Terrestrial Wetlands

Prepared by Carl Trettin,¹ Judy Drexler,² Randall Kolka,¹ Scott Bridgham,³ Sheel Bansal,² Brian Tangen,² Brian Bescoter,⁴ Wenwu Tang,⁵ and Steven Campbell^6
1USDA Forest Service; ²U.S. Geological Survey; ³University of Oregon; ⁴Florida Atlantic University; ⁵University of North Carolina, Charlotte; ⁶USDA Natural Resources Conservation Service

13B.1 Introduction

This chapter used published observational studies and recent syntheses to develop the basis for estimating both the net uptake of atmospheric carbon dioxide (CO₂) by terrestrial wetlands, which is equal to negative net ecosystem exchange (NEE), and the net fluxes of methane (CH₄) from terrestrial wetlands to the atmosphere. The primary source documents were the First State of the Carbon Cycle Report (SOCCR1; CCSP 2007) and the recent Intergovernmental Panel on Climate Change (IPCC) Wetlands Supplement (IPCC 2013). That information was augmented where possible with additional references. There were very few recent reports of measured NEE in comparison to reports on CH₄ flux. Accordingly, there was reliance on the previously published synthesis, with considerable uncertainty remaining in the NEE estimates. Tropical wetland fluxes were derived from the recent synthesis by Sjögersten et al. (2014).

Section 13B.2 summarizes the observational data used as the basis for the area density flux factors. The flux estimates were based on those data and specific references, depending on the assessment area. Section 13B.3 presents the area density flux factors used for each country and region.

13B.2 Literature Review

13B.2.1 Peat Soils

The mean CH₄ and NEE are presented in Table 13B.1. The mean CH₄ flux rate for nonforested and forested wetlands are 23.6 and 8.9 grams (g) of CH₄-C per m² per year, respectively. In comparison, the mean CH₄ flux rate used for peatlands in SOCCR1 was 1.9 g CH₄-C per m² per year. The difference in CH₄ flux rates is attributable to the additional references and the wide range in conditions from the reported studies. The mean NEE for the nonforested and forested wetlands are –135.0 and –124.7 g C per m² per year, respectively. However, there are relatively few reports of measured NEE from peatlands; hence, the basis provided by the published studies is relatively weak. For SOCCR1, NEE was estimated on the basis of net changes in soil and plant carbon, yielding an NEE of –19.0 to –121.0 g C per m² per year for northern and temperate peatlands (CCSP 2007). Plant carbon accumulation was considered negligible for the northern biomes, due the paucity of data. Accordingly, soil carbon accumulation accounted for 100% of the gain in the northern peatlands and 58% in the temperate peatlands.

13B.2.2 Mineral Soils

The mean CH₄ and NEE fluxes for mineral soil wetlands are presented in Table 13B.2. The mean CH₄ flux rate for nonforested and forested wetlands are 26.1 and 26.9 g CH₄-C per m² per year, respectively. In comparison, the mean CH₄ flux rate used for mineral wetlands in SOCCR1 (CCSP 2007) was 6 g CH₄-C per m² per year. As was the case with the peatlands, the variation in CH₄ flux rates is due to the wide range in conditions from the reported studies. The mean NEE for the nonforested areas is –102.1 g C per m² per year. There were too few reports of measured NEE for mineral soil forests; hence, another metric was used. In SOCCR1, NEE was estimated on the basis of net changes in soil and plant carbon, yielding an NEE of –17 to –67 g C per m² per year, for northern and temperate mineral soil wetlands, respectively (CCSP 2007). For that analysis, plant carbon accumulation was considered negligible for the northern biomes, due in large part to the paucity of data. Accordingly, soil carbon accumulation accounted for 100% of the gain in the northern mineral soil wetlands and 25% in the temperate mineral soil wetlands.

13B.3 Country and Regional Density Factors

13B.3.1 Conterminous United States

Carbon flux within the conterminous United States (CONUS) was estimated using area carbon flux density factors (see Table 13B.3). The NEE flux density factors are based on the mean for the peat soil nonforested wetland and mineral soil nonforested wetlands (see Tables 13B.1 and 13B.2). To estimate NEE for the forested wetlands, the SOCCR1 values (Bridgham et al., 2007) were used due to the small number of field-based reports. The estimate in SOCCR1 was based on the annual change in soil and plant carbon; the conservative estimate of 50 g C per m² per year sequestered in forests was used for both peat and mineral soil wetlands (Bridgham et al., 2007). The small number of studies that directly measure NEE in wetlands remains a constraint; hence, the segmented approach used by Bridgham et al. (2007) provides a functional basis.

The CH₄ flux density factors are based on the mean of data reported for the four wetland categories (see Section 13B.2). These mean flux factors are similar to those used in SOCCR1 (CCSP 2007), where the mean for freshwater wetlands was 5.3 g CH₄-C per m² per year.

13B.3.2 Alaska

The available data for establishing the carbon flux for Alaska is very limited. The area density factor for NEE employs the values reported by He et al. (2016), which are based on simulation results (see Table 13B.4). For the CH₄ flux, the mean values used were derived from the literature compilation (see Section 13B.2). In comparison, He et al. (2016) estimated the CH₄ flux at 47.5 g C per m² per year, an amount which is almost twice the value used here; the paucity of data determined use of the more conservative CH₄ flux estimate based on field measurement data.

13B.3.3 Puerto Rico

Estimates of NEE and CH₄ fluxes (see Table 13B.5) were obtained using area density factors for mineral and organic soils derived from the synthesis of tropical wetlands provided by Sjögersten et al. (2014). The same area density factors were used for forested and nonforested wetlands.

13B.3.4 Canada

Carbon flux for Canada was estimated using area carbon flux density factors (see Table 13B.6) calculated on the basis of reported values. The area density factor for NEE in nonforested peatlands and mineral soil wetlands uses the mean reported from measurement studies (see Section 13B.2). For forested wetlands, the value reported in SOCCR1 was used, reflecting the soil carbon accretion, to which was added 31 g C per m² per year sequestered in vegetation, an amount which is based on an 18-year assessment of Canadian forests (Stinson et al., 2011). The analyses of Stinson et al. (2011) did not include changes in soils as a result of bryophytes or sedimentation; hence, adding the soil component seemed appropriate because it was the only component used in SOCCR1 (CCSP 2007).

The CH₄ flux density factors are based on the data average reported for the four categories (see Section 13B.2). These mean flux factors for peatlands are higher than the factor used in SOCCR1 (2.8 g C per m² per year). For freshwater wetlands, the SOCCR1 CH₄ flux was 5.3 g CH₄-C per m² per year, which is considerably lower than the forested and nonforested values (CCSP 2007).

13B.3.5 Mexico

Estimates of NEE and CH₄ fluxes (see Table 13B.7) were obtained using area density factors for mineral and organic soils derived from the synthesis of tropical wetlands developed by Sjögersten et al. (2014). The negative number for NEE indicates net uptake by the ecosystem. The same area density factors were used for forested and nonforested wetlands.

Appendix 13B Supplement: Carbon Pools and Fluxes