Greenhouse gas fluxes from drained organic forestland in Sweden

Scandinavian Journal of Forest Research

ISSN: 0282-7581 (Print) 1651-1891 (Online) Journal homepage: http://www.tandfonline.com/loi/sfor20

Greenhouse gas fluxes from drained organic forestland in Sweden Karin von Arnold , Björn Hånell , Johan Stendahl & Leif Klemedtsson To cite this article: Karin von Arnold , Björn Hånell , Johan Stendahl & Leif Klemedtsson (2005) Greenhouse gas fluxes from drained organic forestland in Sweden, Scandinavian Journal of Forest Research, 20:5, 400-411, DOI: 10.1080/02827580500281975 To link to this article: http://dx.doi.org/10.1080/02827580500281975

Published online: 18 Feb 2007.

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Date: 29 April 2016, At: 05:58

Scandinavian Journal of Forest Research, 2005; 20: 400 /411

ORIGINAL ARTICLE

Greenhouse gas fluxes from drained organic forestland in Sweden

˚ NELL2, JOHAN STENDAHL3 & LEIF ¨ RN HA KARIN VON ARNOLD1, BJO KLEMEDTSSON4 Department of Water and Environmental Studies, Linko¨ping University, Linko¨ping, Sweden, 2Department of Silviculture, Swedish University of Agricultural Sciences, Umea˚, Sweden, 3Department of Forest Soils, Swedish University of Agricultural Sciences, Uppsala, Sweden, and 4Botanical Institute, Go¨teborg University, Go¨teborg, Sweden

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Abstract The objective of this study was to estimate the contribution of drained organic forestlands in Sweden to the national greenhouse gas budget. Drained organic forestland in Sweden collectively comprises an estimated net sink for greenhouse gases of /5.0 Mt carbon dioxide (CO2) equivalents year1 (range /12.0 to 1.2) when default emission factors provided by the Good practice guidance for land use, land-use change and forestry are used, and an estimated net source of 0.8 Mt CO2 equivalents year1 (range /6.7 to 5.1) when available emission data for the climatic zones spanned by Sweden are used. This discrepancy is mainly due to differences in the emission factors for heterotrophic respiration. The main uncertainties in the estimates are related to carbon changes in the litter pool and releases of soil CO2 and nitrous oxide.

Keywords: Carbon dioxide, good practice guidance, greenhouse gas budget, methane, nitrous oxide, peat, scaling.

Introduction The concentration of greenhouse gases (GHG) in the atmosphere is a result of the net strength of diverse sinks and sources. A problem we are facing at present is that the source strength is larger than the sink strength, resulting in increasing atmospheric concentrations of GHG. To assist countries to produce inventories for activities coupled to emissions or removals of GHG, the International Panel on Climate Change has developed guidelines for various socioeconomic sectors. In this context forests are of particular interest because of their ability to accumulate carbon dioxide (CO2), and the role of forestry is specifically considered in the Good practice guidance for land use, land-use change and forestry (GPG-LULUCF) (Penman et al., 2003). Organic soils are formed under wet conditions, since low oxygen concentrations inhibit decomposition (Clymo, 1984). If such a system is drained for forestry, the accumulated organic matter becomes available for aerobic decomposition, leading to

higher soil CO2 release rates (Laine et al., 1996; Silvola et al., 1996a ; Wideń et al., 2001; von Arnold et al., 2005a , b ). At some sites, the net primary production (NPP) of trees and forest floor vegetation can compensate for the soil CO2 release and the forest becomes a net sink (Hargreaves et al., 2003), while other sites become net sources (Lindroth et al., 1998; Lohila et al., 2004). In addition, the nitrogen (N) in the organic matter becomes available for nitrous oxide (N2O)-producing organisms after drainage, and organic soils drained for forestry have been found to be significant sources of N2O (e.g. Martikainen et al., 1993; Maljanen et al., 2003a ; von Arnold et al., 2005a , b ). Moreover, drained organic forest soils can be either sources or sinks for methane (CH4) (e.g. Laine et al., 1996; Nyka¨nen et al., 1998; Maljanen et al., 2003b ; von Arnold et al., 2005a , b ). Some of the most important factors determining the size of GHG fluxes are the climatic zone, soil moisture (Silvola et al., 1996a ), site fertility (Martikainen et al., 1993; Silvola et al., 1996a ; Nyka¨nen et al., 1998) and the tree species present (von Arnold et al., 2005b ).

