Impact of Ground-Level Ozone on Tropical Forest Productivity

By Muhammad OsamaReviewed by Laura ThomsonSep 18 2024

An article recently published in the journal Nature Geoscience comprehensively explored the impact of elevated ground-level ozone (O₃) on tropical forests' productivity and carbon absorption. The researchers aimed to understand how increased O₃ exposure, caused by human activities, affects tropical forests and the global carbon cycle. They conducted experiments to measure the O₃ susceptibility of different tropical tree species and integrated the data into a vegetation model.

Background

Ground-level O₃ is a secondary air pollutant primarily formed by human activities such as urbanization, industrialization, and high energy consumption, which increase emissions of volatile organic compounds (VOCs) and nitrogen oxides (NOx). Over the past century, surface-level O₃ concentrations have risen significantly due to atmospheric chemistry and climate change changes. Monitoring data from the Northern Hemisphere shows substantial increases in O₃ throughout the 20th century.

While the harmful effects of O₃ on plant growth are well-documented in temperate regions, their impact on tropical ecosystems is less understood. Tropical forests, responsible for about 60% of global photosynthesis, play a critical role in carbon sequestration. Despite stricter emission controls in higher-income countries, rapid land-use changes and population growth in emerging economies have shifted most O₃ precursor emissions to tropical and subtropical regions. Rising O₃ levels are expected to further reduce plant productivity in tropical forests, crucial for maintaining the global carbon cycle.

About the Research

In this paper, the authors aimed to determine the susceptibility of various tropical tree species to O₃ and use this data to improve a dynamic global vegetation model "Joint UK Land Environment Simulator (JULES)" for assessing regional and global impacts on the carbon cycle. Experiments were conducted at the TropOz research facility in Queensland, Australia, where 10 tropical tree species were exposed to realistic daytime O₃ concentrations using open-top chambers. These species were selected to represent different successional stages and leaf traits. The study measured total plant biomass in response to O₃ exposure and developed dose-response functions for each species.

Using these experimental data, the researchers updated the JULES model to include spatially explicit atmospheric chemistry data. This allowed them to evaluate how changing O₃ levels affect tropical forest productivity and the global carbon cycle. The model simulations, covering the period from 1900 to 2014, provided insights into the impact of anthropogenic O₃ emissions on tropical forests and their role in carbon dynamics.

Research Findings

The outcomes showed that anthropogenic O₃ significantly reduces annual net primary productivity (NPP) across all tropical forests, with some regions experiencing severe declines. For example, Asian tropical forests saw a 10.9% decrease in NPP, while Central African forests experienced a smaller decline of 1.5%. On average, tropical forests faced a 5.1% reduction in NPP, assuming moderate O₃ susceptibility across species. This decline has resulted in a cumulative loss of 0.29 petagrams of carbon per year (PgC/yr) since 2000, representing about 17% of the tropical land carbon sink in the 21st century. Additionally, areas targeted for current and future forest restoration have been disproportionately affected by elevated O₃ levels.

The study also found significant variation in O₃ susceptibility among tree species. While some species showed minimal impacts, others experienced major reductions in biomass. This variability highlights the importance of considering species-specific O₃ responses when evaluating the effects on diverse tropical ecosystems.

Furthermore, simulations indicated that O₃ concentrations have caused a significant reduction in the land carbon sink, totaling 21.1 PgC since 1900. In the 21st century alone, losses represent approximately 17% of the contemporary tropical carbon sink and 10% of the global natural land carbon sink. The authors emphasized the crucial role of air pollution in disrupting global carbon cycling and its ongoing impact on tropical forests.

Applications

The research has significant implications for global climate policies and forest management strategies. By incorporating O₃ susceptibility into vegetation models, the study helps develop more accurate predictions for the future carbon sequestration potential of tropical forests. This advancement improves understanding of how air pollution affects tropical forest productivity and the global carbon budget, which is crucial for creating effective climate change mitigation strategies.

The findings also highlight the need for targeted air quality management in areas undergoing forest restoration. Policies aimed at reducing O₃ precursor emissions, especially in tropical and subtropical regions, are essential for preserving the carbon sequestration potential of these forests. Additionally, the study provides valuable data for researchers and policymakers working on nature-based climate solutions, such as reforestation and afforestation, helping to refine their strategies.

Conclusion

In summary, the paper demonstrated that elevated ground-level O₃ significantly impacts tropical forest productivity and the global carbon cycle. It emphasized the importance of considering species-level variation in O₃ susceptibility when modeling its effects on tropical forests. The results highlighted the need for future socioeconomic pathways that reduce O₃ formation in the tropics to support the global carbon budget. Future Earth System models should better represent the direct impacts of O₃ on tropical forest productivity and include regional air quality considerations in carbon budgets and forest restoration efforts to combat climate change effectively.

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