The transition between wintertime net carbon loss and springtime net carbon assimilation has an important role in controlling the annual rate of carbon uptake in coniferous forest ecosystems. We studied the contributions of springtime carbon assimilation to the total annual rate of carbon uptake and the processes involved in the winter-to-spring transition across a range of scales from ecosystem CO2 fluxes to chloroplast photochemistry in a coniferous, subalpine forest. We observed numerous initiations and reversals in the recovery of photosynthetic CO2 uptake during the initial phase of springtime recovery in response to the passage of alternating warm- and cold-weather systems. Full recovery of ecosystem carbon uptake, whereby the 24-h cumulative sum of NEE (NEEdaily) was consistently negative, did not occur until 3–4 weeks after the first signs of photosynthetic recovery. A key event that preceded full recovery was the occurrence of isothermality in the vertical profile of snow temperature across the snow pack; thus, providing consistent daytime percolation of melted snow water through the snow pack. Interannual variation in the cumulative annual NEE (NEEannual) was mostly explained by variation in NEE during the snow-melt period (NEEsnow-melt), not variation in NEE during the snow-free part of the growing season (NEEsnow-free). NEEsnow-melt was highest in those years when the snow melt occurred later in the spring, leading us to conclude that in this ecosystem, years with earlier springs are characterized by lower rates of NEEannual, a conclusion that contrasts with those from past studies in deciduous forest ecosystems. Using studies on isolated branches we showed that the recovery of photosynthesis occurred through a series of coordinated physiological and biochemical events. Increasing air temperatures initiated recovery through the upregulation of PSII electron transport caused in part by disengagement of thermal energy dissipation by the carotenoid, zeaxanthin. The availability of liquid water permitted a slightly slower recovery phase involving increased stomatal conductance. The most rate-limiting step in the recovery process was an increase in the capacity for the needles to use intercellular CO2, presumably due to slow recovery of Rubisco activity. Interspecific differences were observed in the timing of photosynthetic recovery for the dominant tree species. The results of our study provide (1) a context for springtime CO2 uptake within the broader perspective of the annual carbon budget in this subalpine forest, and (2) a mechanistic explanation across a range of scales for the coupling between springtime climate and the carbon cycle of high-elevation coniferous forest ecosystems.

Climatic influences on net ecosystem CO2 exchange during the transition from wintertime carbon source to springtime carbon sink in a high-elevation, subalpine forest

We estimated the isoprene and monoterpene source strengths of a pristine tropical forest north of Man- aus in the central Amazon Basin using three different mi- crometeorological flux measurement approaches. During the early dry season campaign of the Cooperative LBA Air- borne Regional Experiment (LBA-CLAIRE-2001), a tower- based surface layer gradient (SLG) technique was applied simultaneously with a relaxed eddy accumulation (REA) system. Airborne measurements of vertical profiles within and above the convective boundary layer (CBL) were used to estimate fluxes on a landscape scale by application of the mixed layer gradient (MLG) technique. The mean daytime fluxes of organic carbon measured by REA were 2.1 mg C m 2 h 1 for isoprene, 0.20 mg C m 2 h 1 for - pinene, and 0.39 mg C m 2 h 1 for the sum of monoterpenes. These values are in reasonable agreement with fluxes deter- mined with the SLG approach, which exhibited a higher scat- ter, as expected for the complex terrain investigated. The ob- served VOC fluxes are in good agreement with simulations using a single-column chemistry and climate model (SCM). In contrast, the model-derived mixing ratios of VOCs

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Isoprene and monoterpene fluxes from Central Amazonian rainforest inferred from tower-based and airborne measurements, and implications on the atmospheric chemistry and the local carbon budget

. To monitor the continental carbon cycle, a fully automated low maintenance measurement system is installed at the Zotino Tall Tower Observatory in Central Siberia (ZOTTO, 60°48' N, 89°21' E) since April 2009. A cavity ring-down spectroscopy (CRDS) analyzer continuously measures carbon dioxide (CO2) and methane (CH4) from six heights up to 301 m a.g.l. Buffer volumes in each air line remove short term CO2 and CH4 mixing ratio fluctuations associated with turbulence, and allow continuous, near-concurrent measurements from all tower levels. Instead of drying the air sample, the simultaneously measured water vapor is used to correct the dilution and pressure-broadening effects for the accurate determination of dry air CO2 and CH4 mixing ratios. The stability of the water vapor correction was demonstrated by repeated laboratory and field tests. The effect of molecular adsorption in the wet air lines was shown to be negligible. The low consumption of four calibration tanks that need recalibration only on decadal timescale further reduces maintenance. The measurement precision (accuracy) of 0.04 ppm (0.09 ppm) for CO2 and 0.3 ppb (1.5 ppb) for CH4 is compliant with the WMO recommendations. The data collected so far (until April 2010) reveals a seasonal cycle amplitude for CO2 of 30.4 ppm at the 301 m level.

