Forest Degradation without Drainage Increases Tropical Peat Greenhouse Gas Emissions
Erin Swails1, Kristell Hergoualc’h1, Jia Deng2, Steve Frolking2
Introduction Methods Results
Conclusions
Tropical peat swamp forests play an important role in regulating global
climate change through their capacity to store tremendous amounts of
carbon in biomass and waterlogged soils. Forest degradation can
substantially decrease biomass stocks, alter organic matter dynamics,
and modify peat greenhouse gas (GHG) emissions (Hergoualc’h et al.,
2020; Swails et al., 2021; Sanchez et al., 2017) (Fig 1) even without
drainage (Fig 2). Despite extensive areas of undrained degraded peat
swamp forests across the tropics, accounting of their soil GHG
emissions is lacking or inaccurate as current Intergovernmental Panel
on Climate Change (IPCC) guidelines (Drösler et al., 2014) do not
provide default emission factors (EF) for anthropogenically degraded
undrained organic soils. We address the following questions:
- How do peat onsite CO2 and net GHG budgets differ between
undrained tropical peat forests that are undegraded and
degraded, and between geographic regions?
- How do environmental variables control peat GHG fluxes in these
ecosystems?
- What are research needs for further refinement of EF for
anthropogenically degraded undrained tropical peat forests?
We reviewed the literature on field measurements of peat GHG fluxes
and controlling environmental variables in undrained degraded (DF)
and undegraded (UF) peat swamp forests in Southeast Asia (SEA) and
Latin America and the Caribbean (LAC) (Fig 3). We calculated peat
onsite CO2 budgets as the difference of mean annual CO2-C outputs
from heterotrophic soil respiration (SRh) and mean annual C inputs
from litterfall and root mortality. Net onsite peat GHG budgets were
calculated as the balance of peat annual GHG emissions using the rate
of onsite peat CO2 emission or uptake and the N2O and CH4 emission
rates. We investigated relationships among total soil respiration, peat
CH4 and N2O fluxes and water table level, air and soil temperature, soil
pH, C:N ratio, cation exchange capacity, base saturation, mineral
nitrogen content, and peat minerotrophy/ombrotrophy status as
indicated by the Ca:Mg ratio. The process-based model DeNitrification
DeComposition (DNDC) (Fig 4) was used to investigate the relationship
between heterotrophic respiration and vegetation C inputs in
undrained secondary peat forest and pristine peat forest in Central
Kalimantan, Indonesia.
- In both regions, degradation without drainage tended to shift the
peat from a net CO2 sink to a source (Fig 5a).
- In SEA the peat in degraded forests tended to be a net GHG source
as compared to undegraded conditions, while In LAC peat was a net
GHG source in both degraded and undegraded forests (Fig 5).
- Across forest conditions and regions, total soil respiration was
negatively correlated with peat ammonium content and Ca:Mg ratio
(Fig 6a and 6d, respectively). It was positively correlated with peat
nitrate content and C:N ratio (Fig 6b and 6c, respectively). Peat CH4
fluxes increased with increasing base saturation (Fig 6e).
- In undrained Indonesian peat swamp forests, DNDC predicted that
heterotrophic respiration was related to vegetation C inputs to the
soil (Fig 7).
- The observed increase in peat GHG emissions in degraded
undrained tropical peat swamp forest as compared to undegraded
conditions calls for inclusion of undrained degraded organic soils as
a new class in the IPCC guidelines to support countries in refining
their GHG inventories.
- Although water table is widely considered the dominant
biogeophysical control on soil GHG emissions in tropical peatlands,
variations in peat GHG fluxes in undrained peat swamp forests were
linked to peat chemistry (observed) and vegetation inputs of C to
peat (simulated).
- Additional measurements that adequately cover spatial and
temporal variability in peat GHG fluxes and controlling
environmental variables in anthropogenically degraded undrained
tropical peatlands are needed, particularly in Africa.
Acknowledgements: This research was conducted under the Sustainable Wetlands Adaptation and Mitigation Program
(SWAMP) and was generously supported by the governments of the United States of America (Grant MTO-069033) and Norway
(QZA-21/-124). It was undertaken as part of the CGIAR research program on Climate Change, Agriculture and Food Security
(CCAFS).
References Drösler, M., et al. (2014). Drained inland organic soils. In 2013 Supplement to the 2006 IPCC Guidelines for National
Greenhouse Gas Inventories: Wetlands: Methodological Guidance on Lands with Wet and Drained Soils, and Constructed
Wetlands for Wastewater Treatment. Intergovernmental Panel on Climate Change; Gumbricht, T., et al. (2017). An expert
system model for mapping tropical wetlands and peatlands reveals South America as the largest contributor. Global Change
Biology, 23(9), 3581-3599.Hergoualc’h, K., et al. (2020). Spatial and temporal variabilityof soil N2O and CH4 fluxes along a
degradation gradient in a palm swamp peat forest in the Peruvian Amazon. Global Change Biology, 26(12), 7198-7216; Sánchez,
M. E., et al. (2017). Carbon dioxide and methane fluxes in grazed and undisturbed mountain peatlands in the Ecuadorian Andes.
Mires and Peat, 19(20); Swails, E., et al. (2021). Spatio-temporal variabilityof peat CH4 and N2O fluxes and their contribution to
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Figure 2. Conceptual diagram of anthropogenic activities that degrade
tropical peat swamp forests. Not all degradation includes drainage.
Figure 1. Summary of peat onsite GHG fluxes as defined by the IPCC.
Figure 3. Study locations (green circles) and peatland extent (black
areas, (Gumbricht et al., 2017) in Southeast Asia (n = 16) (a) and Latin
America and the Caribbean (n = 2) (b). Study locations in Thailand and
Micronesia are not shown (n = 2). There were no data for African peat
swamp forests.
Figure 4. DeNitrification DeComposition (DNDC) model conceptual
framework.
Figure 5. Peat onsite CO2 and net GHG budgets for undegraded (UF) and
degraded (DF) undrained peat swamp forests in Southeast Asia (SEA) and Latin
America and the Caribbean (LAC). The number of sites is indicated in
parentheses. 20-yr GWP values are used to convert CH4 and N2O into CO2-
equivalent. Negative values indicate an emission reduction or removal.
Figure 7. Relationship between modeled heterotrophic respiration (SRh) and C
inputs to peat from litterfall and root mortality in undrained secondary peat forest
(DF) and pristine peat forest (UF) in Central Kalimantan, Indonesia (p < 0.05).
Figure 6. Relationships among peat GHG fluxes and controlling environmental
variables (p < 0.05). Relationships between total soil respiration (SR) and peat
NH4
+ and NO3
- content and C:N and Ca:Mg ratio are shown in (a), (b), (c), and (d),
respectively. The relationship between peat CH4 fluxes and base saturation
(BaseSat) is shown in (e).
a b
1Center for International Forestry Research, 2University of New Hampshire
a b