Correspondence: L. Klemedtsson, Botanical Institute, Go¨ teborg University, PO Box 461, SE 405 30 Go¨ teborg, Sweden. E-mail: [email protected]

(Received 13 December 2004; accepted 17 July 2005) ISSN 0282-7581 print/ISSN 1651-1891 online # 2005 Taylor & Francis DOI: 10.1080/02827580500281975


Greenhouse gas fluxes from drained organic forests Consequently, the net GHG flux from drained organic forestland depends on the site type involved. The overall objective of this study was to estimate the contribution of GHG fluxes from drained organic forestland to the Swedish GHG budget. Accurate scaling up of estimates from small-scale measurements to regional or national scales requires high-quality data on fluxes from different site types. In Sweden, only a few studies have been performed on GHG fluxes from drained organic forestland (Lindroth et al., 1998; Wideń, 2001; von Arnold et al., 2005a , b ). The GPG-LULUCF guidelines present default emission factors, which should be used if country-specific data are not available, and owing to the paucity of Swedish data an estimate was made based on GPG-LULUCF’s default emission factors. However, several studies of GHG fluxes from drained organic forestland have been performed in Finland (Martikainen et al., 1993; Laine et al., 1996; Regina et al., 1996; Silvola et al., 1996a ; Nyka¨nen et al., 1998; Maljanen et al., 2003a , b ; Lohila et al., 2004), and these emission data were assumed to be applicable to forestland in northern Sweden. Therefore, an alternative estimate was also made based on emission factors obtained from Swedish and Finnish studies. Through this procedure, with two different estimates, the aim was to assess the uncertainties involved and to identify aspects where more research is needed.

Materials and methods Area of drained forest land To estimate the total area of organic soil (soils with a carbon content /12% according to criteria provided by the Food and Agriculture Organization, 1998) that had been drained and could support forest production (B/1.0 m3 ha1), were used data from the Swedish National Forest Inventory (S-NFI) for the period 1998 /2002. Owing to limitations of the data in the S-NFI, the selection of sites to include in the analysis had to be based on assumptions about the characteristics of drained forestland. S-NFI covers all forestland influenced by any kind of drainage, including forest road drains, cleaned or enlarged natural streams and excavated slopes of highways (S-NFI, 2000). Consequently, not all sites categorized as drained could be assumed to have organic soils. Unfortunately, the organic content of the soil is not determined in the S-NFI. Instead, the soils are classified in four different categories based on the depth and cover of peat: peat soils with a peat layer thicker than 30 cm over the whole site, over /50% of the site, over B/50% of the site, and

401

mineral soils (S-NFI, 2000). All soils with a peat layer /30 cm are organic, but some of the soils with a peat layer B/30 cm, i.e. soils classified as mineral soils in the S-NFI, also have a carbon content /12%. It was assumed that such soils have been, or are, wet and thus could be recognized by the presence of bottom-layer species typical of wet soils (e.g. Sphagnum spp.). Consequently, the study included 100% of the area with a peat layer over the whole site, 75% of the area of sites with /50% peat coverage, 25% of the area with B/50% peat coverage, and mineral soils where indicator species for wet soils were present. As only productive forested sites were of interest, the non-productive forest area was also excluded (approximately 0.3 Mha with potential forest productivity B/1.0 m3 ha 1). Having estimated the total area of drained organic forestland the areas of various types of site were determined, differing with respect to climatic zone, soil moisture, site fertility and tree species, i.e. factors assumed to be major influences on GHG fluxes (Martikainen et al., 1993; Silvola et al., 1996a ; Nyka¨nen et al., 1998; von Arnold et al., 2005a , b ). The climatic zones used were: northern Sweden, corresponding to regions 1 and 2 in the S-NFI; central Sweden, region 3 in the S-NFI; and southern Sweden, regions 4 and 5 in the S-NFI (Figure 1). Five different categories are used for classifying the drainage status of the sampled sites in the S-NFI: dry, mesic, mesic /moist, moist and wet (S-NFI, 2000). In the estimates the aim was to use only two moisture classes: poorly and well drained. Lowering the groundwater depth to /35 cm below the soil surface is regarded as optimal for nutrient-poor sites and to /55 cm for nutrient-rich sites (Paavilainen & Païva¨nen, 1995). It is, however, difficult to distinguish accurately between poorly and well-drained sites using the information in the S-NFI as there are no strict boundaries between the classes. In general, mesic /moist sites are assumed to have a groundwater table between 0.5 and 1 m (S-NFI, 2000). Therefore, wet and moist sites were classified as poorly drained, and mesic /moist, mesic and dry sites as well drained. Three fertility classes were used, based on potential productivity. Sites with an annual potential forest productivity of 1 /4 m3 ha 1 were classified as lowproductivity, and those with a potential productivity of 4 /8 m3 ha 1 as medium-productivity sites. All sites with a potential productivity over 8 m3 ha1 were classified as highly productive. Finally, the sites in the S-NFI were divided into four classes with respect to the dominant tree species present: pine, spruce, mixed and deciduous (S-NFI, 2000). Two classes were used: coniferous, for areas with ]/ 45%

402

K. von Arnold et al. organic matter in the soil, and root respiration. The following text also uses the term forest floor respiration, defined as the soil respiration plus the respiration of above-ground parts of the forest floor vegetation. Further GHG emissions from forest soils include CH4 released from anaerobic decomposition and N2O from nitrification and denitrification. Change in carbon storage in tree biomass GPG-LULUCF. GPG-LULUCF (Penman et al., 2003) supplies a formula (eq. 1) for estimating carbon uptake by tree biomass (DCG):


DCG Iv BEF1 (1R)DCF A

Figure 1. Percentage of the total productive forestland classified as drained and organic.

