Abstract
Abstract. Boreal forests play a key role in the global carbon cycle and are facing rapid shifts in nitrogen availability with poorly understood consequences for ecosystem function and global climate change. We quantified the effects of increasing nitrogen availability on carbon fluxes from a relatively understudied component of these forests – the forest floor – at three intervals over the summer growing period in a northern Swedish Scots pine stand. Nitrogen addition altered both the uptake and release of carbon dioxide from the forest floor, but the magnitude and direction of this effect depended on the time during the growing season and the amount of nitrogen added. Specifically, nitrogen addition stimulated net forest floor carbon uptake only in the late growing season. We find evidence for species-specific control of forest floor carbon sink strength, as photosynthesis per unit ground area was positively correlated only with the abundance of the vascular plant Vaccinium myrtillus and no others. Comparison of understorey vegetation photosynthesis and respiration from the study site indicates that understorey vegetation photosynthate was mainly supplying respiratory demands for much of the year. Only in the late season with nitrogen addition did understorey vegetation appear to experience a large surplus of carbon in excess of respiratory requirements. Further work, simultaneously comparing all major biomass and respiratory carbon fluxes in forest floor and tree vegetation, is required to resolve the likely impacts of environmental changes on whole-ecosystem carbon sequestration in boreal forests.
Highlights
Boreal forests store around 25 % of global terrestrial carbon (C) (Gower et al, 2001) and are currently estimated to be a net C sink (Myneni et al, 2001; Pan et al, 2011)
By comparison relatively little is known about the effects of fertilization on forest floor CO2 fluxes despite evidence that the forest floor accounts for a large proportion of whole forest photosynthesis and respiration (Goulden and Crill, 1997; Heijmans et al, 2004; Kolari et al, 2006), and exerts a powerful influence over soil nutrient and C cycling (Wardle et al, 1998; Nilsson and Wardle, 2005)
We find evidence for speciesspecific effects on forest floor CO2 fluxes, since GPP was significantly positively correlated with the abundance of V. myrtillus but not related to the amount of V . vitis-idaea (Table 2)
Summary
Boreal forests store around 25 % of global terrestrial carbon (C) (Gower et al, 2001) and are currently estimated to be a net C sink (Myneni et al, 2001; Pan et al, 2011). These ecosystems are critically constrained across large areas by the availability of N (Tamm, 1991; Vitousek and Howarth, 1991) which is greatly increased across managed forests due to intensive fertilization and, across all systems, via elevated N inputs from atmospheric deposition (Galloway et al, 2008; Reay et al, 2008).
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