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Continuous low-maintenance CO 2 /CH 4 /H 2 O measurements at the Zotino Tall Tower Observatory (ZOTTO) in Central Siberia

We present a numerical model capable of computing the physical processes within both plant canopy and planetary boundary layer (PBL), offering the potential benefit of wide applicability due to reduced computational requirements. The model, named SCADIS (scalar distribution), synthesizes existing knowledge of boundary and surface layer turbulence and surface layer vegetative processes and was tested using several data sets from the European part of Russia and Siberia obtained as part of the EUROSIBERIAN CARBONFLUX project. Despite simplifications which were necessary in order to simulate the natural processes, the first version of the model presented here demonstrated a satisfactory agreement between modelled and observed data for different surface features and weather conditions. For example, the model successfully predicted the diurnal patterns of concentration profiles of CO2, water vapour and potential temperature profiles both within the summer atmospheric boundary layer and within the plant canopy itself. The very different effects of the surface energy characteristics of bog versus forest on convective boundary layer (CBL) structure and development are also illustrated. The model was applied to evaluate the effective footprints for eddy covariance measurements above non-uniform plant canopies, the case study here being a mixed spruce forest in European Russia. The model also demonstrates the likely variations in the above-canopy turbulence and surface layer fluxes as dependent on the presence of patches of deciduous broadleaf forest within a predominantly evergreen coniferous stand.

https://www.tandfonline.com/doi/pdf/10.3402/tellusb.v54i5.16729

A simple three-dimensional canopy - planetary boundary layer simulation model for scalar concentrations and fluxes

[1] We have built the first comprehensive global three-dimensional model of delta(18)O in atmospheric CO2. The constructed model goes beyond all other approaches made until now, by simulating the diurnal variations and transport of CO2, delta(18)O of water, and delta(18)O of CO2. The (COO)-O-18 fluxes are thereby dependent on the atmospheric (COO)-O-18 composition. We have validated the model surface processes, showing that it compares well to other estimates and measurements of NPP, NEE, and stomata-internal CO2 mixing ratio (c(i)), except for high northern latitudes. Here, the model is considerably lower in NPP and higher in ci than other model estimates. However, estimates derived indirectly from observations tend to support our model findings. The water isotopes of rain are reproduced very well at all latitudes. The soil bucket model used in the model integrates incoming rain in one single value. The bucket approach overattenuates the isotopic variations of rain, and hence our isotopic source signature of respiration shows almost no seasonal cycle and is thus isotopically too depleted during summer.

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A comprehensive global three‐dimensional model of δ18O in atmospheric CO2: 1. Validation of surface processes

Using light aircraft and at intervals of approximately 14 days, vertical profiles of temperature, humidity, CO 2 concentration and 13 C/ 12 C and 18 O/ 16 O ratio, as well as concentrations of CH 4 , CO, H 2 and N 2 O, from about 80 to 3000 m above ground level have been determined for the atmosphere above a flux measurement tower located near the village of Zotino in central Siberia (60°45′N, 89°23′E). As well as being determined from flask measurements (typically at heights of 100, 500, 1000, 1500, 2000, 2500 and 3000 m) continuous CO 2 concentration profiles at 1 Hz have also been obtained using an infrared gas analyser. This measurement program is ongoing and has been in existence since July 1998. Data to November 2000 are presented and show a seasonal cycle for CO 2 concentration of about 25 μmol mol −1 within the atmospheric boundary layer (ABL) and about 15 μmol mol −1 in the free troposphere. Marked seasonal cycles in the isotopic compositions of CO 2 are also observed, with that of oxygen-18 in CO 2 being unusual: always being depleted in the ABL with respect to the free troposphere above. This is irrespective of whether the CO 2 concentration is higher or lower in the free troposphere. We interpret this as indicating a net negative discrimination being associated with the net terrestrial carbon exchange, irrespective of whether photosynthesis or respiration dominates the net carbon flux in this region. During winter flights, large fluctuations in CO 2 concentration with height are often observed both within and above the stable ABL. Usually (but not always) these variations in CO 2 concentrations are associated with more or less stoichiometrically constant variations in CO and CH 4 concentrations. We interpret this as reflecting the frequent transport of polluted air from Europe with very little vertical mixing having occurred, despite the large horizontal distances traversed. This notion is supported by back-trajectory analyses. Vertical profiles of CO 2 concentration with supplementary flask measurements allow more information on the structure and composition of an air mass to be obtained than is the case for flask measurements or for ground-based measurements only. In particular, our data question the notion that there is usually anything like “well mixed background air” in the mid-to-high northern latitudes during the winter months. DOI: 10.1034/j.1600-0889.2002.01335.x

Vitaly Zrazhewske

Papers

A trace-gas climatology above Zotino, central Siberia