pine and spruce; and deciduous, for areas with /55% deciduous tree species (as customary for the S-NFI). The plots in the S-NFI are organized in clusters (tracts) and the calculation of the error of the estimates was based on the frequency (P ) of the estimated class within each tract (P/SRAclass/ SRAtract). The standard error was estimated for the P-values and then scaled to area units. Finally, ranges, i.e. 95% confidence intervals, were calculated.

where DCG /average annual carbon increment in living tree biomass (t C year1), Iv /average annual net increment in the volume suitable for industrial processing (m3 ha 1 year1), BEF1 /biomass expansion factor above-ground/stem increment (dimensionless), R/shoot/root ratio (dimensionless), D /density (t dry mass m3), CF /carbon fraction of dry matter (t t1), and A /area (ha). S-NFI provides data on stem production, i.e. potential productivity (Table I), which were used for Iv. Mean default values and ranges for BEF1, R and D given in GPG-LULUCF were used (Table II) (Penman et al., 2003). Finally, the carbon increase at each specific site type was multiplied by the area (A ) occupied by the respective site types, derived from the S-NFI (Table I).

Alternative estimates. The equation used in the alternative estimate (eq. 2) differed from the GPGLULUCF equation (eq. 1), as allometric relationships (see below) were used to convert stem volume to whole tree volume directly. As in eq. (1), data on potential productivity and areal estimates from S-NFI were used (Table I). DCG Iv BEF2 DCF A

Estimation of fluxes CO2 is incorporated into tree biomass by NPP. It was assumed that the proportion of carbon stored in needles, leaves, fine roots and forest floor vegetation is more or less constant over the years and that the fraction of carbon annually taken up by NPP that is allocated to these parts affects the amount of carbon in the litter pool, but not the amount of carbon stored in tree biomass. CO2 generated by soil respiration originates from two different sources: heterotrophic respiration, i.e. the CO2 released from decomposition of

(1)

(2)

where BEF2 /biomass expansion factor total increment/stem increment (dimensionless); for explanation of other parameters, see eq. (1). According to Marklund (1988), 60% of the whole tree biomass is allocated to the stem in both coniferous and deciduous species. The present calculations included a range of 50 /70%. Consequently, a BEF2 of 1.7 (range 1.4 /2.0) was used. Country-specific data for densities were derived from Boutelje and Rydell (1995) and the default CF value from GPG-LULUCF was used (Penman et al., 2003) (Table II).

Greenhouse gas fluxes from drained organic forests

403

Table I. Productivity data for Swedish drained organic forestland based on Swedish National Forest Inventory data. Average potential productivity (m3 ha 1 year1)

Area9/SE (ha) Moisture

Peat layer

Zone

Productivity class

Coniferous

n

Deciduous

n

Coniferous

Deciduous

Wet/moist

/30 cm

North

Low Medium Low Medium High Low Medium High Low Medium Low Medium High Medium High Low Medium Low Medium High Low Medium High Low Medium Low Medium High Low Medium High Low Medium Medium Low Medium High Low Medium Medium High Low Medium High

42,1139/5629 8339/753 80279/1895 93619/2146 18039/671 65619/1727 10,8159/2277 86179/1768 10,1619/3084 8399/665 18999/1078 35219/1736 8959/482 29399/1213 51119/1645 117,2559/8786 16,2259/3049 30,1419/4532 28,9319/3920 29079/1089 34,1669/4007 80,9459/5138 119,7339/5296 110,6529/11,358 12,7309/3708 62119/2166 42,7479/6257 42749/1538 15279/915 31,0459/4148 37,7629/4224 43789/1772 18469/888 15419/802 19149/980 67799/1493 85499/1449 58559/3039 16619/902 53839/1879 0 3739/481 6259/789 10049/788

65 1 15 24 6 14 25 19 13 1 4 7 3 5 8 188 28 61 73 7 73 214 324 127 19 12 77 9 4 58 79 7 6 3 4 21 32 10 2 9 0 1 4 3

15,4869/3093 33049/1178 4979/542 46829/1269 5149/800 7249/519 56129/1314 12,9809/1764 47589/2031 7759/822 5459/564 16049/861 0 0 24759/1094 39,2359/4211 67649/1283 14649/650 48709/1248 8939/773 3739/457 11,1899/1830 49,6029/3083 12,7599/3069 72709/2102 0 39239/1480 5769/526 0 17499/859 80059/1730 48129/1508 0 0 0 18439/0 20999/655 30619/1596 0 5149/496 473 0 0 21

22 5 1 12 1 1 11 26 7 1 1 3 0 0 6 54 10 2 10 2 1 27 132 18 9 0 9 2 0 4 18 5 0 0 0 3 7 3 0 1 1 0 0 1

2.5 4.6 3.0 5.8 9.6 3.1 6.5 10.4 3.0 4.9 3.2 6.3 9.4 6.9 9.7 2.7 4.6 2.9 5.6 9.5 3.1 6.0 10.6 2.8 4.7 3.4 5.9 9.7 3.0 6.6 9.9 2.7 4.7 5.5 3.8 6.5 10.6 2.9 4.6 6.0 / 3.9 7.1 8.7

2.6 4.3 3.9 5.9 8.7 4.0 6.0 10.6 3.0 5.5 3.8 5.5 / / 10.0 2.8 4.5 2.4 6.4 8.4 2.2 6.7 10.7 3.0 4.8 / 7.0 8.4 / 6.7 9.8 2.0 / / / 7.8 10.3 3.4 / 7.1 9.4 / / 11.3

Central

South

B/30 cm

North Central


South Mesic /moist

/30 cm

North Central

South

B/30 cm

North Central

South

Mesic/dry

/30 cm

North Central South

B/30 cm

North Central South

Litter input to soil GPG-LULUCF. In GPG-LULUCF litter inputs are thought to be in balance with decomposition and, consequently, are neglected in the estimates.

