1. Degradation increases peat greenhouse gas
emissions in undrained tropical peat swamp
forests
Erin Swails, Steve Frolking, Jia Deng, Kristell Hergoualc’h
9th International Symposium on Non-CO2 Greenhouse Gases
Amsterdam, 21 – 23 June 2023

2. Tropical peat swamp forests
• High litter inputs + waterlogged soils →
substantial soil carbon and nitrogen storage
• Tropical peat soils are a globally important C
pool (350 Gt C) (Gumbricht et al. 2017)
• Disturbance alters peatland function and
triggers high and long-term soil GHG
emissions (Leifeld et al. 2019, Hergoualc’h & Verchot, 2011)
Tropical peatland extent, Gumbricht et al. 2017

3. Drivers of degradation in tropical peat swamp
forests
• Drainage and conversion of tropical peat swamp forests is an
important source of anthropogenic GHG emissions
• Degradation without drainage can also increase soil GHG emissions
in tropical peatlands (Sánchez et al. 2017, Swails et al. 2021)
o Alters litter inputs (e.g., Allison and Vitousek 2004) and forest microclimate
(e.g., Blonder et al. 2018)
o Modifies soil microbial communities (Nurulita et al. 2016)
o Peat fire induces extreme physical and chemical changes (Könönen et
al. 2015, Certini 2005)
Conversion
Fire
Harvest
Drainage
Grazing Agroforestry
GHG
GHG

4. Tropical peat greenhouse gas emissions
in national inventories
• Intergovernmental Panel on Climate Change 2013 Wetlands
Supplement (WS) provides default Tier 1 emission factors
(EF) for tropical peat soils
o No default EF for anthropogenically-degraded undrained
organic soils
o EF based on data from Southeast Asia ombrotrophic
peatlands

5. Literature review
• Two undrained forest conditions: undegraded, degraded
• Two regions: Latin America and the Caribbean (LAC) (4
locations) and Southeast Asia (SEA) (17 locations)
o No data for African peat swamp forests
• Ombrotrophic (20 locations) + minerotrophic peats (1
location)
• Logging, secondary forest regrowth following partial
clearing, felling for fruit harvest
• Minimum of 3 spatial replicates per site, measured every 2
months, over a period of 1 year (16 of 39 studies discarded)
Peatland extent, Gumbricht et al. 2017
Study location
Study locations in Thailand and Micronesia not shown

6. Data calculation and presentation
• Each site considered replicate, single mean value for multi-
year studies
• Mean annual GHG fluxes (N2O, CH4, onsite CO2) and
controlling variables (rainfall, air and soil temperature,
water table level, soil pH, C:N ratio, cation exchange
capacity, base saturation, mineral N content, Ca:Mg)
• Rainfall seasonality: Difference in water table level in 3
shallowest and deepest consecutive months
• Peat GHG budgets: Annual CH4 and N2O fluxes converted to
CO2-equivalent considering 20-year global warming
potentials (CH4 = 86, N2O = 268) (Myhre et al. 2013)

7. Peat N2O fluxes
• Mean annual peat N2O fluxes tended to be greater in degraded
compared undegraded conditions in SEA
• LAC sites not compared (limited data)
• Mean annual N2O fluxes increased with increasing soil pH in SEA
0
2
4
6
8
kg
N
ha
-1
yr
-1
(n = 3)
(n = 6)
(n = 1) (n = 2)
Southeast Asia
Undegraded
Forest
Latin America and Caribbean
Degraded
Forest
Undegraded
Forest
Degraded
Forest

8. Peat CH4 fluxes
• Mean annual peat CH4 fluxes tended to increase in degraded
compared to undegraded conditions in SEA, opposite trend in LAC
• Mean annual peat CH4 fluxes 5x greater in LAC than SEA in
undegraded forests (p = 0.04), similar trend in degraded forests
0
100
200
300
kg
C
ha
-1
yr
-1
(n = 3) (n = 12)
(n = 6)
(n = 2)
Southeast Asia
Undegraded
Latin America and Caribbean
Degraded Undegraded Degraded
a
b

9. Controlling variables of peat GHG fluxes
• In SEA mean annual water table higher in degraded
compared to undegraded conditions (p = 0.02)
• In undegraded forests mean annual water table higher in
LAC than SEA (p = 0.002), similar trend in degraded forests
• Annual precipitation 10% greater at LAC sites than SEA sites
(p = 0.01)
• Water table seasonality tended to be greater in SEA (36.0 ±
8.1 cm) than LAC (25.1 ± 10.7 cm)
Southeast Asia Latin America and the Caribbean
Undegraded
Forest
Degraded Forest Undegraded
Forest
Degraded Forest
Water table -31.3 ± 4.5 (5) a, β -14.8 ± 3.9 (13) α 0.1 ± 1.2 (4) b 6.5 ± 0.4 (2)

10. Peat GHG budgets
• Degradation shifted peat from net GHG sink to GHG source in SEA, increased
peat GHG emissions in LAC
o Compare to sum of default peat onsite CO2 and CH4 and N2O IPCC EF expressed in CO2
equivalents = 21.7 (using a 20-year GWP)
• SEA: Enhancement of N2O and CH4 in degraded compared to undegraded
forests made substantial contribution to increased peat GHG emissions
• LAC: CH4 accounted for >50% of peat GHG budget regardless of forest condition
-20
-10
0
10
20
30
Mg
CO
2
-equivalent
ha
-1
yr
-1
N2O
CH4
CO2
-7.9 ± 6.9 (n = 13)
20.7 ± 7.4 (n = 22)
9.8 ± 9.0 (n = 8) 24.3 ± 8.2 (n = 4)
Southeast Asia
Undegraded
Forest
Latin America and Caribbean
Degraded
Forest
Undegraded
Forest
Degraded
Forest

11. Conclusions
• Further investigations of links between vegetation disturbance
and changes in organic matter dynamics, soil nutrient cycling, and
peat GHG emissions needed
• Additional measurements that adequately cover spatial and
temporal variability in peat GHG fluxes are critical, particularly in:
o Minerotrophic peats
o Africa
• Regional differences between SEA, LAC highlight key challenges to
the development of globally relevant soil EF for tropical peatlands
• Observed increases in peat GHG emissions in undrained degraded
tropical peat swamp forest as compared to undegraded
conditions call for their inclusion as a new class in the IPCC
guidelines to support countries in their development of GHG
inventories

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