| dc.description.abstract | Extensive areas of tropical peat swamp forests (PSFs) have been deforested, degraded, and converted to other land uses. In degraded and drained conditions, the carbon emission is greater than the carbon sequestered, making this ecosystem a significant contributor to greenhouse gas (GHG) emissions from forestry and land-use sectors. Rewetting interventions as part of restoration management have been strongly recommended to mitigate the GHG emissions from degraded and drained tropical PSFs.
GHG emission reductions or carbon benefits from the rewetting intervention is calculated by subtracting the GHG emissions from rewetted PSFs with GHG emissions from drained PSFs. Due to the lack of site-specific emission factors (EFs) from rewetted tropical PSFs, default EFs derived from Tier 1 methodology proposed by the Intergovernmental Panel on Climate Change (IPCC) have been widely used to quantify carbon benefit from the rewetting intervention in tropical PSFs. However, GHG emissions are not only affected by the climatic zone and soil nutrient status but also by the site-specific characteristics such as vegetation cover, previous land use, and restoration management being implemented. Therefore, the application of default EFs to a specific land cover could under- or over-estimate the carbon benefit, mainly if applied to a specific tropical PSF cover at the national and project levels.
To assess this situation, we conducted a study with the intent of contributing to the restoration of tropical PSFs, particularly by providing site-specific GHG EFs for undrained and rewetted sites in tropical PSFs in Central Kalimantan Province, Indonesia. Both sites had been selectively logged prior being restored. The restoration was started in 2016. The study was divided into three phases: the first was a systematic review and meta-analysis of peer-reviewed literature and data, the second was an assessment of the study sites’ biophysical characteristics and carbon stocks, the third evaluated the variability of GHG EFs in the study sites.
In the first phase (Chapter two), a review and meta-analysis were conducted to evaluate the effect of rewetting on carbon emissions by comparing the carbon emissions (CO2, CH4, and Dissolved Organic Carbon [DOC]) from rewetted peatlands (treatment sites) with drained peatlands (control sites) in various climatic zones and at different restoration times. Twenty-seven articles passed the inclusion criteria and were used for meta-analysis. Three of these articles were from tropical climates while the remainder were from boreal and, predominantly, temperate climates. The overall effect size indicates that peatland rewetting can reduce the CO2 emission rate by -1.343 ± 0.358 Mg CO2-C ha-1 yr-1, while significantly increasing the CH4 emission rate by 0.033 ± 0.003 Mg CH4-C ha-1 yr-1. However, there was no discernible effect of rewetting on DOC. The findings highlight the lack of primary data from rewetted tropical peatlands which indicates an opportunity to conduct more research on carbon emissions.
In the second phase (Chapter three), biophysical characteristics, carbon stock, and fluxes of the study sites were assessed. The biophysical characteristics of both sites were not significantly different, except for the C/N ratio. The number of species per ha and basal areal in the undrained site were 78, and 23.2 3.1m2/ha While in the rewetted site were 53 and 23.4 1.7 m2/ha. The average peat depth, bulk density, carbon, nitrogen content, and C/N ratio in the undrained sites were 434.6 ± 5.4 cm, 0.071 ± 0.01 gr/cm3, 52.7%, 2.3% and 25.4.1, while in the rewetted sites, they were 396.7 ± 3.5 cm, 0.073 ± 0.02 gr/cm3, 51.2%, 2.8% and 19.1. The total carbon stock at the rewetted location was 1,886.7 ± 87.7 Mg C ha−1 which is lower than the undrained at 2,106.2 ± 214.3 Mg C ha−1. The soil organic carbon was the largest component with approximately 90% of total carbon stock, with SOC at 1913 ± 190 dan 1685 ± 61 Mg C ha-1 in undrained and rewetted sites, respectively. Litterfall productions were 4.68 ± 0.30 and 3.92 ± 0.34 Mg C ha−1 yr−1, respectively, while CO2 emissions from peat decomposition (Rh) were 4.05 ± 0.02 and 3.96 ± 0.16 Mg CO2-C ha−1yr−1 on the rewetted and undrained sites; respectively. Additionally, the CH4 emissions were 0.0015 ± 0.00 Mg C ha−1 yr−1 at the rewetted site and 0.056 ± 0.000 Mg C ha−1 yr−1 at the undrained site. The DOC were 70.6 ± 2.56 and 69.1 ± 1.74 mg/L in the rewetted and undrained sites, respectively. Overall, the results suggested that the rewetted site had been more severely degraded than the undrained site. In addition, since the carbon absorbed from the litterfall will be stable over time, the soil carbon balance will depend on the magnitude of Rh and CH4 emissions from the tropical PSFs.
In the third phase (Chapter four), carbon emission measurements from heterotrophic respiration (Rh), total soil respiration (Rs), and CH4 were conducted monthly from September 2019 to December 2020. Measurements were taken at 64 chambers evenly distributed between the rewetted and undrained sites. We observed that temporal variation of GHG emissions was strongly influenced by GWL fluctuation. Rh and Rs were negatively correlated with WL, while CH4 emissions were positively correlated. Meanwhile, spatial variation was influenced by microbes and organic matter from litterfall at the plot scale. The mean annual CO2 emissions from Rh were 2.3 ± 0.2 Mg CO2-C ha-1yr-1, 3.1 ± 0.2 Mg CO2-C ha-1yr-1 and CH4 emissions were 57.5 ± 11.0 Kg CH4-C ha-1yr-1, 60.2 ± 11.2 Kg CH4-C ha-1yr-1 in the rewetted and the undrained sites, respectively. These values are greater than the default EFs of rewetted tropical peatland proposed by the IPCC for CO2 and CH4.
Synthesizing the findings from the previous chapter, we found that both sites reduced the peat decomposition between 70% to 90% when compared to the peat decomposition from degraded and drained PSFs, oil palm, acacia, and rubber plantations. In addition, both sites demonstrated soil carbon accumulations of 2.28 0.21 and 0.76 0.11 Mg C ha-1 yr-1 in the rewetted and undrained sites, respectively. The total GHG emissions calculated using global warming potential 20 year (GWP20) for the rewetted site were -3.32 Mg CO2-eq ha-1yr-1 and for the undrained site were 2.98 Mg CO2-eq ha-1yr-1. Subtracting the GHG emissions in the rewetted and undrained PSFs by degraded PSFs would reduce GHG emissions approximately 358.5 to 484.4 Mg CO2-eq ha–1 over 20 years or 17.92 to 24.20 Mg CO2-eq ha–1yr–1. As recommended by the IPCC, the quantification of carbon benefits from the project level should use higher tier (specific) EFs which derived from project location. The study offers specific EFs to quantify carbon benefits from tropical PSFs conservation and restoration projects on previously degraded and drained tropical PSFs. | id |
