Jed Calvert, Abbey R Yatsko, Judy Bresgi, Alexander W Cheeseman, Keith Cook, James Crowe, Indigo Gambold, Caleb Jones, Liam O'Connor, Tony Peter, Pedro Russell-Smith, Elisha Taylor, Blair Trigger, Baptiste Wijas, Amy E Zanne

Forests and woodlands are critical carbon stores, and methods for quantifying forest aboveground biomass (AGB) are increasingly relied upon for determining sequestered CO2 traded in carbon markets. AGB is traditionally measured using allometric models, yet terrestrial laser scanning (TLS) is emerging as a highly accurate remote sensing alternative. However, internal tree stem damage from biotic decay is an unresolved source of error for both TLS and allometries, with implications for accurate carbon assessment.

We destructively harvested 63 TLS-scanned trees in an Australian savanna to understand the impact of internal damage on AGB estimation at individual tree- and plot-levels. We tested the performance of TLS versus five allometries in measuring AGB, applying both database and field-measured wood specific gravity. We recorded how internal damage changed throughout the tree and tested if tree size and internal stem damage amount contributed to AGB under or over predictions.

We asked four questions: 1) How accurately does TLS measure AGB in comparison to allometries at both tree- and plot-levels? 2) Does applying field-measured or database wood specific gravity values affect TLS and allometry AGB estimate accuracy? 3) How does internal stem damage vary throughout trees? 4) Does tree size or amount of internal stem damage predict AGB overestimation?

TLS provided closest estimates to aggregated AGB at the plot-level. At the tree-level, all methods were strong at predicting field-measured AGB (R2>0.84), however we found TLS using field-measured wood specific gravity to be most accurate (R2=0.99). Although allometric models were unaffected by internal damage, TLS tended to overpredict AGB of large, damaged trees. Roughly half of the trees in the study sustained 1-10% damage, which was most extensive at the base and main trunk, decreasing into the crown.

Synthesis and applications: For plot-level forest carbon estimation where internal stem damage is low (<10%), we recommend TLS to accurately estimate AGB, as well as in situations where precision is required at the individual tree-level. When quantifying AGB using TLS in more damaged wooded ecosystems (>10%), internal stem damage should be quantified to avoid overestimation and maintain high standards of precision in carbon markets.