A Method for Estimating Crown Weight in Eucalyptus, and Some Implications of Relationships between Crown Weight and Stem Diameter
P. M. Attiwill
School of Forestry, University of Melbourne, Victoria, Australia
Equations relating crown weight to diameter at breast height outside bark (dbhob) are developed for approximately 50-year-old, even-aged stands of Eucalyptus obliqua from initial branch weight-branch girth relationships. These equations are used to predict crown weights of individual trees; the predictions for n' trees per unit area have standard errors of 15%/√n' of the summed leaf weights and 17.5%/√n' of the summed branchwood weights. Although stem weight is a function of dbhob^2, considerations both of (crown weight) - dbhob and (crown weight) -(stem diameter at base of crown) relationships show that crown weight is a function of (stem diameter at base of crown)^2 = constant (dbhob)^3, thus accounting for form factor. There is no logical basis, therefore, for selecting a tree of mean dbhob as a unit for estimating (weight of mean tree X number of trees) dry weight per unit area. The bias (in this sense meaning the deviation from the true value) of estimates so obtained are discussed; an index of the error is provided by the distribution of dbhob^3 with respect to dbhob. The error, one of underestimation, is likely to increase with stand age, and this must be an important deficiency in some of the productivity-age sequences which have been developed.