Overlooked branch turnover creates a widespread bias in forest carbon accounting

Lim, H., Medvigy, D., Mäkelä, A., Kim, D., Albaugh, T.J., Knier, A., Blaško, R., C. Campoe, O., Deshar, R., Franklin, O. ORCID: https://orcid.org/0000-0002-0376-4140, Henriksson, N., Littke, K., Lutter, R., Maier, C.A., Palmroth, S., Rosenvald, K., Slesak, R.A., Tullus, A., & Oren, R. (2024). Overlooked branch turnover creates a widespread bias in forest carbon accounting. Proceedings of the National Academy of Sciences 121 (42) e2401035121. 10.1073/pnas.2401035121.

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Abstract

Significance
Net primary production of forests, a major land carbon flux, is estimated in the field as the sum of biomass increment, i.e., net growth of live biomass over time, and biomass turnover, i.e., the production of biomass replacing loss through mortality. Despite its importance in forest carbon dynamics, both measurements and models have largely overlooked turnover of branch biomass in live trees. Synthesizing field-based data across global biomes and incorporating branch turnover in state-of-the-art models, our study demonstrates that the prevailing neglect of branch turnover leads to widespread biases in carbon flux estimates across global datasets and model simulations. Modifications to field measurement protocols and model simulations are needed to eliminate the systematic biases in projection of land carbon dynamics.

Abstract
Most measurements and models of forest carbon cycling neglect the carbon flux associated with the turnover of branch biomass, a physiological process quantified for other organs (fine roots, leaves, and stems). Synthesizing data from boreal, temperate, and tropical forests (184,815 trees), we found that including branch turnover increased empirical estimates of aboveground wood production by 16% (equivalent to 1.9 Pg Cy−1 globally), of similar magnitude to the observed global forest carbon sinks. In addition, reallocating carbon to branch turnover in model simulations reduced stem wood biomass, a long-lasting carbon storage, by 7 to 17%. This prevailing neglect of branch turnover suggests widespread biases in carbon flux estimates across global datasets and model simulations. Branch litterfall, sometimes used as a proxy for branch turnover, ignores carbon lost from attached dead branches, underestimating branch C turnover by 38% in a pine forest. Modifications to field measurement protocols and existing models are needed to allow a more realistic partitioning of wood production and forest carbon storage.

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