ForestGEO scientists quantified the proportion of total biomass losses from damaged but surviving trees across seven tropical forests.
Damage (i.e., branchfall, trunk breakage, wood decay) is a ubiquitous feature in forest ecosystems. Yet, traditional forest inventories assume tree mortality as the only source of biomass losses. While previous studies have shown that damage is an important condition preceding tree death, the contribution of non-lethal damage (i.e., from surviving trees) to total forest biomass (and therefore carbon) losses remained unclear. ForestGEO scientists combined field-based measurements of tree completeness with vertical volume profile models obtained from terrestrial laser scanning to show that 42% (range 12%–76% across forests) of total aboveground biomass loss is due to damage to living trees across seven tropical forests.
Ground-based biomass stocks and fluxes are widely used to estimate carbon budgets, to quantify forest carbon offsets, and to calibrate and validate remote sensing products employed to obtain biomass estimates at regional and global scales. This study shows that biomass loss from damage to living trees constitutes an important and overlooked component of biomass loss. These results contrast with the 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 the biomass loss to damage is likely to become more important.
Forest carbon losses constitute a significant source of uncertainty in vegetation models. These estimates are typically calculated based on the biomass of dead trees, without accounting for losses via damage to living trees: branchfall, trunk breakage, 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 overestimate stand-level AGB stocks by 4% (1-17% range across forests) because assume structurally complete trees, underestimate total AGB loss by 29% (6-57%) due to overlooked damage-related AGB losses, and overestimate AGB loss via mortality by 22% (7-80%) because of the assumption that trees are undamaged before dying. These results indicate that forest carbon fluxes are higher than previously thought. Damage on 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.
Figure. Damaged but surviving trees are highly frequent across forest ecosystems. Photo credits: C.Y. Chia-Hao (left), D. Zuleta (center, right).
Contact: Daniel Zuleta, Ph.D., Ecologist (Postdoctoral fellow) at Forest Global Earth Observatory, Smithsonian Tropical Research Institute in Washington, DC (email@example.com)
This project was supported as part of the Next Generation Ecosystem Experiments–Tropics and was funded by the Office of Biological and Environmental Research (BER) within the U.S. Department of Energy’s (DOE) Office of Science. Data collection was supported by the Forest Global Earth Observatory (ForestGEO) of the Smithsonian Institution.
Zuleta, D., et al. “Damage to living trees contributes to almost half of the biomass losses in tropical forests” Global Change Biology in press (2023). https://doi.org/10.1111/gcb.16687.