Derivation of compatible crown width equations for some important tree species of Spain

Crown width is an important feature in describing stand density and individual tree competition. Two different approaches to using crown width in competition measures are reported in the literature: (i) crown width of open-grown trees, assumed to describe the zone of influence, and thus the potentially available stand area attributable to an individual tree and (ii) crown width of stand-grown trees, describing individual tree competition in distance-dependent individual tree growth models. The crown width measurements of the second Spanish National Inventory are used to derive (i) open-grown tree crown width equations and (ii) general tree crown width equations. The results show that the expectation that open-grown tree crown width may be derived as a special case of a general crown width equation is not met. Therefore, the authors sought a method of obtaining “compatible” tree crown width equations (modelled to a specific region and for individual tree species) that cover the range from open-grown to stand-grown trees with acceptable accuracy. As a practical example, the authors' compatible tree crown width equations with their species-dependent coefficients are then compared for 11 major tree species in Spain in order to see if these equations give the open-grown tree crown width when open-grown tree height diameter ratio is inserted as independent variable. For conifers and for Eucalyptus globulus, the authors' procedure does indeed provide equations valid for open-grown trees to stand-grown trees with an acceptable loss of efficiency over the range. For the other broad-leaved tree species, no recommendable compatible crown width equation could be derived.