What Is the Contribution of Nonlethal Tree Damage to Forest Carbon Losses?

Forest Global Earth Observatory scientists quantified the proportion of total biomass losses from damaged but surviving trees across seven tropical forests.

Three damaged but surviving trees.

Damaged but surviving trees are highly frequent across forest ecosystems.

[Courtesy Chia-Hao Chang-Yang (left) and Daniel Zuleta (middle, right).]

The Science

Damage (e.g., branchfall, trunk breakage, and wood decay) is a ubiquitous feature in forest ecosystems. However, traditional forest inventories assume tree mortality is the only source of biomass losses. While previous studies show damage is an important condition preceding tree death, contribution of nonlethal damage (i.e., from surviving trees) to total forest biomass (and therefore carbon) losses remained unclear. Forest Global Earth Observatory (ForestGEO) scientists combined field-based measurements of tree completeness with vertical volume profile models obtained from terrestrial laser scanning to show 42% (range 12% to 76% across forests) of total aboveground biomass loss is due to damage to living trees across seven tropical forests.

The Impact

Ground-based biomass stocks and fluxes are widely used to estimate carbon budgets, quantify forest carbon offsets, and calibrate and validate remote sensing products employed to obtain biomass estimates at regional and global scales. This study shows biomass loss from damage to living trees constitutes an important and overlooked component of biomass loss. These results contrast with typically low forest biomass losses estimated only from tree mortality and suggest that forest carbon turnover may be higher than previously thought. Since forest disturbance rates are expected to increase under climate change, biomass loss from damage is likely to become more important.

Summary

Forest carbon losses constitute a significant source of uncertainty in vegetation models. These estimates are typically calculated based on dead tree biomass without accounting for losses via damage to living trees: branchfall, trunk breakage, and wood decay. In this study, forest ecologists employ multiple annual records of tree survival and structural completeness to compare aboveground biomass (AGB) loss via damage to living trees to total AGB loss (mortality + damage) in seven tropical forests widely distributed across environmental conditions. Researchers find that 42% (3.62 Mg ha-1 yr-1; 95% CI 2.36–5.25) of total AGB loss (8.72 Mg ha-1 yr-1; CI 5.57–12.86) is due to damage to living trees. They also find that conventional forest inventories: (1) overestimate stand-level AGB stocks by 4% (1 to 17% range across forests) because they assume structurally complete trees; (2) underestimate total AGB loss by 29% (6 to 57%) because they overlook damage-related AGB losses; and (3) overestimate AGB loss via mortality by 22% (7 to 80%) because they assume that trees are undamaged before dying. These results indicate that forest carbon fluxes are higher than previously thought. Damage to living trees is an underappreciated component of the forest carbon cycle that is likely to become even more important as the frequency of forest disturbances increases.

Principal Investigator

Daniel Zuleta
Smithsonian Tropical Research Institute
[email protected]

Program Manager

Brian Benscoter
U.S. Department of Energy, Biological and Environmental Research (SC-33)
Environmental System Science
[email protected]

Funding

This project was supported as part of the Next-Generation Ecosystem Experiments Tropics and funded by the Biological and Environmental Research (BER) program within the U.S. Department of Energy’s (DOE) Office of Science. Data collection was supported by the ForestGEO of the Smithsonian Institution.

References

Zuleta, D., et al. "Damage to Living Trees Contributes to Almost Half of the Biomass Losses in Tropical Forests." Global Change Biology 29 (12), 3409–20  (2023). https://doi.org/10.1111/gcb.16687.