Alternative estimate. The litter mainly consists of needles, leaves and fine roots from trees and forest floor vegetation. Bray and Gorham (1964) estimated the needle and leaf production in cool temperate regions to amount to around 30 and

50% of the stem production for coniferous and deciduous tree species, respectively. Further, they reported that on average 9% of the total litter input originates from the forest floor vegetation. Consequently, litter-input data of 33 and 55% of stem production were used for coniferous and deciduous forests, respectively (including ranges of 30/40% for coniferous forests and 50 /60% for deciduous forests). The fine root litter is considered in the estimates of soil emissions (see Soil emission factors , Alternative estimate , below).

404

K. von Arnold et al.

Table II. Default factors for input parameters for eqs (1) and (2).

BEF R D CF

Boreal coniferous species

Boreal deciduous species

1.15 (1.00 /1.30) 0.32a (0.12 /1.06) 0.41b 0.5

1.10 (1.00 /1.30) 0.26a (0.12 /0.93) 0.51c 0.5

Temperate coniferous Temperate deciduous species species 1.10 (1.00 /1.30) 0.32a (0.12 /1.06) 0.41b 0.5

1.20 (1.10 /1.30) 0.26a (0.12 /0.93) 0.51c 0.5

Coniferous species 1.7 (1.4 /2.0)

Deciduous species 1.7 (1.4 /2. 0)

0.44d (0.37 /0.50) 0.60c (0.58 /0.62) 0.5 0.5

Note: Data are the mean values used in the calculations and ranges in parentheses. BEF/biomass expansion factor (dimensionless); R/shoot/root ratio (dimensionless); D /density (t dry mass m 3); CF /carbon fraction of dry matter (t t 1). a R values for stands with a standing biomass of 50 /150 t ha 1 for coniferous and 75 /150 t ha 1 for deciduous species are shown as an average and range for all of the different stands; b mean value of Picea abies (0.40 t dry mass m 3) and Pinus sylvestris (0.42 t dry mass m 3); c mean value of Betula , as birch is the dominating deciduous species in Swedish forestland; d mean value of P. abies (0.37 /0.44 t dry mass m 3) and P. sylvestris (0.45 /0.50 t dry mass m 3).


Soil emission factors GPG-LULUCF. The GPG-LULUCF guidelines present default factors for heterotrophic respiration and N2O emissions, while emission factors for CH4 are not available owing to a paucity of data (Penman et al., 2003).

Alternative estimates. To calculate alternative emission factors for the different drained organic forestland site types, annual GHG emissions data reported in Finnish and Swedish studies were used (Table II). Although the standard errors of the literature data were sometimes large, only the mean values were used. However, if emissions had been measured during more than one year and the annual emissions were reported separately for each of the years, the different years were used as independent samples to cover the temporal variation (Table III). The three sites from which previously unpublished data had been collected by von Arnold were all situated in southern Sweden (57808? N, 14845? E). The tree layer was dominated by Picea abies at all three sites, and the forest floor vegetation was dominated by Vaccinium myrtillus at two of the sites and by low herbs, such as Viola palustris , at the third. The mean annual groundwater table differed among sites, ranging from 12 to 60 cm (Table III). At the sites, four, five or nine chambers were sampled once a month, over a 2 year period, using the method described in von Arnold et al. (2005a , b ). The site for which unpublished data, collected by Weslien, are presented was also situated in southern Sweden (58820? N, 13830? E). It was dominated by Betula pendula , Rubus idaeus and Urtica dioica . Seven chambers were sampled weekly or biweekly, using the method described in von Arnold et al. (2005a , b ) and the mean annual groundwater table ranged from 48 to 70 cm during two sampling years (Table III).

