Tropical Forests After Logging: A Persistent Carbon Source, Not a Sink

Tropical forests are often heralded as the lungs of our planet, crucial for regulating the global carbon cycle. They store an estimated 55% of terrestrial forest carbon and absorb nearly 40% of global terrestrial carbon. However, a groundbreaking study published in 2023 in the Proceedings of the National Academy of Sciences (PNAS) challenges a widely held assumption about the carbon balance of these vital ecosystems, particularly after logging. Contrary to the common belief that logged or degraded tropical forests quickly rebound into net carbon sinks, this research reveals they can remain a persistent and substantial net source of carbon to the atmosphere for at least a decade.

The Hidden Carbon Cost of Logging

For years, many climate models and conservation strategies have operated under the assumption that tropical forests, even after selective logging, rapidly recover their capacity to absorb carbon from the atmosphere. This belief largely stemmed from observations of woody biomass recovery—the regrowth of trees. What was often overlooked, however, were the simultaneous and significant carbon losses from other critical ecosystem components, such as the decomposition of soil organic matter and necromass (deadwood). This incomplete carbon accounting has potentially led to a substantial overestimation of these forests' ability to sequester carbon, with significant implications for global carbon budgets and climate change mitigation efforts.

The central question driving this research was: What is the complete carbon balance in logged tropical forests, taking into account both biomass gains and carbon losses from soil and deadwood? Are these forests truly net carbon sinks, or do they remain a persistent source of carbon to the atmosphere? By providing direct and comprehensive measurements of ecosystem-level CO2 exchange in a degraded tropical forest landscape, the study aimed to bridge this critical knowledge gap.

Unveiling the True Carbon Dynamics: A Dual Approach

To thoroughly investigate the carbon balance in post-logging tropical forests, the researchers employed a robust combination of two primary methodologies in a structurally degraded tropical forest landscape in Malaysian Borneo, a region grappling with high rates of deforestation and degradation.

First, they utilized an eddy covariance tower. This advanced technique allows for the direct, large-scale quantification of the net CO2 exchange between the forest ecosystem and the atmosphere. The tower was strategically placed within the SAFE (Stability of Altered Forest Ecosystems) project area, in a forest that had undergone intensive logging. Measurements were meticulously collected over a 7-year period, spanning different post-logging phases: a 10-year recovery period after previous logging, a period of active "salvage" logging, and a subsequent 2-3 year recovery period after this latest logging event. This provided continuous, real-time monitoring of carbon fluxes.

Second, the study conducted exhaustive plot-based biometric estimations. Eleven one-hectare plots were established, covering a gradient of logging intensity, from unlogged to intensely logged forests. These plots facilitated a complete accounting of ecosystem carbon components, including carbon stored in live and growing woody biomass, as well as carbon losses from soil organic matter and deadwood. Biometric estimations were compared between logged and unlogged plots, and a plot located within the eddy covariance tower's footprint was included for cross-validation of the data, enhancing the study's reliability.

The synergy of these two methodologies—large-scale flux measurements via eddy covariance and plot-level stock balance via biometrics—enabled the researchers to construct a comprehensive and coherent picture of the carbon budget. Unlike previous studies that often focused almost exclusively on biomass recovery, this work included all carbon source and sink terms (gains from tree growth and losses from soil decomposition and necromass), offering a far more accurate perspective. While the data originated from Malaysian Borneo, the rigor of the methods and the underlying ecological principles suggest high relevance for other degraded tropical forests globally.

Startling Findings: Logged Forests are Carbon Emitters

The study's principal findings directly challenge the pervasive assumption that degraded and recovering tropical forests are net carbon sinks. Instead, the research emphatically reveals that these ecosystems are a persistent and substantial net source of carbon to the atmosphere.

