CO2 exchange

An overview of estimates of the carbon sequestration in European forests is given in Table 17.1. Apart from a distinction in the type of flux and forest compartment, a differentiation has been made in the quality of the upscaling methods, going from individual sites to the European scale. A direct comparison of the data in Table 17.1 is hampered, because of the differences in the NEP, NEE and net biome production (NBP). The NEP and NEE stand for the total uptake of CO2 by photosynthesis, corrected for plant and soil respiration. The NBP is the NEP corrected for CO2 emissions due to harvest and forest fires. The latter term is critical with respect to long-term carbon storage, since an aggrading forest may sequester large amounts of carbon, but most of it is emitted again to the atmosphere after logging. For a more detailed discussion about productivity terminology (gross primary productivity (GPP), NPP, NEP, NBP, etc.) we refer to Hymus and Valentini (Chapter 2, this volume). A systematic discussion related to the various approaches and results is given in De Vries et al. (2001, 2005b).

De Vries et al. (2005b) estimated the carbon sequestration of European forests during 1960-2000 by using data from the 6000 level I plots, which are assumed to be representative of 162 million hectares of forests. Data for the stand age and site quality were used to estimate the actual forest growth, using yield tables from the early 1960s, when nitrogen deposition was still low (reference deposition) (Klap et al., 1997). The NEP was calculated by using a carbon content of 50%. The impact of elevated nitrogen deposition during 1960-2000 was accounted for by a method described in Section 17.4. The carbon pool change in stem wood due to forest growth thus derived equalled 0.281 Pg/year, which is comparable to the estimates given in Table 17.1. Dividing this estimate by 162 million hectares of European forests leads to a mean carbon pool change in tree stem wood of ~1730 kg/ha/year. The net carbon sequestration rate or carbon sink in stem wood was calculated by assuming that NBP equals 33% of the NEP. This percentage is based on an estimated average NBP/NEP ratio for Europe, implying a net increase in standing forest biomass of 33% of the growth since 67% is removed by harvesting or forest fires (Nabuurs and Schelhaas, 2003). This translates to an NBP of 0.094 Pg/year, which is comparable to estimates listed in Table 17.1, based on repeated forest inventories using country inventory data (Kauppi et al., 1992; Nabuurs et al., 1997) and by modelling forest growth (Liski et al., 2002). Dividing this estimate by 162 million heatares of European forests leads to a mean net carbon sequestration rate of ~575 kg/ha/year.

Apart from sequestration in the trees, carbon can also be sequestered in the soil. The variation in soil carbon sequestration estimates is larger than for trees due to the difficulty of its assessment. For example, changes in the carbon pool in forest soils from repeated soil inventories are hard to detect within a short period of time, considering the large size of the soil carbon pools, with the possible exception of the organic layer (see also De Vries et al., 2000). An estimate of the net carbon sequestration in the soil from direct measurements of the carbon input to the soil by litterfall and root decay, as well as by carbon release by mineralization (e.g. Schulze et al., 2000), is hampered by the fact that the result is based

Table 17.1. Overview of different estimates of carbon sequestration on a European wide scale.

Type of Estimated sink Upscaling carbon flux Compartment Method (Pg/year) method Reference

NBP landscape

NBP Landscape

NEE/NEP whole forest/trees

Whole forest

Total

(aboveground biomass)

NBP whole forest/trees

NEP contribution

Trees (stem wood)

Trees (stem wood) Trees

(aboveground biomass)

Inversion modelling measurements

Tree growth measurements

Repeated forest inventories Modelling forest growth Nitrogen retention

0.30

0.10

0.06-0.10b 0.039c

Good

Bousquet et al. (1999)

Neural networks Papale and Valentini

Forest maps

Multiply with forested area

(2003) Martin et al.

Country Kauppi et al. (1992)

inventory data Nabuurs et al. (1997)

Country inventory data World average values

Nadelhoffer et al.

CO2 net flux

NBP forest soil NBP

Forest soil (belowground biomass)

Forest soil (belowground biomass)

Forest soil

Carbon soil input minus carbon mineralization Modelling forest growth and decomposition Nitrogen

(belowground retention biomass)

0.14a

0.031-0.049b

Multiply with forested area

Country inventory data

Schulze et al. (2000)

0.034c World average Nadelhoffer et al.

values

NBP = net biome production; NEP = net ecosystem production; NEE = net ecosystem exchange.

aThe estimates derived by Schulze et al. (2000) were slightly lower based on a forested area in Europe of 149 million hectares, but the estimates were scaled to an area of 162 million hectares, used in this study.

bThese estimates were originally limited to the EU + Norway and Switzerland (~138 million hectares) but results were scaled to the European forested area, excluding most of Russia (~162 million hectares).

cThese estimates were originally global but were scaled to the European nitrogen deposition on forests of 1.54 Mt/year. Actually, the estimate by Nadelhoffer et al. (1 999) for carbon sequestration in trees refers to the contribution of nitrogen deposition to NEP by trees and not to the total NEP by forest growth.

on subtracting large numbers with relative high uncertainties. In this context a modelling exercise seems most reasonable (e.g. Liski et al., 2002).