Estimating heterotrophic respiration is problematic, as most studies have reported either soil or forest floor respiration (Table II). It was assumed that the contribution of respiration in above-ground parts of the forest floor vegetation was negligible, making soil and forest floor respiration comparable. Owing to the inhibition techniques used for measuring the contribution of root respiration, the estimates of CO2 released from roots include not only root respiration but all root-derived activity (i.e. root respiration and decomposing activity associated with root exudates and recently dead root tissues, e.g. fine roots). Around 50% of the soil respiration in forests has been reported to originate from rootderived activity (Silvola et al., 1996b ; Hanson et al., 2000). Consequently, the soil respiration was divided by 2 to obtain an estimate of the heterotrophic respiration. This procedure subtracts not only the root respiration but also the litter input from fine root turnover. The sites were classified according to the climatic zone in which they are situated, the mean annual water table, site fertility and tree species (Table III). For site fertility classifications, the classification system presented by Ha˚nell (1991) was used. Herb-dominated sites and bilberry/ horsetail-dominated sites characterized by spruce and deciduous trees were classified as highly productive. Bilberry /horsetail-dominated sites characterized by pine and tall-sedge and dwarf shrub types were classified as medium-productivity sites, and Carex globularis , low sedge and marsh andromeda /cranberry types as low-productivity sites. To determine whether the soil fluxes of CO2, CH4 and N2O differed among sites that differed in terms of climatic zones, mean annual groundwater tables, fertility or dominant tree species, non-parametric Mann /Whitney and Kruskal/Wallis tests were performed using SPSS 10.0 software (SPSS, Chicago, IL, USA). For both climatic zone and site fertility,

Greenhouse gas fluxes from drained organic forests Table III. Soil emissions of carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O) (g m sites. CO2

Method

880 890 1173 1346 1254 1397

SR SR SR SR SR SR

Heterotrophic respiration 440 449 587 673 627 699

CH4

N2O

2.81 2.77 0.11 0.01 0.79 0.84

0.00 0.01 0.01 0.09 0.00 0.02 0.66

/0.52


0.00 0.00 0.00 0.00 0.21 0.08 0.10 0.22 0.20 3.28 2.78 3.17 2.21 3.49 3.45 1.07 1.13 /0.05 0.02 0.20 0.03 0.87 1.41 0.56 /0.09 /0.08 /0.24 0.82 1001 1445 862 880 1243 1386 1195 1217 1272 1177 1327 1412 1632 513 656 2222 970 1918 1356 1527 938 1108 1329 1460 2224 1535

SR SR SR SR SR SR SR SR SR SR SR SR SR SR SR SR FR FR FR FR FR FR FR FR FR FR

501 723 431 440 622 693 598 609 636 589 664 706 816 257 328 1111 395 869 678 764 469 554 665 730 1112 768

0.47 1.61 0.06 /0.01 /0.01 0.37 0.48 0.51 1.27 0.11

0.03 0.04 0.09 0.07 0.05 0.06 0.04 0.19 0.19 1.13

Groundwater level /20 /20 /30 /30 /30 /30 129 129 /20 /30 /30 /30 /30 /40 /40 /40 /50 22 35 21 32 12 17 29 45 30 40 32 48 43 18 30 43 41 45 32 23 23 20 20 14 14 30 31 31 36 36 30 43 21 21 55 14 17 22 25 23 20 21 16 15 17

2

year

1

405

) reported from drained forest

Zone

Productivity

Tree species

Ref.

Central Central Central Central Central Central North North Central Central Central Central Central Central Central North North Central Central Central Central Central Central Central Central Central Central Central Central Central North North North North North North Central Central Central Central Central Central Central Central Central Central Central Central Central North North North South South South South South South South South South South

Low Low Medium Medium Medium Medium High High Low Low Low Low Medium Medium Medium Medium Medium Low Low Low Low Low Low Medium Medium Medium Medium Medium Medium High Low Low High High High Medium Low Low Low Low Low Low Medium Medium Medium Medium Medium Medium High Low Low Medium Low Low High High High High High High High High

Con Con Con Con Con Con Dec Dec Con Con Con Con Con Con Dec Con Con Con Con Con Con Con Con Con Con Dec Dec Dec Dec Dec Con Con Dec Dec Dec Dec Con Con Con Con Con Con Con Con Con Con Con Con Dec Con Con Con Con Con Con Con Con Con Con Dec Dec Dec

a a a a a a b c d d d d d d d d d e e e e e e e e e e e e e e e e e e f g g g g g g g g g g g g g g g g h h h h h h h i i i

406


Table III (Continued ) CO2

Method

1871 1123 1103 1929 2200 2760 6516 2800

FR FR FR FR FR FR FR FR

Heterotrophic respiration 936 562 552 965 1100 1380 3138 1181

CH4

N2O

Groundwater level

1.72 4.73 0.59 /0.11 /0.02 /0.10

0.61 0.06 0.01 0.02 2.20 2.90

18 13 12 60 48 70 17 70

Zone South South South South South South South South

Productivity

Tree species

Ref.

High High High High High High Medium High

Dec Con Con Con Dec Dec Con Con

i j j j k k l l


Note: The CO2 flux measurements refer either to soil respiration (SR) or forest floor respiration (FR). Con/coniferous; Dec/deciduous; a/Laine et al. (1996); b/Maljanen et al. (2003a ); c /Maljanen et al. (2003b ); d/Martikainen et al. (1993); e/Nyka¨nen et al. (1998); f /Regina et al. (1996); g/Silvola et al. (1996a ); h/von Arnold et al. (2005a ); i/von Arnold et al. (2005b ); j/von Arnold (unpublished data); k/Weslien (unpublished data); l /Wideń (2001).