Key results include:

• Logged Forests as Carbon Sources: Both the eddy covariance method and terrestrial biometric estimations independently confirmed that logged forests consistently acted as a net carbon source. The eddy covariance tower recorded an average net CO2 exchange of 3.36 ± 1.76 g C m−2 d−1 over the 7-year observation period, clearly indicating a net release of carbon to the atmosphere. Biometric measurements in logged plots showed an average net CO2 exchange of 3.85 ± 1.13 Mg C ha−1 year−1, whereas unlogged plots were nearly carbon neutral (−0.71 ± 1.23 Mg C ha−1 year−1).
• Persistent Emissions: Carbon emissions were found to persist for at least a decade after logging. Estimated emission rates ranged from 1.75 ± 0.94 Mg C ha−1 year−1 in moderately logged plots to 5.23 ± 1.23 Mg C ha−1 year−1 in unsustainably and severely degraded plots, demonstrating a direct correlation between logging intensity and carbon release.
• Balance of Gains and Losses: While logged forests did exhibit significantly higher woody biomass gains compared to unlogged areas (consistent with existing literature on tree regrowth), these gains were overwhelmingly offset by much larger carbon losses from soil organic matter and deadwood. This critical imbalance was the deciding factor in shifting the overall carbon balance from a sink to a source.

The theoretical and conceptual implications of these findings are profound. The study fundamentally challenges the premise that post-logging tree biomass growth automatically translates into an ecosystem-level carbon sink. It underscores that including all carbon fluxes, particularly ecosystem respiration emissions and necromass decomposition, is indispensable for accurate carbon accounting. This suggests that previous assessments of carbon sequestration capacity in degraded forests have been incomplete and, consequently, overly optimistic. The study emphasizes the urgent need for a more holistic understanding and more comprehensive carbon models that account for all sources and sinks.

Compared to previous studies, this work moves beyond assessments focused solely on woody biomass. While it confirms that biomass recovery does occur in logged forests, it unequivocally demonstrates that this is insufficient to compensate for the significant carbon losses from other reservoirs, thereby contradicting the general assumption that these forests are net carbon sinks. The study validates concerns that heterotrophic soil respiration and deadwood decomposition are elevated in logged forests, an aspect that had been highlighted in more recent investigations.

Policy and Societal Implications: A Call for Reassessment

The findings of this study carry far-reaching practical implications, especially for public policy and society's understanding of the role of tropical forests in climate change mitigation.

In terms of public policy applications, the revelation that logged tropical forests are a persistent net carbon source, not a sink, represents a paradigm shift. This means that "the amount of carbon sequestered in the world's tropical forests could be considerably lower than currently estimated." This conclusion necessitates an urgent reevaluation of global and national carbon accounting strategies. Programs such as REDD+ (Reducing Emissions from Deforestation and Forest Degradation) or landscape restoration initiatives, which often rely on the assumed capacity of logged forests to sequester carbon, must adjust their projections and expectations. Sustainable forest management policies need to reconsider their metrics of success, focusing not only on timber volume recovery but also on the full ecosystem-level carbon balance. This could lead to stricter regulations on logging intensity and a greater emphasis on protecting carbon stocks in soil and deadwood.

The implications for society are significant. If the carbon absorption capacity of tropical forests has been overestimated, it suggests that the path to achieving global climate goals (such as the target of limiting warming to 1.5°C) may be more challenging than previously thought. This could translate into a need for even deeper and faster cuts in fossil fuel emissions and other anthropogenic sources. For local communities and Indigenous peoples who depend on these forests, continued degradation and carbon emissions not only affect biodiversity but can also alter local hydrological and climatic patterns, impacting their food security and quality of life. The persistence of post-logging emissions indicates that a forest's "recovery" in terms of visible biomass does not always equate to its functional recovery as a carbon sink.

While explicit recommendations from the authors are not detailed in the provided excerpt, the study's "Significance" section implicitly underscores the necessity to reevaluate assumptions about the role of logged forests in global carbon budgets. The authors implicitly urge for more accurate and comprehensive accounting of all carbon fluxes in these ecosystems. This leads to the implicit recommendation to invest in long-term research and monitoring in logged forests to fully understand their carbon dynamics and develop management practices that genuinely promote net carbon capture.

Topics of interest

Biodiversity Climate