Stand Density Index (Reineke)
Computes Reineke's stand density index, SDI = trees per hectare × (quadratic mean diameter ÷ 25)^1.605, which expresses the stocking of a forest stand as the equivalent number of trees per hectare it would hold if every one measured 25 cm DBH — the 10 inches of the original 1933 work, rounded in the metric version. The exponent 1.605 is the slope of the self-thinning line Reineke fitted empirically, and it is precisely what makes the index nearly independent of age and site quality, unlike a plain trees-per-hectare count. The number guides thinning decisions when compared with the species maximum SDI, which for most species falls between 1,000 and 1,200: competition mortality typically starts around 55% to 60% of the maximum, and the recommended management zone runs from 35% to 55%. In the example, 559 against a maximum of 1,100 gives about 51%, meaning the stand is still below the self-thinning threshold but already at the top of the management zone. Enter the number of trees per hectare and the quadratic mean diameter.
Result
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Reading Reineke's SDI before you thin
Forest inventory hands you two columns: trees per hectare and mean diameter. On their own they will not say whether the stand is crowded. A block with 1,600 slim stems and another with 700 fat ones can sit under the same competition, and a plain head count cannot tell them apart. Reineke's index folds density and diameter into a single scale that holds across ages and site classes. Without it the thinning date becomes a hunch, and thinning late costs growth that never comes back.
The arithmetic is SDI = N × (Dg ÷ 25)^1.605, with N in trees per hectare and Dg, the quadratic mean diameter, in centimetres. The 25 cm reference stem stands in for the 10 inches of the 1933 paper, rounded in the metric version. The exponent 1.605 is the slope Reineke fitted to fully stocked plots, and it explains why the index barely moves with age or site quality. With the screen defaults, 800 stems at 20 cm, the answer reads 559.2, roughly 51% of a maximum SDI of 1,100. Below 35% of that maximum the site sits idle; above 55% competition starts killing stems.
The index assumes an even-aged, single-species stand. In mixed or uneven-aged forest a single Dg hides very different structures and the number stops meaning much. Maximum SDI is species-specific and has been revised upward for improved plantations, so checking against the wrong ceiling shifts the whole management band. The classic unit slip is entering diameter in inches: 8 in place of 20 cm drops the reading from 559.2 to 128.5 and paints an empty stand where thinning is already overdue.
Frequently asked questions
What does the 559.2 from the default values mean?
May I use the arithmetic mean diameter instead of the quadratic one?
Why will the tool refuse a zero stem count or a zero diameter?
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The results provided by this tool are for general informational and educational purposes only and do not constitute professional, financial, medical, legal, tax or accounting advice. Always confirm important decisions with a qualified professional and official sources.