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Tree taper is the degree to which a tree's stem or bole decreases in diameter as a function of height above ground. Within Forestry and for the purposes of timber production, trees with a high degree of taper are said to have poor form, while those with low taper have good form. The opposite is the case for open-grown amenity trees. The form of a tree is sometimes quantified by the Girard form class, which is the ratio, expressed as a percentage, of the butt-log scaling diameter to diameter at breast height.
Taper is often represented by mathematical functions fitted to empirical data, called taper equations. One such function, attributed to Ormerod, is
d 2 = D 2 1.6 {\displaystyle d^{2}={D^{2}}\left^{1.6}}
where: