Interannual variability and timing of growing‐season CO₂ exchange in a boreal forest

Abstract

[1] We studied the interannual variability of cumulative net ecosystem CO 2 exchange (NEE) and its connection with cumulative or average climatic variables during five growing seasons. The analysis was based on a 5-year-long time series of CO 2 flux measured from April 1996 to April 2001 in a Scots pine forest in southern Finland by the eddy covariance technique. The onset of the ecosystem growing season was best connected with air temperature, and the end of the growing season was best connected with day length. With these variables we were able to predict the timing and the length of each growing season within 0-3 days. The forest was a sink of carbon with little interannual variability: The uptake during the four full growing seasons varied by 80 g C m À2 , ranging from 230 to 310 g C m À2 . The estimated winter release each year varied between 60 and 90 g C m À2 . The interannual variation in seasonal (spring, summer, autumn) carbon exchange ranged from 30 g C m À2 in autumn and spring to 80 g C m À2 in summer. The average climatic variables explained the variability of the seasonal or growing-season cumulative NEE only partly. Both the daytime and the nighttime CO 2 fluxes contributed markedly to the variability in carbon exchange, indicating that photosynthesis and respiration have an equally important influence on NEE.

Carbon accumulation in Finland's forests 1922–2004 – an estimate obtained by combination of forest inventory data with modelling of biomass, litter and soil

“…Our estimate for the mean NEP in the 1990s (0.10 kg/m 2 /year) is in the midrange of NEP measurements taken at six forest sites in the Nordic countries, varying from a carbon source equal to 0.09 kg/m 2 /year to a carbon sink equal to 0.25 kg/m 2 /year [73]. On the other hand, our estimate is lower than measurements taken at a 40-year-old Scots pine stand in southern Finland, where they ranged from 0.23 to 0.31 kg/m 2 /year between 1997 and 2000 [63]. The measurements were high for this site probably because the tree stand was still young and growing vigorously in the most productive part of the country.…”

“…Such inter annual variability is important on a site scale based on measurements of carbon fluxes [63] and on a global scale based on ecosystem modelling [40], inverse modelling [2] or satellite observations [53]. This variability has not, however, been accounted for in earlier regional scale studies based on field inventories because it has not been possible to derive the annual estimates from these inventories [9].…”

Greenhouse gas fluxes in a drained peatland forest during spring frost-thaw event

“…This finding is in line with the study by Lohila et al (2007) where they found that an afforested boreal peatland turned from a net sink to a source of carbon during rainy days in the summer. Total NEE at the Kalevansuo drained peatland forest from spring to early summer (25 April-21 June, −102 g C m −2 ) is comparable to NEE values reported from boreal forests growing on mineral or peat soils (Suni et al, 2003;Lohila et al, 2007).…”

Ten years of fluxes and stand growth in a young beech forest at Hesse, North-eastern France

“…The main source of this variability was attributed to the growing season length (GSL). In a boreal Scots pine forest, Suni et al (2003) also reported a low interannual variability in NEE, which was mainly related to the temperature. In a coniferous -deciduous mixed forest, Carrara et al (2003; attributed the large internannual variability to both GSL and to the annual temperature.…”