three different groupings were used, i.e. north, central and south of Sweden, and low, medium and high productivity, respectively. For mean annual groundwater table, three different tests were performed using two groupings in each case: above and below 30, 40 and 50 cm. For tree species two groupings were used: coniferous and deciduous tree species. The emissions were regarded as differing among groupings only in cases where the difference was statistically significant (p B/0.05). For climatic zone and site fertility, where three groupings were used, a difference was only considered to be significant if it was found among all three groups and followed the same trend. For example, if emissions were significantly higher from mediumthan from low-productivity sites, but the emissions from low-productivity sites did not differ significantly from (or were significantly higher than those from) high-productivity sites, the impact of site fertility was considered to be insignificant. In cases where more than one characteristic could significantly separate the emissions, e.g. the emissions differed among both sites with different mean annual groundwater table and sites dominated by different tree species, the most significant separation was considered first. Thereafter, the separation capacity of the second characteristic was checked within the subgroups (e.g. if there was a significant, p B/0.05, difference between emissions from sites with a groundwater table /40 cm dominated by coniferous species and sites with a groundwater table /40 cm dominated by deciduous species). If no significant differences were found within these subgroups, the second parameter was ignored. When the characteristics on which the site type separations were to be based had been determined, the mean values for each type of site were calculated, and used as alternative emission factors for them.

Standard errors were then calculated for the alternative emissions factors and ranges, i.e. 95% confidence intervals as in GPG-LULUCF (Penman et al., 2003). Net ecosystem flux The total change in carbon storage in tree biomass and litter input to soil was calculated by adding together the data, i.e. the change in carbon times the average area, for all different site types. The range for the total change in carbon in biomass and litter was, thereafter, calculated by dividing the highest and lowest estimate by the total average area and multiplying the numbers by the highest and lowest areal estimates, respectively. The total emissions from the soil were calculated by multiplying the emission factor for a certain site type by the area occupied by the site type. The ranges were calculated by multiplying the lowest number of the range for the emission factor by the lowest number of the range for the areal estimate and the highest number of the range for the emission factor by the highest number of the range for the areal estimate. To calculate the net ecosystem flux the heterotrophic respiration, CH4 and N2O fluxes were added together using GWP values of 1 for CO2, 23 for CH4 and 296 for N2O (Houghton et al., 2001). Thereafter, the carbon change in tree biomass and litter was subtracted from the soil emissions. Consequently, negative net emissions are reported for net sinks and positive values for net sources. The ranges for the net ecosystem flux were calculated by subtracting the highest number of the range for the soil emissions from the lowest number of the range from carbon change in litter and biomass and vice versa. The highest number in the range for areal estimate was used in both cases.


Greenhouse gas fluxes from drained organic forests

407

Results

Carbon change in biomass and litter

Area estimates

The two estimates of carbon uptake by tree biomass did not differ significantly. Based on default values and functions provided by GPG-LULUCF the carbon increase in the trees on drained organic forest soil in Sweden amounted to 6.9 Mt CO2 year 1 (range 5.0 /13.2), while the alternative values gave a figure of 8.8 CO2 Mt year 1 (range 6.0 /12.2) (Table IV). The litter fraction, which was not included in the GPG-LULUCF, was estimated to amount to 2.0 Mt CO2 year1 (range 1.6 /2.8) in the alternative estimate (Table IV).

The total estimated area of drained organic forestland based on S-NFI data was 1,046,0009/SE 32,000 ha (Table I), about 70% of which consisted of soils with a /30 cm peat layer (710,000 ha) and about 30% of soils with a B/30 cm peat layer (336,000 ha). In northern Sweden, the area of drained organic forestland was estimated as 423,000 ha, and the areas were concentrated close to the east coast (Figure 1). In central Sweden there were 168,000 ha and in southern Sweden 455,000 ha of drained organic forestland. In the southernmost region most of the drained areas were found in the south-western part (Figure 1). Only about 16% of the drained area was classified as moist/wet (about 167,0009/ 12,000 ha), and was thus placed in the poorly drained category (Table I). The rest was classified as well drained (about 879,0009/29,000 ha), i.e. mesic /moist (about 826,000 ha) or mesic/dry (53,000 ha) (Table I). Coniferous species dominated 79% and deciduous 21% (832,0009/28,000 ha and 215,0009/13,000 ha, respectively) (Table I). Over 50% (581,000 ha) of the area was classified as highly or medium productive. The potential productivity of all tree species was higher in the southern part of the country and the most fertile sites were dominated by spruce or deciduous tree species (Figure 2).

Soil emission factors GPG-LULUCF. The GPG-LULUCF guidelines give different default factors for heterotrophic respiration in temperate and boreal drained organic forest soils, of 0.68 t C ha 1 year 1 (range 0.41 /1.91) and 0.16 t C ha 1 year 1 (range 0.08 /1.09), respectively (Penman et al., 2003). In the present calculations the forests were assumed to be temperate in the southernmost region and boreal in the other two regions (Figure 1). Similarly, different N2O emission factors are provided for nutrient-rich and nutrient-poor drained organic forest soils, and default emission factors of 0.1 kg N2O-N ha1 year 1 (range 0.02 /0.30) were used for nutrientpoor organic soils and 0.6 kg N2O-N ha 1 year 1 (range 0.16 /2.40) for nutrient-rich organic soils (Penman et al., 2003). In the present calculations,

Figure 2. Average potential forest productivity (stem m3 ha 1) for different tree species. The map to the left shows areas dominated by spruce, the map in the middle areas dominated by pine and the map to the right areas dominated by deciduous tree species.

