Small-tree volume equations for subtropical hardwood plantation species

ABSTRACT

Estimation of stem volume is needed to determine timber yield and forest carbon, but equations to estimate volume from easily assessed inventory measures are lacking for hardwood plantation species in the subtropics. We assessed the merits of three volume equations that combine measurements of tree diameter at breast height (DBH) and total tree height for five hardwood plantation taxa (Corymbia citriodora subsp. variegata, Corymbia hybrids, Eucalyptus cloeziana, E. longirostrata and E. dunnii). Goodness-of-fit statistics indicated that model 1 (comprehensive combined variable equation) and model 2 (the Schumacher and Hall equation) fitted the data well (e.g. adjusted R² > 98). Analysis of bias in different diameter classes indicated that these models did not show any systematic bias patterns with tree size. Model 3 (combined variable volume equation) is the least desirable as it showed evidence of overestimation of volume for larger trees in some taxa. Models 1 and 2 provide a useful initial set of volume equations for five potentially important hardwood plantation species in subtropical Queensland. These equations are based on stems with a DBH range of 5−30 cm and hence there is a need to collect data for larger stems (>30 cm DBH) as existing plantations mature.

KEYWORDS:

• comprehensive combined variable equation
• combined variable equation
• Schumacher and Hall equation
• regression
• forest inventory
• model performance

Acknowledgements

The volume equations derived here were based on measurements made by numerous past and present staff in the Department of Agriculture and Fisheries (Queensland Government). The authors thank anonymous reviewers for their comments, which helped improve the paper.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

The authors declare no external funding or grant-awarding bodies.