Abstract
Two systems of additive equations were developed to predict aboveground stand level biomass in log products and harvest residue from routinely measured or predicted stand variables for Pinus radiata plantations in New South Wales, Australia. These plantations were managed under three thinning regimes or stand types before clear-felling at rotation age by cut-to-length harvesters to produce sawlogs and pulpwood. The residue material following a clear-fell operation mainly consisted of stumps, branches and treetops, short off-cut and waste sections due to stem deformity, defects, damage and breakage. One system of equations did not include dummy variables for stand types in the model specification and was intended for more general use in plantations where stand density management regimes were not the same as the stand types in our study. The other system that incorporated dummy variables was for stand type-specific applications. Both systems of equations were estimated using 61 plot-based estimates of biomass in commercial logs and residue components that were derived from systems of equations developed in situ for predicting the product and residue biomass of individual trees. To cater for all practical applications, two sets of parameters were estimated for each system of equations for predicting component and total aboveground stand biomass in fresh and dry weight respectively. The two sets of parameters for the system of equations without dummy variables were jointly estimated to improve statistical efficiency in parameter estimation. The predictive performances of the two systems of equations were benchmarked through a leave-one-plot-out cross validation procedure. They were generally superior to the performance of an alternative two-stage approach that combined an additive system for major components with an allocative system for sub-components. As using forest harvest residue biomass for bioenergy has increasingly become an integrated part of forestry, reliable estimates of product and residue biomass will assist harvest and management planning for clear-fell operations that integrate cut-to-length log production with residue harvesting.
Highlights
Building upon and extending the work of Wang et al (2017) on individual trees, this paper aims to develop additive systems of stand biomass equations for predicting the fresh and dry weights of commercial logs and harvest residues and the weights of their respective sub-components in rotation age P. radiata stands under three thinning regimes in New South Wales (NSW), Australia
For neither parameter was the difference between the estimates for fresh and dry weight predictions significant at α = 0.05 according to the t-statistics that could be derived from the standard errors of the parameter estimates
The use of harvest residue biomass for renewable energy generation has become an integral part of the sustainable management of P. radiata plantations in Australia and other major growing countries
Summary
Following its introduction and domestication elsewhere in the world over much of the last century (Burdon et al 2017), it has been transformed from an agroforestry, landscaping, ornamental and Christmas tree species that has little economic importance within its home countries to a major softwood plantation species outside of its native range with much improved growth performance and responsiveness to management and cultivation (Lewis et al 1993; Rogers 2004; Mead 2013). The harvesting and wood processing residues are increasingly used for bioenergy generation (Acuña et al 2010; Mead 2013; Burdon et al 2017; Ghaffariyan et al 2017; Lock and Whittle 2018; Van Holsbeeck et al 2020)
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