Biometric and eddy covariance-based assessment of decadal carbon sequestration of a temperate Scots pine forest

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Abstract

In recent years, the status of forests as sources or sinks of carbon has received much attention. Nonetheless, evidence-based long-term estimates of the magnitude of the carbon sequestration in forests are still scarce. In this study we present two independent estimates of net carbon sequestration in a temperate Scots pine dominated forest ecosystem over a 9-year period (2002–2010). First, the net ecosystem carbon balance (NECB) was estimated from net ecosystem CO2 exchange as measured by the eddy covariance technique (NECBEC). To this end, the eddy covariance estimates were combined with non-CO2 carbon fluxes such as DOC leaching and VOC emissions. The second approach to determine the carbon sequestration was based on the changes in the ecosystem carbon stocks over time (NECBSC). For this NECBSC estimate, two assessments of the ecosystem carbon stocks (2002 and 2010) were compared. Results showed that the eddy covariance approach estimated a net uptake of 2.4 ± 1.25 tC ha−1 yr−1, while the stock based approach suggested a carbon sink of 1.8 ± 1.20 tC ha−1 yr−1. No significant change was observed in the mineral soil carbon, while the carbon stock of the litter layer slightly decreased. Phytomass was thus the main carbon sink (2.1 tC ha−1 yr−1) in the pine forest, predominantly in the stems (1.3 tC ha−1 yr−1). The fact that stem wood is the main carbon sink within the ecosystem implies that the future harvesting has the potential to fully offset the CO2 uptake by this Scots pine forest.

Highlights

Eddy covariance based carbon balance over a nine-year period. ► Extensive stock based carbon balance including thorough soil carbon stock assessment. ► Eddy covariance estimate was within the range of the stock change estimate. ► No significant change in soil carbon stock over the nine-year period.

Introduction

Temperate forests store up to several orders of magnitude more carbon than grasslands or shrubland vegetation (Whittaker and Likens, 1975). Consequently, much recent attention has focused on the status of temperate forests as sources or sinks in the global carbon cycle (Canadell et al., 2000, Fan et al., 1998). However accurate estimates on the magnitude of carbon sequestration of temperate forests are still scarce.

The effort to increase our understanding of the carbon balance has resulted in a global network of eddy covariance towers (Baldocchi, 2008). These towers continuously measure the exchange of CO2 and H2O between the ecosystem and the atmosphere and as such, monitor the net ecosystem productivity NEP (i.e. the net balance between CO2 uptake and release) and few of them have now gathered long term datasets (Granier et al., 2008, Pilegaard et al., 2011). The eddy covariance NEP estimates (NEPEC) do, however, not account for non-CO2 lateral carbon losses from the ecosystem. Thus, to estimate the actual net carbon accumulation or net ecosystem carbon balance (NECB, i.e. the amount of carbon that is sequestered or lost from the ecosystem) from the eddy covariance measurement (NECBEC), the NEPEC should be corrected for harvest, DOC and VOC losses (Chapin et al., 2006). Furthermore, due to the large year-to-year variability in the NEPEC, multi-year time series are required to determine the long term average trend in carbon exchange. A second, commonly used approach for estimating NECB is based on the change in carbon stocks in live and dead biomass and soil (NECBSC) (Kolari et al., 2004). However, it is difficult to detect a shift in the soil carbon stock against the considerably higher background stock and due to the very high spatially variability (Schrumpf et al., 2011). Thus, to detect a significant trend in the carbon balance of a forest, a long study period is needed for both NECBSC and NECBEC approaches. This explains in part why data on forest carbon sequestration, considered as the difference between CO2 uptake and release, accounting for carbon losses as DOC and VOC, are so scarce in the literature. A third frequently used method to determine carbon sequestration in forest ecosystems is based on the difference between the net gain of CO2 due to autotrophic processes and the losses due to heterotrophic respiration (Curtis et al., 2002, Ehman et al., 2002). This method was not applied in this study due to the lack of heterotrophic respiration estimates. Although both NECBEC and NECBSC represent both the difference between gross ecosystem productivity on the one hand and the sum of total ecosystem respiration and all non-CO2 losses on the other hand, they are independent assessments with unrelated errors (Curtis et al., 2002, Falge et al., 2002). These errors, however, cause large uncertainties in both approaches. Estimates of NECB based on stock changes are uncertain due to the high spatial heterogeneity of soil carbon stocks and consequently require a large amount of samples. Therefore, in most short-term carbon balance assessments the change in soil carbon stock is not measured and thus assumed to be zero (Kolari et al., 2004).

In this study we present data on the carbon sequestration of a temperate Scots pine forest in Belgium over a period of 9 years. We estimate the total carbon accumulation by comparing an eddy covariance based approach (NECBEC) to a stock based approach (NECBSC) which includes inventories of above- and below ground phytomass, litter and an extensive soil carbon stock assessment. In addition we present a detailed analysis of the uncertainty on the different carbon stocks changes. We are not aware of studies reporting on these forest dynamics in a comparable level of detail. Additionally, by using the stock based approach we get insight into the fate of the accumulated carbon, net primary productivity (NPP) and the carbon allocation pattern in the forest ecosystem.

Section snippets

Plot description and study period

The experimental forest “De Inslag” is located in Brasschaat, 20 km NE of Antwerp in the Belgian Campine region (51°18′N, 4°31′E). The study site consists of a 2.0 ha, 80-year-old even-aged Scots pine (Pinus sylvestris L.) stand situated within a 150 ha mixed coniferous/deciduous forest dominated by Scots pine. The stand is part of the ICP Forests level II and Fluxnet/CarboEurope-IP networks. Historic inventory data showed a tree density of 538 trees ha−1 in 1995 (Janssens et al., 1999). In the

Stock based approach

The total carbon storage in the pine forest was estimated at 194.5 tC ha−1 in 2002 and increased to 210.6 tC ha−1 in 2010, which equals an increase of 16.0 tC ha−1 or 1.8 tC ha−1 yr−1 (Table 1).

NECBSC

In general, forests are considered to sequester carbon in both biomass and soil (Janssens et al., 2003). Schulze et al. (2009) recently reported that European forest soils sequester on average 0.2 tC ha−1 yr−1. However, due to a lack of direct measurements, this estimate is derived from a combination of model results and input–outputs balances. Our forest stand did not show a significant increase of carbon in the mineral soil over the study period. This could be related to the forest age as in

Conclusion

The carbon sequestration of a temperate Scots pine forest over a period of 9 years was assessed using two commonly used approaches. The eddy covariance estimate, after correction for non CO2 carbon fluxes estimated that the ecosystem gained 2.4 tC ha−1 yr−1. Additionally, the measured changes in the carbon stock indicated a carbon sink of 1.8 tC ha−1 yr−1. Most of the sequestered carbon was retrieved in the phytomass, while little to no changes in mineral soil carbon stocks could be ascertained.

Acknowledgement

We are grateful to Fred Kockelberg (UA), Nadine Calluy (UA) and Marc Schuermans (ANB) for technical support. We would also like to thank the reviewers for their helpful comments and suggestions. Sampling of soil water and soil moisture was carried out within the framework of the UN/ECE intensive monitoring of forest ecosystems (ICP-Forests). This research was supported by the Centre of Excellence ECO (UA-Methusalem), the Research Foundation-Flanders (FWO-Vlaanderen) and GHG-Europe (Grant No.

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