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Table IV. Total flux estimates (Mt CO2 equivalents year1) based on Good practice guidance for land use, land-use change and forestry (GPG-LULUCF) and alternative emission factors. GPG-LULUCF GHG flux Carbon increase in tree biomass Carbon increase in litter Heterotrophic respiration CH4 emissions N2O emissions Sum

Alternative estimate

Average

Range

Average

Range

/6.9

/13.2 to /5.0

/8.8 /2.0 10.8 0.07 0.7 0.8

/12.2 to /6.0 /2.8 to /1.6 8.3 to 13.7 0.00 to 0.16 0.2 to 1.3 /6.7 to 5.1

1.7

1.0 to 6.3

0.2 /5.0

0.0 to 0.6 /12.0 to 1.2


Note: GHG/greenhouse gas; CH4 /methane; N2O/nitrous oxide.

medium- and high-productivity sites were considered to be nutrient-rich, and low-productivity sites nutrient-poor. Applying the emission factors supplied by GPGLULUCF, the estimated annual rate of CO2 release from heterotrophic respiration in Swedish drained organic forestland amounted to 1.7 Mt CO2 year 1 (range 1.0 /6.3) and the N2O emissions totalled 0.2 Mt CO2 equivalents year 1 (range 0.0 /0.6) (Table IV).

(Table V). Finally, the derived N2O emission factors were 0.3 kg N2O-N ha 1 year 1 (range 0.12 /0.48) for sites dominated by coniferous species and 6.2 kg N2O-N ha1 year1 (range 2.18 /10.22) for sites dominated by deciduous tree species (Table V); three to nine times higher than the GPG-LULUCF default emission factors. Applying the alternative emission factors, heterotrophic respiration caused a release of 10.8 Mt CO2 year 1 (range 8.3 /13.7), the CH4 fluxes contributed another 0.07 Mt CO2 equivalents year 1 (range 0.00 /0.16) and the N2O emissions 0.7 Mt CO2 equivalents year 1 (range 0.2 /1.3) (Table IV).

Alternative estimate. In the calculation of alternative emission factors, the most significant factor affecting heterotrophic respiration and CH4 emissions was found to be the groundwater table, the threshold of 40 cm being particularly significant, while N2O emissions were most strongly affected by tree species (Table V). Separations based on the second most significant characteristic did not result in significant differences between the subgroups in any case. The emission factors for heterotrophic respiration derived from the calculations were 3.0 t CO2-C ha1 year 1 (range 2.49 /3.51) for the well drained sites and 1.9 t CO2-C ha 1 year 1 (range 1.45 / 2.35) for the poorly drained sites (Table V). These values are higher than the default values presented in GPG-LULUCF. The derived CH4 emissions factors were 0.6 kg CH4-C ha1 year1 (range /1.46 / 2.66) for well drained and 10.2 kg CH4-C ha1 year1 (range 6.57 /13.83) for poorly drained sites

Net ecosystem emissions The estimate based on data provided by GPGLULUCF suggested that the area of drained organic forestland in Sweden represents a net sink of /5.0 Mt CO2 equivalents year1 (range /12.0 to 1.2) (Table IV). Using the alternative emission factors, in contrast, indicated that the drained organic forestland in Sweden is a net source of 0.8 Mt CO2 equivalents year 1 (range /6.7 to 5.1) (Table IV). Discussion The two estimates of net exchange GHG rates from the drained organic forestland presented here differ substantially, and the ranges in both cases are large. To improve the quality of regional or national

Table V. Asymptotic significance (two-tailed) for the statistical tests.

Groundwater level

Zone Productivity Tree species

CO2

CH4

N2O

B/30B/ /30, 0.025 B/40B/ /40, 0.001 B/50B/ /50, 0.003 North B/centralB/south, 0.033 LowB/mediumB/high, 0.003 ConB/Dec, 0.002

/30B/ B/30, 0.002 /40B/ B/40, 0.000 /50B/ B/50, 0.006 North B/centralB/south, 0.015 HighB/mediumB/low, 0.001 DecB/Con, 0.002

B/30B/ /30, 0.338 B/40B/ /40, 0.010 B/50B/ /50, 0.102 CentralB/south B/north, 0.002 LowB/mediumB/high, 0.001 ConB/Dec, 0.000

Note: CO2 /carbon dioxide; CH4 /methane; N2O/nitrous oxide; Con/coniferous; Dec/deciduous.

Greenhouse gas fluxes from drained organic forests estimates of these rates, both more detailed data on the differences between different types of sites and more emission data are needed.


