Can crown variables increase the generality of individual tree biomass equations

Crown variables and height can improve biomass predictions and may help to quantify how tree biomass is influenced by inter-tree competition (stand density, species composition), climate, edaphic conditions, and age. Tree biomass is influenced by tree age, stand structural characteristics, and climatic and edaphic site factors. However, most allometric equations for predicting tree biomass do not quantify these effects and when these characteristics are included as variables, they typically only reflect the average or current conditions even though tree allometry is the result of past growing conditions as well. An alternative to using site and stand characteristics is to use tree variables that have been influenced by site and stand structure for the whole lifetime of the tree. The objective of this study was to compare stem diameter-only equations that predict above-ground biomass or foliage mass with equations that also contain a climatic variable (aridity index) and stand variables, such as age and basal area (as a proxy for competition), or equations containing another tree size variable, such as crown diameter, crown length and tree height. This was done using 348 trees from 24 sites and 10 Eucalyptus species in southern Australia. Stand basal area and age were significant in 2 of 23 final equations and the aridity index was not significant in any equations. Adding another tree size variable to the diameter-only equations reduced the bias and increased the precision in 6 of the final equations. This study shows that the addition of crown variables (or height) to diameter-only biomass equations can quantify some of the effects that site and stand structural characteristics have on biomass. Variables such as crown diameter and height could also be useful because they are easier to measure accurately using remote sensing methods than stem diameters and related stand structural characteristics, such as basal area.