Notes: The timing of the commencement of photosynthesis (P*) in spring is an important determinant of growing-season length and thus of the productivity of boreal forests. Although controlled experiments have shed light on environmental mechanisms triggering release from photoinhibition after winter, quantitative research for trees growing naturally in the field is scarce. In this study, we investigated the environmental cues initiating the spring recovery of boreal coniferous forest ecosystems under field conditions. We used meteorological data and above-canopy eddy covariance measurements of the net ecosystem CO2 exchange (NEE) from five field stations located in northern and southern Finland, northern and southern Sweden, and central Siberia. The within- and intersite variability for P* was large, 30–60 days. Of the different climate variables examined, air temperature emerged as the best predictor for P* in spring. We also found that ‘soil thaw’, defined as the time when near-surface soil temperature rapidly increases above 0°C, is not a useful criterion for P*. In one case, photosynthesis commenced 1.5 months before soil temperatures increased significantly above 0°C. At most sites, we were able to determine a threshold for air-temperature-related variables, the exceeding of which was required for P*. A 5-day running-average temperature (T5) produced the best predictions, but a developmental-stage model (S) utilizing a modified temperature sum concept also worked well. But for both T5 and S, the threshold values varied from site to site, perhaps reflecting genetic differences among the stands or climate-induced differences in the physiological state of trees in late winter/early spring. Only at the warmest site, in southern Sweden, could we obtain no threshold values for T5 or S that could predict P* reliably. This suggests that although air temperature appears to be a good predictor for P* at high latitudes, there may be no unifying ecophysiological relationship applicable across the entire boreal zone.

Notes: We present carbon stable isotope, δ13C, results from air and organic matter samples collected during 98 individual field campaigns across a network of Carboeuroflux forest sites in 2001 (14 sites) and 2002 (16 sites). Using these data, we tested the hypothesis that δ13C values derived from large-scale atmospheric measurements and models, which are routinely used to partition carbon fluxes between land and ocean, and potentially between respiration and photosynthesis on land, are consistent with directly measured ecosystem-scale δ13C values. In this framework, we also tested the potential of δ13C in canopy air and plant organic matter to record regional-scale ecophysiological patterns.Our network estimates for the mean δ13C of ecosystem respired CO2 and the related ‘discrimination’ of ecosystem respiration, δer and Δer, respectively, were −25.6±1.9‰ and 17.8 ±2.0‰ in 2001 and −26.6±1.5‰ and 19.0±1.6‰ in 2002. The results were in close agreement with δ13C values derived from regional-scale atmospheric measurement programs for 2001, but less so in 2002, which had an unusual precipitation pattern. This suggests that regional-scale atmospheric sampling programs generally capture ecosystem δ13C signals over Europe, but may be limited in capturing some of the interannual variations.In 2001, but less so in 2002, there were discernable longitudinal and seasonal trends in δer. From west to east, across the network, there was a general enrichment in 13C (∼3‰ and ∼1‰ for the 2 years, respectively) consistent with increasing Gorczynski continentality index for warmer and drier conditions. In 2001 only, seasonal 13C enrichment between July and September, followed by depletion in November (from about −26.0‰ to −24.5‰ to −30.0‰), was also observed. In 2001, July and August δer values across the network were significantly related to average daytime vapor pressure deficit (VPD), relative humidity (RH), and, to a lesser degree, air temperature (Ta), but not significantly with monthly average precipitation (Pm). In contrast, in 2002 (a much wetter peak season), δer was significantly related with Ta, but not significantly with VPD and RH. The important role of plant physiological processes on δer in 2001 was emphasized by a relatively rapid turnover (between 1 and 6 days) of assimilated carbon inferred from time-lag analyses of δer vs. meteorological parameters. However, this was not evident in 2002. These analyses also noted corresponding diurnal cycles of δer and meteorological parameters in 2001, indicating a rapid transmission of daytime meteorology, via physiological responses, to the δer signal during this season.Organic matter δ13C results showed progressive 13C enrichment from leaves, through stems and roots to soil organic matter, which may be explained by 13C fractionation during respiration. This enrichment was species dependent and was prominent in angiosperms but not in gymnosperms. δ13C values of organic matter of any of the plant components did not well represent short-term δer values during the seasonal cycle, and could not be used to partition ecosystem respiration into autotrophic and heterotrophic components.

Notes: This paper discusses the advantages and disadvantages of the different methods that separate net ecosystem exchange (NEE) into its major components, gross ecosystem carbon uptake (GEP) and ecosystem respiration (Reco). In particular, we analyse the effect of the extrapolation of night-time values of ecosystem respiration into the daytime; this is usually done with a temperature response function that is derived from long-term data sets. For this analysis, we used 16 one-year-long data sets of carbon dioxide exchange measurements from European and US-American eddy covariance networks. These sites span from the boreal to Mediterranean climates, and include deciduous and evergreen forest, scrubland and crop ecosystems.We show that the temperature sensitivity of Reco, derived from long-term (annual) data sets, does not reflect the short-term temperature sensitivity that is effective when extrapolating from night- to daytime. Specifically, in summer active ecosystems the long-term temperature sensitivity exceeds the short-term sensitivity. Thus, in those ecosystems, the application of a long-term temperature sensitivity to the extrapolation of respiration from night to day leads to a systematic overestimation of ecosystem respiration from half-hourly to annual time-scales, which can reach 〉25% for an annual budget and which consequently affects estimates of GEP. Conversely, in summer passive (Mediterranean) ecosystems, the long-term temperature sensitivity is lower than the short-term temperature sensitivity resulting in underestimation of annual sums of respiration.We introduce a new generic algorithm that derives a short-term temperature sensitivity of Reco from eddy covariance data that applies this to the extrapolation from night- to daytime, and that further performs a filling of data gaps that exploits both, the covariance between fluxes and meteorological drivers and the temporal structure of the fluxes. While this algorithm should give less biased estimates of GEP and Reco, we discuss the remaining biases and recommend that eddy covariance measurements are still backed by ancillary flux measurements that can reduce the uncertainties inherent in the eddy covariance data.

Notes: Abstract Trends in the precipitation-weighted average concentrations and in the deposition of inorganic ions in daily precipitation samples at three Finnish background stations were calculated for a period extending from the seventies to 1988. During the period 1973–1988, S04 2− concentrations in wet deposition at the station in Central Finland decreased by 39% in accordance with the emission decrease of SO1 in Finland of 50% over the same period, and the 20% decrease in Europe from 1980 to 1988. However, due to the more significant decrease in alkaline ions (80% for Mg2+), H+ concentrations increased by 30%. At the two southernmost stations, overall concentrations are higher and the decrease of Mg2+ and Ca2+ is less important, but at the station in southeastern Finland the increase of N03 − relative to NH4 + has doubled H+ concentrations during the period 1977–1988. Division of the data according to wind direction sectors showed generally decreasing concentrations in the sectors related to Scandinavia, but unchanging or increasing concentrations in the sectors related to eastern Europe. Acid deposition trends were estimated as H+, S04 2− - (Ca2+ + Mg2+), and H+ + NH4 + - N03 +. All these trends showed increasing acid deposition in central Finland and decreasing trends in western Finland. In eastern Finland the trends were opposing due to the increasing N03 + but decreasing NH4 − trends. It is concluded that S04 2− deposition trends do not reflect acid deposition trends, and SO4 2− or N03 − deposition do not geographically represent acid deposition.