Areal estimates First of all, the areal estimates are clearly important. Estimates of the Swedish land area consisting of drained organic forestland are presented in GPGLULUCF (i.e. 524,000 ha), but only drained peatland is included (Penman et al., 2003). However, high rates of soil respiration have also been found in forests situated on organic soils with a peat layer thinner than 30 cm (Lindroth et al., 1998; von Arnold et al., 2005b ). Therefore, the authors believe that all drained organic forestland should be included in the estimates, as in this study. As soil organic content is not included in the S-NFI data (S-NFI, 2000) the areal estimate of organic soils with a peat layer B/30 cm involves large uncertainties. Consequently, the present S-NFI database is not detailed enough to allow these sites to be accurately identified. The way in which the drained organic forestland is classified also affects the calculations. The alternative emission factors differed both among sites that differed in their mean annual groundwater tables and among sites dominated by different tree species. The use of mean annual groundwater tables for classifying sites instead of climatic zone poses problems, since the soil moisture determinations are very rough in the S-NFI (S-NFI, 2000). The assumptions used to estimate the areas of sites with mean groundwater levels above and below this threshold may have significant effects on the calculated net impact. For example, the estimated total annual heterotrophic respiration from drained organic forestland would be 30% lower if mesic /moist sites had been assumed to have a mean annual groundwater level in the upper 40 cm of the soil, rather than below this level. At forested sites the soil nutrient conditions are closely associated with tree species (Menyailo et al., 2002), since litter quality differs among species (Johansson, 1995). In the calculation of the alternative emission factor, tree species was found to be a better predictor of N2O emissions than productivity. However, accurate scaling up cannot be based on tree species alone as N2O emissions from drained organic forestland, which have been previously used for agriculture, have been found to be very high (i.e. 5/21 kg N2O-N ha1 year1) regardless of the tree species they support (Maljanen et al., 2004). In the 1920s to 1940s about 600 /700 kha of land with organic soils was used for agriculture in Sweden, whereas the current area is about 250 /300 kha. It is very likely that a major part

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of the land area formerly used for agriculture is currently forested and, thus, between 30 and 45% of the organic drained forestland in Sweden may have been used for agriculture. This area will not be accounted for if the estimate is only based on tree species, which may result in the national N2O emissions from drained organic forestland being underestimated by a factor of up to 4. Greenhouse gas fluxes The most important fluxes are those of CO2. For the carbon change in tree biomass it is unknown whether the use of GPG-LULUCF default values or the alternative values provide the most accurate estimate. However, they were not significantly different and the errors involved (if any) are unlikely to be major. The litter pool, in contrast, is associated with large uncertainties. The litter input has been measured at three coniferous forests in Sweden (Berggren et al., 2004), including a drained organic forested site used in the present emission factor calculations (von Arnold et al., 2005a ). Within this forest the total litter input (i.e. including needles, leaves and litter input from forest floor vegetation) was estimated to be equivalent to around 30% of the carbon change in stem biomass at dry and mesic plots and around 50% of the changes in moist plots (Berggren et al., 2004). Consequently, these values are similar to those reported by Bray and Gorham (1964) used in the scaling in this study. However, much more research is needed to estimate accurately litter inputs in drained organic forests. The largest uncertainties in these estimates are associated with the emission factors. A potential source of error was the application of Finnish data to northern Sweden. Further, all of the data from southern Sweden are measurements of forest floor respiration, whereas all of the data from central and northern Sweden are soil respiration (i.e. excluding forest floor respiration). This may have introduced a systematic error. However, a better estimate cannot be provided at present. The CO2 release rates from Swedish organic forestland based on the alternative estimates are more than seven times higher than estimates based on the GPG-LULUCF data (Table IV). The authors believe that the alternative estimates are more accurate than those based on GPGLULUCF values. To their knowledge only two studies have measured the net annual ecosystem CO2 exchange at drained forests using micrometeorological techniques. The sites investigated in these studies were both well drained according to the present classification. One was reported to be a net source of between 240 and 800 g CO2 m 2 year 1

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(Lindroth et al., 1998), while preliminary data from the other site, a formerly agricultural pine site in Finland, suggest that it was a net emitter of about 50 CO2 g m 2 year1 (Lohila et al., 2004). Applying these net fluxes to the total area of drained organic forestland in Sweden makes it an estimated net annual source of between 0.5 and 8.0 Mt CO2, i.e. a source even larger than the alternative estimate (Table IV). Similarly, the net N2O emission from drained organic forestland was higher according to the alternative estimates than when GPG-LULUCF default values were used. However, the estimated net N2O release from drained organic forest soil is still 30% lower than earlier estimates of Swedish N2O emissions from drained and harvested forests (Rodhe, 1995).

study involve large uncertainties concerning both carbon increases in biomass and litter, and heterotrophic respiration. Moreover, more accurate scaling up of N2O emissions from drained organic forestland is required as the emissions may be much higher from the drained organic forestland in Sweden formerly used for agriculture.

Acknowledgements This work formed part of the ‘‘Land use strategies for reducing net greenhouse gas emissions’’ project, supported by the Foundation for Strategic Environmental Research (MISTRA) and part of the ‘‘Emission from drained forest soils’’ project, supported by the Swedish Energy Agency.

Contribution of drained organic forestland to the Swedish greenhouse gas budget

References

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