Reineke Stand Density Index
The thin-now metric: SDI = TPA x (QMD/10)^1.605 and its percent of the species maximum (35% onset of competition, 55-60% lower management zone, ~100% self-thinning). Uses the QUADRATIC mean diameter (the diameter of the tree of average basal area, always >= the arithmetic mean); a plain average understates density and thins too late.
Formula and source
SDI = TPA x (QMD / 10)^1.605; percent_max = SDI / SDI_max x 100.
Reineke Stand Density Index (Reineke 1933; USDA FS RMRS), by name.
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