Bakwin, P.; Berbigier, P.; Davis, K.; Dolman, A. J.; Falk, M.; Fuentes, J. D.; Goldstein, A.; Granier, A.; Grelle, A.; Hollinger, D.; Janssens, I. A.; Jarvis, P.; Jensen, N. O.; Katul, G.; Mahli, K.; Matteucci, G.; Meyers, T.; Monson, R.; Munger, W.; Oechel, W.; Olson, R.; Pilegaard, K.; Paw U, K. T.; Thorgeirsson, H.; Valentini, R.; Verma, Shashi; Vesala, T.; Wilson, K.; and Wofsy, S., "Environmental controls over carbon dioxide and water vapor exchange of terrestrial vegetation" (2002 B.E. Law et al. / Agricultural and Forest Meteorology 113 (2002) 97-120 AbstractThe objective of this research was to compare seasonal and annual estimates of CO 2 and water vapor exchange across sites in forests, grasslands, crops, and tundra that are part of an international network called FLUXNET, and to investigating the responses of vegetation to environmental variables. FLUXNETs goals are to understand the mechanisms controlling the exchanges of CO 2 , water vapor and energy across a spectrum of time and space scales, and to provide information for modeling of carbon and water cycling across regions and the globe. At a subset of sites, net carbon uptake (net ecosystem exchange, the net of photosynthesis and respiration) was greater under diffuse than under direct radiation conditions, perhaps because of a more efficient distribution of non-saturating light conditions for photosynthesis, lower vapor pressure deficit limitation to photosynthesis, and lower respiration associated with reduced temperature. The slope of the relation between monthly gross ecosystem production and evapotranspiration was similar between biomes, except for tundra vegetation, showing a strong linkage between carbon gain and water loss integrated over the year (slopes = 3.4 g CO 2 /kg H 2 O for grasslands, 3.2 for deciduous broadleaf forests, 3.1 for crops, 2.4 for evergreen conifers, and 1.5 for tundra vegetation). The ratio of annual ecosystem respiration to gross photosynthesis averaged 0.83, with lower values for grasslands, presumably because of less investment in respiring plant tissue compared with forests. Ecosystem respiration was weakly correlated with mean annual temperature across biomes, in spite of within site sensitivity over shorter temporal scales. Mean annual temperature and site water balance explained much of the variation in gross photosynthesis. Water availability limits leaf area index over the long-term, and inter-annual climate variability can limit carbon uptake below the potential of the leaf area present.

[1] We present an analysis framework and illustrate its potential to constrain terrestrial carbon fluxes at the regional scale using observations of CO 2 and CO over North America acquired during the CO 2 Budget and Rectification Airborne (COBRA) study in 2000. The COBRA data set, presented in detail in a companion paper provides dense spatial coverage and extensive profiling in the lower atmosphere, revealing strong CO 2 signatures of land surface fluxes in the active and relic mixed layers of the atmosphere. We introduce a ''receptor-oriented'' analysis framework designed to quantitatively interpret the atmospheric signatures of surface processes by linking concentrations at measurement locations (receptors) to surface fluxes in upwind regions. The framework incorporates three main components: (1) the Stochastic Time-Inverted Lagrangian Transport (STILT) model, driven with assimilated winds and running backward in time to map out the source-receptor relationship (footprint) at high temporal and spatial resolution; (2) an observation-based lateral boundary condition for CO 2 , resolving vertical and meridional gradients; and (3) a simple parameterization for biosphere-atmosphere fluxes that uses eddy covariance observations from the AmeriFlux network as prior estimates for fluxes. This framework allows quantitative comparison between the top-down constraint on fluxes from airborne observations of CO 2 with the bottom-up constraint of eddy flux measurements in a Bayesian synthesis inversion. The model is used to investigate the observed representation error (mismatch between point measurements and grid-cell-averaged values in models), evaluated in the companion paper, showing that unresolved spatial variability of surface fluxes gives rise to most of the representation error over the continent. Thus the representation error reflects the effect of aggregation errors. Discrepancies between simulated and observed CO 2 distributions are assessed to indicate where improvements are needed, including improved empirical representation of biosphere-atmosphere exchange process and better simulation of convective processes in atmospheric transport models. Citation: Gerbig, C., J. C. Lin, S. C. Wofsy, B. C. Daube, A. E. Andrews, B. B. Stephens, P. S. Bakwin, and C. A. Grainger, Toward constraining regional-scale fluxes of CO 2 with atmospheric observations over a continent: 2. Analysis of COBRA data using a receptor-oriented framework,

