Comparison of approaches and functions for estimating greenhouse gas emissions from long-term harvested wood products in carbon abatement projects

Abstract

Long-term harvested wood products (Lt-HWPs) are not released to the atmosphere immediately after tree harvesting, but retire over a period of time depending on the types of their uses. Two different approaches, one accounting for carbon emissions from possible end of life pathways after retirement of HWPs and another assuming immediate carbon release, were applied to the Verified Carbon Standard (VCS) approved Improved Forest Management-Logged Protected Forest project in Australia. Exponential and logistic functions using Tier 2 parameters were used to track the annual retirement from Lt-HWPs and compared the results to exponential (Winjum et al. parameters) function and linear 20 years (VCS method). Exponential (Winjum et al. parameters) function with the immediate release of carbon that retire/decay annually generated the lowest greenhouse gas (GHG) emissions (69,382 tCO2-e), followed by exponential (Tier 2) function with 234,628 tCO2-e. Linear 20-year (VCS method) generated the highest GHG emissions among all methods (367,299 tCO2-e) over 25 project crediting period. For a project producing sawnwood (30%), exponential function (Winjum et al. parameters) also produced the lowest estimation of GHG emissions from Lt-HWPs. Although the exponential annual decay (Winjum et al. parameters) function which assumes immediate carbon release, generated the lowest GHG emission from Lt-HWPs in the temperate climatic region, the end of life pathway approach employing exponential or logistic functions with Tier 2 parameters, can simulate carbon emissions from Lt-HWPs more accurately and yield an unbiased estimation of GHG emission from HWPs, Interestingly, linear 20-year (VCS method) using a fixed timeframe of a 20-year linear function was neither found conservative in terms of net GHG emissions, nor flexible enough to take into account different wood product types and climatic region specific decay rates that may vary with project location and forest types.