Species-Specific Allometric Equations for Predicting Belowground Root Biomass in Plantations: Case Study of Spotted Gums (CORYMBIA citriodora subspecies variegata) in Queensland

Species-Specific Allometric Equations for Predicting Belowground Root Biomass in Plantations: Case Study of Spotted Gums (Corymbia citriodora subspecies variegata) in Queensland

Forests 2021, 12(1210): 1-17. https://doi.org/10.3390/f12091210

or link: https://www.mdpi.com/1999-4907/12/9/1210/htm

by Trinh Huynh^1,2,*, Grahame Applegate¹, Tom Lewis^1,3, Anibal Nahuel A. Pachas ^1,3, Mark A. Hunt ⁴, Mila Bristow ⁵ and David J. Lee¹

¹ Forest Research Institute, University of the Sunshine Coast, Maroochydore, QLD 4556, Australia; gapples@usc.edu.au (G.A.); tom.lewis@daf.qld.gov.au (T.L.); nahuel.pachas@daf.qld.gov.au (A.N.A.P.); dlee@usc.edu.au (D.J.L.)

     ² Forest Science Institute of Central Highlands and South of Central, Da Lat City 670000, Vietnam

³  Department of Agriculture and Fisheries, Queensland Government, 1 Cartwright Road, Gympie, QLD 4570, Australia

     ⁴  University of Tasmania, Churchill Ave, Hobart, TAS 7001, Australia; m.hunt@utas.edu.au

     ⁵  Plant Health Australia, Canberra, ACT 2600, Australia; mbristow@phau.com.au

     *   Correspondence: Trinh.Huynh@research.usc.edu.au

Abstract: Spotted gum (Corymbia citriodora spp. variegata; CCV) has been widely planted, has a wide natural distribution, and is the most important commercially harvested hardwood species in Queensland, Australia. It has a great capacity to sequester carbon, thus reducing the impact of CO2 emissions on climate. Belowground root biomass (BGB) plays an important role as a carbon sink in terrestrial ecosystems. To explore the potential of biomass and carbon accumulation belowground, we developed and validated models for CCV plantations in Queensland. The roots of twenty-three individual trees (size range 11.8–42.0 cm diameter at breast height) from three sites were excavated to a 1-m depth and were weighed to obtain BGB. Weighted nonlinear regression models were most reliable for estimating BGB. To evaluate the candidate models, the data set was cross-validated with 70% of the data used for training and 30% of the data used for testing. The cross-validation process was repeated 23 times and the validation of the models were averaged over 23 iterations. The best model for predicting spotted gum BGB was based on a single parameter, with the diameter at breast height (D) as an independent variable. The best equation BGB = 0.02933  × D2.5805  had  an adjusted R2 of 0.854 and a mean absolute percentage error of 0.090%. This equation was tested against published BGB equations; the findings from this are discussed. Our equation is recommended to allow improved estimates of BGB for this species.

Keywords: allometric equation; belowground root biomass; cross-validation; spotted gum planta- tions; weighted nonlinear models