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Foresters have produced many measures of stand density. Yet, none of these is entirely satisfactory. A majority of the measures (stand density index, basal area, and leaf area) present number of trees per unit area as a function of one factor: average tree size. This paper identifies the second factor driving self-thinning: the accumulation of gaps between tree crowns inevitable even in dense stands with a sizeable overlap of crowns. A model accounting for both factors allows us to quantify stand density and find a single number characterizing the density of undisturbed stands. The number changes with species, being higher for more shade tolerant ones. It is found that the second factor affects survival of trees but not their growth. This means that there are two kinds of stand density.

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