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Understanding the relationships between climate and carbon exchange by terrestrial ecosystems is critical to predict future levels of atmospheric carbon dioxide because of the potential accelerating effects of positive climate-carbon cycle feedbacks. However, directly observed relationships between climate and terrestrial CO 2 exchange with the atmosphere across biomes and continents are lacking. Here we present data describing the relationships between net ecosystem exchange of carbon (NEE) and climate factors as measured using the eddy covariance method at 125 unique sites in various ecosystems over six continents with a total of 559 site-years. We find that NEE observed at eddy covariance sites is (1) a strong function of mean annual temperature at mid-and high-latitudes, (2) a strong function of dryness at mid-and low-latitudes, and (3) a function of both temperature and dryness around the mid-latitudinal belt (45 • N). The sensitivity of NEE to mean annual temperature breaks down at ∼16 • C (a threshold value of mean annual temperature), above which no further increase of CO 2 uptake with temperature was observed and dryness influence overrules temperature influence.

Grasslands and agroecosystems occupy one-third of the terrestrial area, but their contribution to the global carbon cycle remains uncertain. We used a set of 316 site-years of CO₂ exchange measurements to quantify gross primary productivity, respiration, and light-response parameters of grasslands, shrublands/savanna, wetlands, and cropland ecosystems worldwide. We analyzed data from 72 global flux-tower sites partitioned into gross photosynthesis and ecosystem respiration with the use of the light-response method (Gilmanov, T. G., D. A. Johnson, and N. Z. Saliendra. 2003. Growing season CO₂ fluxes in a sagebrushsteppe ecosystem in Idaho: Bowen ratio/energy balance measurements and modeling. Basic and Applied Ecology 4:167-183) from the RANGEFLUX and WORLDGRASSAGRIFLUX data sets supplemented by 46 sites from the FLUXNET La Thuile data set partitioned with the use of the temperature-response method (Reichstein, M., E. Falge, D. Baldocchi, D. Papale, R. Valentini, M. Aubinet, P. Berbigier, C. Bernhofer, N. Buchmann, M. Falk, T. Gilmanov, A. Granier, T. Grunwald, K. Havrankova, D. Janous, A. Knohl, T. Laurela, A. Lohila, D. Loustau, G. Matteucci, T. Meyers, F. Miglietta, J.M. Ourcival, D. Perrin, J. Pumpanen, S. Rambal, E. Rotenberg, M. Sanz, J. Tenhunen, G. Seufert, F. Vaccari, T. Vesala, and D. Yakir. 2005. On the separation of net ecosystem exchange into assimilation and ecosystem respiration: review and improved algorithm. Global Change Biology 11: 1.424-1439). Maximum values of the quantum yield (alpha = 75 mmol.mol(-1)), photosynthetic capacity (A(max) = 3.4 mg CO₂ . m(-2).s-1), gross photosynthesis (P-g,P-max = 1.16 g CO₂ . m(-2).d(-1)), and ecological light-use efficiency (epsilon(ecol) = 59 mmol . mol(-1)) of managed grasslands and high-production croplands exceeded those of most forest ecosystems, indicating the potential of nonforest ecosystems for uptake of atmospheric CO₂. Maximum values of gross primary production (8 600 g CO₂ . m(-2).yr(-1)), total ecosystem respiration (7 900 g CO₂ . m(-2).yr(-1)), and net CO₂ exchange (2 400 g CO₂ . m(-2).yr(-1)) were observed for intensively managed grasslands and high-yield crops, and are comparable to or higher than those for forest ecosystems, excluding some tropical forests. On average, 80% of the nonforest sites were apparent sinks for atmospheric CO₂, with mean net uptake of 700 g CO₂ . m(-2).yr(-1) for intensive grasslands and 933 g CO₂ . m(-2).d(-1) for croplands. However, part of these apparent sinks is accumulated in crops and forage, which are carbon pools that are harvested, transported, and decomposed off site. Therefore, although agricultural fields may be predominantly sinks for atmospheric CO₂, this does not imply that they are necessarily increasing their carbon stock

A B S T R A C TWe examine the atmospheric budget of CO 2 at temperate continental sites in the Northern Hemisphere. On a monthly time scale both surface exchange and atmospheric transport are important in determining the rate of change of CO 2 mixing ratio at these sites. Vertical differences between the atmospheric boundary layer and free troposphere over the continent are generally greater than large-scale zonal gradients such as the difference between the free troposphere over the continent and the marine boundary layer. Therefore, as a first approximation we parametrize atmospheric transport as a vertical exchange term related to the vertical gradient of CO 2 and the mean vertical velocity from the NCEP Reanalysis model. Horizontal advection is assumed to be negligible in our simple analysis. We then calculate the net surface exchange of CO 2 from CO 2 mixing ratio measurements at four tower sites. The results provide estimates of the surface exchange that are representative of a regional scale (i.e. ∼10 6 km 2 ). Comparison with direct, local-scale (eddy covariance) measurements of net exchange with the ecosystems around the towers are reasonable after accounting for anthropogenic CO 2 emissions within the larger area represented by the mixing ratio data. A network of tower sites and frequent aircraft vertical profiles, separated by several hundred kilometres, where CO 2 is accurately measured would provide data to estimate horizontal and vertical advection and hence provide a means to derive net CO 2 fluxes on a regional scale. At present CO 2 mixing ratios are measured with sufficient accuracy relative to global reference gas standards at only a few continental sites. The results also confirm that flux measurements from carefully sited towers capture seasonal variations representative of large regions, and that the midday CO 2 mixing ratios sampled in the atmospheric surface layer similarly capture regional and seasonal variability in the continental CO 2 budget.

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We analysed climate change impacts on the growth and natural mortality of forest tree species and forest carbon (C) balance along an elevation gradient extending from the Pannonian lowland to the West Carpathian Mountains (Central Europe). Norway spruce Picea abies, European beech Fagus sylvatica, and oak Quercus sp. were investigated for 2 future time periods: 2021-2050 and 2071-2100. The period 1961-1990 was used as reference. Forest growth simulations were based on the SIBYLA tree growth simulator (an empirical model), and C cycle-related simulations were performed using BIOME-BGC (a process-based biogeochemical model). Growth simulations indicated that climate change will substantially affect the growth of spruce and beech, but not of oak, in Central Europe. Growth of spruce and beech in their upper distribution ranges was projected to improve, while drought-induced production decline was projected at the species' receding edges. Beech was the only species projected to decline critically at lower elevations. C cycle simulations performed for the zone of ecological optima of the 3 tree species indicated that these forests are likely to remain net carbon dioxide sinks in the future, although the magnitude of their sequestration capacity will differ. Increasing nitrogen deposition and atmospheric carbon dioxide concentration were projected to greatly affect the forest C cycle. A multi-model assessment based on SIBYLA and BIOME-BGC simulations performed for the zone of ecological optima suggested that oak production will either remain the same as in the reference period or will increase. Future production of beech seems uncertain and might decline, while spruce production is likely to increase. The results also confirmed the value of multi-model approaches for assessing future forest development under climate change.KEY WORDS: Norway spruce · European beech · Oak · Forest carbon cycle · Tree production · Tree mortality · BIOME-BGC model · SIBYLA tree growth simulator Resale or republication not permitted without written consent of the publisher

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