Abstract
The net ecosystem exchange (NEE) of successional stages of the Abies-dominated dark taiga was measured in central Siberia (61degreesN 90degreesE) during the growing season of the year 2000 using the eddy covariance technique. Measurements started before snow melt and canopy activity in spring on day of year (DOY) 99 and lasted until a permanent snow cover had developed and respiration had ceased in autumn DOY 299, Three stands growing in close vicinity were investigated: 50 yr-old Betula pubescens ("Betula stand", an early successional stage after fire), 250 yr-old mixed boreal forest, representing the transition from Betula-dominated to Abies-dominated canopies, and 200-yr-old Abies sibirica ("Abies stand", representing a late successional stage following the mixed boreal forest). The mixed boreal forest had a multi-layered canopy with dense understory and trees of variable height and age below the main canopy, which was dominated by Abies sibirica, Picea obovata and few old Betula pubescens and Populus tremula trees. The Abies stand had a uniform canopy dominated by Abies sibirica. This stand appears to have established not after fire but after wind break or insect damage in a later successional stage. The stands differed with respect to the number of days with net CO2 uptake (Betula stand 89 days, mixed boreal forest 109 days, and Abies stand 135 days), maximum measured LAI (Betula 2.6 m(2) m(-2), mixed boreal forest 3.5 m(2) m(-2) and Abies stand 4.1 m(2) m(-2)) and basal area (Betula stand 30.2 m(2) ha(-1), mixed boreal forest 35.7 m(2) ha(-1), and Abies stand 46.5 m(2) ha(-1)). In the mixed boreal forest, many days with net daytime CO2 release were observed in summer. Both other sites were almost permanent sinks in summer. Mean daytime CO2 exchange rates in July were -8.45 mumol m(-2) s(-1) in the Betula stand, -4.65 mumol m(-2) s(-1) in the mixed boreal forest and -6.31 mumol m(-2) s(-1) in the Abies stand. Measured uptake for the growing season was -247.2 g C m(-2) in the Betula stand, -99.7 g C m(-2) in the mixed boreal forest and -269.9 g C m(-2) in the Abies stand. The total annual carbon uptake might be slightly lower (i.e. less negative) due to some soil respiration under snow in winter. The study for the first time demonstrates that old forests in the "Dark Taiga" are carbon sinks and that sink activity is very similar in late and early successional stages. Canopy and crown structure with associated self-shading and available radiation are suggested as possible causes for the observed differences.