Differences in the seasonal pattern of assimilatory and respiratory processes are responsible for divergences in seasonal net carbon exchange among ecosystems. Using FLUXNET data (http://www.eosdis.ornl.gov/FLUXNET) we have analyzed seasonal patterns of gross primary productivity (F GPP ), and ecosystem respiration (F RE ) of boreal and temperate, deciduous and coniferous forests, Mediterranean evergreen systems, a rainforest, temperate grasslands, and C 3 and C 4 crops. Based on generalized seasonal patterns classifications of ecosystems into vegetation functional types can be evaluated for use in global productivity and climate change models. The results of this study contribute to our understanding of respiratory costs of assimilated carbon in various ecosystems.Seasonal variability of F GPP and F RE of the investigated sites increased in the order tropical < Mediterranean < temperate coniferous < temperate deciduous < boreal forests. Together with the boreal forest sites, the managed grasslands and crops show the largest seasonal variability. In the temperate coniferous forests, seasonal patterns of F GPP and F RE are in phase, in the temperate deciduous and boreal coniferous forests F RE was delayed compared to F GPP , resulting in the greatest imbalance between respiratory and assimilatory fluxes early in the growing season.F GPP adjusted for the length of the carbon uptake period decreased at the sampling sites across functional types in the order C 4 crops, temperate and boreal deciduous forests (7.5-8.3 g C m −2 per day) > temperate conifers, C 3 grassland and crops (5.7-6.9 g C m −2 per day) > boreal conifers (4.6 g C m −2 per day). Annual F GPP and net ecosystem productivity (F NEP ) decreased across climate zones in the order tropical > temperate > boreal. However, the decrease in F NEP with latitude was greater than the decrease in F GPP , indicating a larger contribution of respiratory (especially heterotrophic) processes in boreal systems.

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[1] We analyze the spatial variability of CO 2 measurements from aircraft platforms, including extensive observations acquired over North America during the CO 2 Budget and Rectification Airborne (COBRA) study in 2000. The COBRA data set is unique in its dense spatial coverage and extensive profiling in the lower atmosphere. Strong signatures of CO 2 fluxes at the land surface were observed in the active and relic mixed layers of the atmosphere (up to $20 ppm gradients). Free tropospheric CO 2 exhibited significantly less variability except in areas affected by convective transport. Statistical analyses of the COBRA data indicate that CO 2 mixed-layer averages can be determined from vertical profiles with an accuracy of approximately ±0.2 ppm, limited by atmospheric variance. Analysis of the associated representation error suggests that models require horizontal resolution smaller than $30 km to fully resolve spatial variations of atmospheric CO 2 in the boundary layer over the continent. To provide a global context for these data, we analyzed the GLOBALVIEW marine boundary layer (MBL) reference CO 2 . Comparison of the MBL reference with extensive aircraft data extending over 20 years, covering the whole troposphere over the northern Pacific, shows significant seasonal biases of up to 2 ppm in the free troposphere, indicating that the MBL reference is a suitable boundary condition only for some applications. The spatial variability of CO 2 revealed by the COBRA-2000 calls for a suitable analysis framework to derive regional and continental fluxes, presented in a companion paper. The problem requires boundary conditions constrained by both surface and upper tropospheric observations and constraints on terrestrial fluxes that exploit the information content of the highly variable CO 2 distribution over land.