Productivity of forests in the Eurosiberian boreal region and their potential to act as a carbon sink - a synthesis

Based on review and original data, this synthesis investigates carbon pools and fluxes of Siberian and European forests (600 and 300 million ha, respe ctively). We examine the productivity of ecosystems, expressed as positive rate when the amount of carbon in the ecosystem increases, while (following micrometeorological convention) downward fluxes from the atmosphere to the vegetation (NEE=Net Ecosystem Exchange) are expressed as negative numbers. Productivity parameters are Net Primary Productivity (NPP=whole plant grow th), Net Ecosystem Productivity (NEP = CO2 assimilation minus ecosystem res piration), and Net Biome Productivity (NBP=NEP minus carbon losses through disturbances bypassing respiration, e.g. by fire and logging). Based on chr onosequence studies and national forestry statistics we estimate a low aver age NPP for boreal forests in Siberia: 123 gC m(-2) y(-1). This contrasts w ith a similar calculation for Europe which suggests a much higher average N PP of 460 gC m(-2) y(-1) for the forests there. Despite a smaller area, Eur opean forests have a higher total NPP than Siberia (1.2-1.6 vs. 0.6-0.9 x 1 0(15) gC region(-1) y(-1)). This arises as a consequence of differences in growing season length, climate and nutrition. For a chronosequence of Pinus sylvestris stands studied in central Siberia during summer, NEE was most negative in a 67-y old stand regenerating after fire (-192 mmol m(-2) d(-1)) which is close to NEE in a cultivated forest of Germany (-210 mmol m(-2) d(-1)). Considerable net ecosystem CO2-uptake w as also measured in Siberia in 200- and 215-y old stands (NEE:174 and - 63 mmol m(-2) d(-1)) while NEP of 7- and 13-y old logging areas were close to the ecosystem compensation point. Two Siberian bogs and a bog in European R ussia were also significant carbon sinks (-102 to - 104 mmol m(-2) d(-1)). Integrated over a growing season (June to September) we measured a total gr owing season NEE of -14 mol m(-2) summer(-1) (-168 gC m(-2) summer(-1)) in a 200-y Siberian pine stand and -5 mol m(-2) summer(-1) (-60 gC m(-2) summe r(-1)) in Siberian and European Russian bogs. By contrast, over the same pe riod, a spruce forest in European Russia was a carbon source to the atmosph ere of (NEE: + 7 mol m(-2) summer(-1) = + 84 gC m(-2) summer(-1)). Two year s after a windthrow in European Russia, with all trees being uplifted and f ew successional species, lost 16 mol C m(-2) to the atmosphere over a 3-mon th in summer, compared to the cumulative NEE over a growing season in a Ger man forest of -15.5 mol m(-2) summer(-1) (-186 gC m(-2) summer(-1); Europea n flux network annual averaged - 205 gC m(-2) y(-1)). Differences in CO2-exchange rates coincided with differences in the Bowen r atio, with logging areas partitioning most incoming radiation into sensible heat whereas bogs partitioned most into evaporation (latent heat). Effects of these different surface energy exchanges on local climate (convective s torms and fires) and comparisons with the Canadian BOREAS experiment are di scussed. Following a classification of disturbances and their effects on ecosystem c arbon balances, fire and logging are discussed as the main processes causin g carbon losses that bypass heterotrophic respiration in Siberia. Following two approaches, NBP was estimated to be only about 13-16 mmol m(-2) y(-1) for Siberia. It may reach 67 mmol m(-2) y(-1) in North America, and about 1 40-400 mmol m(-2) y(-1) in Scandinavia. We conclude that fire speeds up the carbon cycle, but that it results also in long-term carbon sequestration by charcoal formation. For at least 14 ye ars after logging, regrowth forests remain net sources of CO2 to the atmosp here. This has important implications regarding the effects of Siberian for est management on atmospheric concentrations. For many years after logging has taken place, regrowth forests remain weaker sinks for atmospheric CO2 t han are nearby old-growth forests.