Pools and fluxes of carbon in three Norway spruce ecosystems along a climatic gradient in Sweden

Abstract

This paper presents an integrated analysis of organic carbon (C) pools in soils and vegetation, within-ecosystem fluxes and net ecosystem exchange (NEE) in three 40-year old Norway spruce stands along a north-south climatic gradient in Sweden, measured 2001Sweden, measured -2004. A process-orientated ecosystem model (CoupModel), previously parameterised on a regional dataset, was used for the analysis. Pools of soil organic carbon (SOC) and tree growth rates were highest at the southernmost site (1.6 and 2.0-fold, respectively). Tree litter production (litterfall and root litter) was also highest in the south, with about half coming from fine roots (<1 mm) at all sites. However, when the litter input from the forest floor vegetation was included, the difference in total litter input rate between the sites almost disappeared (190-233 g C m À2 year À1 ). We propose that a higher N deposition and N availability in the south result in a slower turnover of soil organic matter than in the north. This effect seems to overshadow the effect of temperature. At the southern site, 19% of the total litter input to the O horizon was leached to the mineral soil as dissolved organic carbon, while at the two northern sites the corresponding figure was approx. 9%. The CoupModel accurately described general C cycling behaviour in these ecosystems, reproducing the differences between north and south. The simulated changes in SOC pools during the measurement period were small, ranging from À8 g C m À2 year À1 in the north to +9 g C m À2 year À1 in the south. In contrast, NEE and tree growth measurements at the northernmost site suggest that the soil lost about 90 g C m À2 year À1 .

Carbon stores and fluxes in even- and uneven-aged Norway spruce stands

“…In addition root litter and field layer litter should be accounted for. Figures on these fluxes lack in the present investigation , but based on results from Sweden, the figures on root litter are in the same order of magnitude as the above ground litterfall (Kleja et al 2008). This means that the present difference in litter fall C input to the soil could be some 1.9 Mg C ha -1 a -1 (65%) higher in the even-aged system.…”

“…In the present investigation the C storage in the vegetation (above ground parts) is some 0.3-0.6% of the C in the trees (Table 2). However, the annual ground vegetation turnover rate is high and it has been demonstrated that C input to soil from field layer can account for more than 25% of total ecosystem production (Kleja et al 2008) in a 40 year old Picea abies stand. …”

“…The dissolved organic carbon (DOC) runoff has not been measured in the experiments, but the magnitude of C leaving as DOC is in the order of a few up to ten per cent of the total CO 2 fluxes (Neff and Hooper 2002). Kleja et al (2008) found that between 6 -19% of the litter input to the O-horizon was lost as DOC in a study of Swedish spruce forests, however, almost all (95%) of the DOC was captured in the upper mineral soil, so loss of C as DOC appears to be minimal from boreal forest soils. Cautions should also be taken due to the lack of real experimental replications.…”

Simulated climate change impacts on fluxes of carbon in Norway spruce ecosystems along a climatic transect in Sweden

“…The study by Mäkipää et al (1999) demonstrates for example how uptake of N and decomposition were coupled with an empirical multiplier that was related to annual values of transpiration as the forcing conditions for the decomposition of soil organic material. In recent work, Svensson et al (2007) applied the CoupModel (Jansson and Karlberg 2004) with a daily time step for estimating decomposition rates and N supply in order to evaluate effects of climate and N on ecosystem productivity in Sweden. They concluded that it was not possible to explain the tree growth rate in northern Sweden with the conventional decomposition mineralization model.…”

“…Simulations were made for well-drained soils in 4 selected regions in Sweden, representing differences in the current climate and N deposition rates, using the CoupModel, previously parameterized on a regional-based dataset (Svensson et al 2007). Specific objectives were to: (i) Identify how a change in climate affected abiotic factors of importance for ecosystem C dynamics; (ii) analyze how a change in climate affected the rate of processes governing the accumulation of carbon in vegetation and litter production, compared to the rate of processes regulating the rate of SOC decomposition; and (iii) identify the extent to which limiting factors for key ecosystem processes are expected to change, for instance the role of moisture and nitrogen.…”