Forest–atmosphere carbon dioxide exchange in eastern Siberia

Abstract

We investigated the daily exchange of CO 2 between undisturbed Larix gmelinii (Rupr.) Rupr. forest and the atmosphere at a remote Siberian site during July and August of 1993. Our goal was to measure and partition total CO 2 exchanges into aboveground and belowground components by measuring forest and understory eddy and storage¯uxes and then to determine the relationships between the environmental factors and these observations of ecosystem metabolism. Maximum net CO 2 uptake of the forest ecosystem was extremely low compared to the forests elsewhere, reaching a peak of only $5 mmol m À2 s À1 late in the morning. Net ecosystem CO 2 uptake increased with increasing photosynthetically active photon ux density (PPFD) and decreased as the atmospheric water vapor saturation de®cit (D) increased. Daytime ecosystem CO 2 uptake increased immediately after rain and declined sharply after about six days of drought. Ecosystem respiration at night averaged $2.4 mmol m À2 s À1 with about 40% of this coming from the forest¯oor (roots and heterotrophs). The relationship between the understory eddy¯ux and soil temperature at 5 cm followed an Arrhenius model, increasing exponentially with temperature (Q 10 $2.3) so that on hot summer afternoons the ecosystem became a source of CO 2 . Tree canopy CO 2 exchange was calculated as the difference between above and below canopy eddy¯ux. Canopy uptake saturated at $6 mmol CO 2 m À2 s À1 for a PPFD above 500 mmol m À2 s À1 and decreased with increasing D. The optimal stomatal control model of Ma Èkela È et al. (1996) was used as a`big leaf' canopy model with parameter values determined by the non-linear least squares. The model accurately simulated the response of the forest to light, saturation de®cit and drought. The precision of the model was such that the daily pattern of residuals between modeled and measured forest exchange reproduced the component storagē ux. The model and independent leaf-level measurements suggest that the marginal water cost of plant C gain in Larix gmelinii is more similar to values from deciduous or desert species than other boreal forests. During the middle of the summer, the L. gmelinii forest ecosystem is generally a net sink for CO 2 , storing $0.75 g C m À2 d À1. Published by Elsevier Science B.V.

Net ecosystem exchange of carbon dioxide and water of far eastern Siberian Larch (<I>Larix cajanderii</I>) on permafrost

“…The values we report here are substantially (a factor 2) higher. Both Hollinger et al (1998) and our work use eddy correlation equipment and obtained good energy balance closure. Assuming that our measurements are correct, there are three possible explanations for this discrepancy.…”

“…A similar and related explanation would take into account age and density differences in the forest. The Spasskaya Pad forest is in fact older than the Hollinger et al (1998) forest: 160 versus 125 year. It appears unlikely that this age difference would explain the current differences: one would expect an older forest to take up less carbon.…”

“…This is likely to be an upper estimate and may vary considerable from year to year. Hollinger et al (1998) took 14 days of measurements over a larch forest south of Yakutsk and reported very low maximum rates of midday net ecosystem exchange. The values we report here are substantially (a factor 2) higher.…”

“…Assuming that our measurements are correct, there are three possible explanations for this discrepancy. One relates to the possibility that the values obtained from Hollinger et al (1998)'s series of only 14 days just randomly happened to be on the low side. We cannot completely rule out this possibility, and our year-long measurements suggest that after the peak in June rather sharp declines in uptake may be observed.…”

Response of central Siberian Scots pine to soil water deficit and long‐term trends in atmospheric CO2concentration

“…It has also been shown over a period of 15 summer days in an eastern Siberian Larix gmeliniicanopy by Hollinger et al [1998], who used the optimal stomatal regulation model of Mäkelä et al [1996] and Berninger et al [1996]. In those models a parameter closely related to l varies with time since last rainfall.…”