The Potential For Carbon Sequestration Through Reforestation Of Abandoned Tropical Agricultural And Pasture Lands

Abstract

Approximately half of the tropical biome is in some stage of recovery from past human disturbance, most of which is in secondary forests growing on abandoned agricultural lands and pastures. Reforestation of these abandoned lands, both natural and managed, has been proposed as a means to help offset increasing carbon emissions to the atmosphere. In this paper we discuss the potential of these forests to serve as sinks for atmospheric carbon dioxide in aboveground biomass and soils. A review of literature data shows that aboveground biomass increases at a rate of 6.2 Mg ha−1 yr−1 during the first 20 years of succession, and at a rate of 2.9 Mg ha−1 yr−1 over the first 80 years of regrowth. During the first 20 years of regrowth, forests in wet life zones have the fastest rate of above-ground carbon accumulation with reforestation, followed by dry and moist forests. Soil carbon accumulated at a rate of 0.41 Mg ha−1 yr−1 over a 100-year period, and at faster rates during the first 20 years

Generated Summary

This research article examines the potential for carbon sequestration through reforestation of abandoned tropical agricultural and pasture lands. The study reviews existing literature to assess the capacity of these forests to serve as sinks for atmospheric carbon dioxide in above-ground biomass and soils. The methodology involves analyzing data on the rates of above-ground biomass accumulation and soil carbon sequestration in secondary forests in the tropics in relation to stand age, climate, and land-use history. The researchers focus on the effects of reforestation on above- and below-ground carbon pools, focusing on secondary forests that colonize the majority of abandoned lands in the tropics. They analyze patterns of carbon accumulation during secondary succession to provide valuable insights for successful forest restoration and explore the potential of reforestation to provide a carbon offset alternative for tropical countries.

Key Findings & Statistics

• Aboveground biomass increases at a rate of 6.2 Mg ha−1 yr−1 during the first 20 years of succession, and at a rate of 2.9 Mg ha−1 yr−1 over the first 80 years of regrowth.
• During the first 20 years of regrowth, forests in wet life zones have the fastest rate of above-ground carbon accumulation with reforestation, followed by dry and moist forests.
• Soil carbon accumulated at a rate of 0.41 Mg ha−1 yr−1 over a 100-year period, and at faster rates during the first 20 years.
• The overall rate of aboveground biomass accumulation was 2.36 Mg ha-1 yr-1.
• The rate of above-ground biomass accumulation was significantly faster during the first 20 years of regrowth (6.17 Mg ha−1 yr−1) than over the subsequent 60 year period (p < 0.01).
• Overall (80 yr), wet forests accumulated biomass at a rate 3.24 Mg ha−1 yr−1 (n = 44) and moist forests at a rate of 2.17 Mg ha-1 yr−1 (n = 91).
• The rate of aboveground regrowth in abandoned agricultural fields was 6.04 Mg ha−1 yr−1 during the first 20 years.
• Rates of above-ground C accumulation in plantations range from 0.8 to 15 Mg C ha−1 yr−1, during the first 26 years following establishment.
• Tropical forests store approximately 206 Pg C in the soil.
• Soil C accumulated at a rate of 1.30 Mg ha−1 yr-1 during the first 20 years and at a rate of 0.20 Mg ha−1 yr−1 for the subsequent 80 year period.
• Overall, soil C accumulated at a rate of 0.41 Mg ha−1 yr-1 over a 100 year period following reforestation.
• During the first 20 years of forest establishment, soil C pools averaged 60 ± 4 Mg C/ha (n = 33); this increased significantly to 74 ± 6 Mg C/ha (n = 24) during the subsequent 20-100 years (p < 0.01).
• Sites that were deforested but not managed prior to forest re-establishment tended to accumulate soil C at a faster rate (1.17 Mg C ha−1 yr−1; n = 12), than pasture sites (0.49 Mg ha−1 yr-1; n = 21), or agricultural sites (0.25 Mg ha−1 yr−1; n = 12).
• The rate of aboveground regrowth in abandoned agricultural fields was 6.04 Mg ha−1 yr−1 during the first 20 years.

Other Important Findings

• Reforestation and restoration have the potential to contribute to C storage directly through biomass and soil C accumulation, and indirectly by providing an alternative to fossil fuels for energy generation.
• Forests where trees have rapid growth rates, such as many tropical plantations and natural successions, are excellent options for mitigating CO2 emissions through C sequestration.
• The relative distribution of C accumulated in soils and plants is not well documented, but C is generally thought to accumulate more rapidly above-ground than belowground, and some belowground C pools are likely to have slower turnover times and thus have the potential for longer-term C storage.
• Forest composition and structure, land-use history, and climate are all likely to affect the rate and character of C sequestration.
• Mature, natural forests in moist life zones tend to have greater aboveground C stores than wet or dry forests.
• The large area of secondary forests in the tropics and the commercialization of C sequestration raise numerous questions of ecological interest.
• Tropical forests are well known for high rates of net primary production and store approximately 216 Pg C in the aboveground biomass.
• Moist tropical forests tend to have more aboveground biomass than wet or dry tropical forests.
• Patterns of biomass accumulation with time differ between plantations and secondary forests. Plantation species are often selected for rapid aboveground growth and, thus, usually gain aboveground biomass faster than secondary forests under the same edaphic and climatic conditions.
• During the first 20 years of regrowth, there was a strong significant increase in aboveground biomass following agricultural use, but not with other land uses.
• Forests regrowing on old agricultural fields accumulated biomass at slightly faster rates than forests grown on abandoned pastures.

Limitations Noted in the Document

• The choice of biomass equation used in individual studies is likely to strongly influence values.
• There are considerably more data points for the first few years of regrowth than for later stages of succession.
• Analyses are limited by some methodological issues.
• The use of biomass products as renewable fuels can provide a longer-term mechanism for offsetting increased atmospheric CO2. This approach requires careful accounting of the loss of C sequestration potential associated with harvesting, and of the amount of fossil fuel conserved by substituting a biomass product for a fossil fuel source.
• Data for dry forests did not vary significantly with age.
• The predictive power of the relationship between soil C content and forest age during the first 100 years following establishment was low.
• Data are limited by some methodological issues, and also due to the fact that plantations are often established on degraded lands.
• The precision in estimates of rate processes is likely to decrease as the time interval of the measurement increases.

Conclusion

The study underscores the significant potential of tropical reforestation as a carbon offset mechanism, both above- and belowground, with the capacity for long-term carbon sequestration. The findings suggest that the rate of carbon accumulation varies with factors such as life zone, land-use history, and the age of the forest. The research emphasizes the importance of considering previous land use when assessing a site’s potential for carbon sequestration, as intensive pasture management can reduce soil fertility and compaction, which can hinder the process. The study also highlights the high resiliency of dry forests, which accumulate soil C faster than wet or moist forests. Additionally, the continuous accumulation of aboveground biomass over time, up to 80 years, further supports the potential for reforestation to sequester carbon. However, the study acknowledges that the rate of carbon accumulation decreases over time, and that the allocation of biomass between above- and belowground is species- and community-specific, impacting the persistence of sequestered carbon. In summary, reforestation efforts can be justified for carbon offsets, but there is a need for standardized protocols, more data from older forests, and consideration of the impact of human activities and natural disturbances on carbon sequestration.