De Vries et al. (2005b) estimated the long-term net soil carbon sequestration in European forest soils by calculating the nitrogen immobilization (sequestration) in soils at ~6000 level I plots, multiplied by the carbon/nitrogen ratio of the forest soils. Nitrogen immobilization (sequestration) was calculated as a fraction of the nitrogen deposition corrected for nitrogen uptake, according to

N immobilization = frNimmobilization

The fraction frNimmobilization was calculated as a function of the carbon/nitrogen ratio of the forest soil using available results on this relationship (Gundersen et al., 1998) and those obtained from budgets for 121 intensive monitoring plots (De Vries et al., 2001). By multiplying the net nitrogen immobilization with the carbon/nitrogen ratio, the variation of the carbon/nitrogen ratio with the depth of the soil profile was accounted for, according to

C sequestration = N immobilization . (fretff. C/Nff + (1 - fretf . C/Nms)

where C/Nff and C/Nms are the carbon/nitrogen ratios of the forest floor and the mineral soil (up to a depth of 20 cm), respectively, and fretff is the nitrogen retention fraction in the forest floor, as it is the ratio of the nitrogen retention in the forest floor to the nitrogen retention in the complete soil profile (forest floor and mineral soil). The carbon sequestration in forest soils was calculated for the period 1960-2000 using site-specific estimates for more than 6000 level I forest plots in a systematic grid of 16 x 16 km, according to:

• Nitrogen (NH4, NO3) deposition: European Monitoring Evaluation Programme (EMEP) model estimates for the period 1960-2000 (data at 5-year intervals that were linearly interpolated);

• Net nitrogen uptake: yield estimates as a function of stand age and site quality as described by Klap et al. (1997), multiplied by deposition-dependent nitrogen contents in biomass;

• Fretff: related to measured carbon/nitrogen ratios in forest soil and modelled fraction NH4 in deposition, based on results of15N tracer experiments (Tietema et al., 1998; Nadelhoffer et al., 1999) as given by De Vries et al. (2005b);

• Carbon/nitrogen ratios for forest soils: measurements, partly extrapolations.

The estimated carbon sequestration using Eq. 17.2 equalled 0.023 Pg/year, which is 50% less than the global estimate derived by Nadelhoffer et al. (1999), scaled to Europe (0.034 Pg/year; De Vries et al., 2005b). This is to be expected since these authors assumed a constant high retention fraction of 0.70 in the forest soil (organic layer with a carbon/ nitrogen ratio of 30), whereas we used a carbon/nitrogen ratio-dependent fraction also occurring below the humus layer with lower carbon/nitrogen ratios. With the assumption that all the net incoming nitrogen is retained (total immobilization, no leaching) we got an estimate (0.042 Pg/year) that is slightly higher than that of Nadelhoffer et al. (1999). The calculated net carbon sequestration in the soil during 1960-2000 of ~0.023 Pg/year translates to an average accumulation of 143 kg/ha/year. This estimate is seven times lower than the value derived by Schulze et al. (2000), based on the estimated carbon retention in 11 sites (0.13-0.17 Pg C/year), but this is likely to be an overestimate, as it would imply that the carbon/nitrogen ratio of European forest soils is strongly increasing and there are no indications that this is the case. Furthermore, the calculated net soil carbon sequestration is in line with a calculated average value of 110 kg/ha/year for 16 typical forest types across Europe, derived by Nabuurs and Schelhaas (2002), and of 190 kg/ha/year, based on a modelling exercise for a large part of Europe by Liski et al. (2002).

The geographic variation in carbon sequestration in trees and soils is illustrated in Fig. 17.3. Carbon sequestration is small in northern Europe, where nitrogen input is low and nearly all incoming nitrogen is

Fig. 17.3. Geographic variation of calculated carbon sequestration in trees and soil over Europe.

retained by the vegetation, and higher in central and eastern Europe where nitrogen input is higher. The finding that carbon sequestration is negligible in the northern boreal forest is in line with results from Martin et al. (1998) based on flux measurements for CO2.

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