems per hectare, is that basal area is directly measurable without measuring anything. Stand at a point with a wedge prism or an angle gauge, sweep a full circle, and count every tree whose stem looks wider than the gauge's fixed offset. Multiply the count by the gauge's basal area factor — typically 2 or 4 m²/ha per tree in metric, 10 or 20 ft²/ac in the US — and that product is the stand's basal area per hectare. No diameters, no plot radius, no boundary decisions. The geometry arranges for each tree to be included with probability proportional to its own basal area, which is why a variable-radius plot is several times faster than the fixed-radius plot this equation describes, and why prism cruising took over North American inventory work.

What basal area buys you is a language for competition. A stand at 40 m²/ha and a stand at 20 m²/ha may carry identical numbers of stems and identical mean heights, but in the first every crown is closed against its neighbours and diameter growth per tree has slowed sharply, while in the second the trees are still growing at close to their free-grown rate. Thinning prescriptions are written as a residual basal area for exactly this reason: it is the quantity that says how much competition is left, independent of whether the stand is many small trees or few large ones.

Pair it with stems per hectare and the pair becomes more useful still. Basal area divided by stem count is the mean basal area per tree; invert the circle formula on that and you have the quadratic mean diameter, which is the entry point to every yield table, stocking chart and stand density index there is.

The main field error is boundary bias. On a fixed-radius plot every borderline tree is a judgement, and the judgements do not cancel: cruisers call borderline trees out more often than in, particularly awkward ones, so fixed-radius tallies run low. A tape stretched honestly, and a rule decided before the plot rather than during it, is most of the fix.