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Abstract: This study examines the hypothesis that surface-atmosphere exchange of CO2 in terrestrial ecosystems always can be interpreted purely in terms of biological processes, neglecting geochemical cycling by karst systems that characterize 22 million km2 in the world [Yuan Daoxian, G., 1997. The carbon cycle in karst, Zeitschrift für Geomorphologie 108 (Supplementbände), 91–102]. Eddy covariance data of net CO2 fluxes are examined for two ecosystems over karstic substrates in contrasting climates in the North and South of Spain. A semi-arid matorral is found to behave similarly to previously studied ecosystems when well watered, but sizeable mid-day CO2 emissions during extended drought and plant senescence are found to be incompatible with ecophysiological interpretations of the flux. For a temperate pasture overlying an accessible cave, afternoon CO2 emissions in summer are likewise inexplicable in a biological context, but coincide with periods when ventilation is observed inside the subterranean cavity. These results suggest direct linkages at times in CO2 exchanges among atmosphere, ecosystems, and carbonate substrates which represent more than a tenth of the Earth''s land surface. [Copyright &y& Elsevier]

The effects of harvest on European forest net ecosystem exchange (NEE) of carbon and its photosynthetic and respiratory components (GPP (gross primary production) and TER (total ecosystem respiration)) were examined by comparing four pairs of mature/harvested sites in Europe via a combination of eddy covariance measurements and empirical modeling. Three of the comparisons represented high coniferous forestry (spruce in Britain, and pines in Finland and France), while a coppice-with-standard oak plantation was examined in Italy.While every comparison revealed that harvesting converted a mature forest carbon sink into a carbon source of similar magnitude, the mechanisms by which this occurred were very different according to species or management practice. In Britain, Finland, and France the annual sink (source) strength for mature (clear-cut) stands was estimated at 496 (112), 138 (239), and 222 (225) g C m−2, respectively, with 381 (427) g C m−2 for the mature (coppiced) stand in Italy. In all three cases of high forestry in Britain, Finland, and France, clear-cutting crippled the photosynthetic capacity of the ecosystem– with mature (clear-cut) GPP of 1970 (988), 1010 (363), and 1600 (602) g C m−2– and also reduced ecosystem respiration to a lesser degree– TER of 1385 (1100), 839 (603), and 1415 (878) g C m−2, respectively. By contrast, harvesting of the coppice oak system provoked a burst in respiration– with mature (clear-cut) TER estimated at 1160 (2220) gC m−2– which endured for the 3 years sampled postharvest. The harvest disturbance also reduced GPP in the coppice system– with mature (clear-cut) GPP of 1600 (1420) g C m−2– but to a lesser extent than in the coniferous forests, and with near-complete recovery within a few years. Understanding the effects of harvest on the carbon balance of European forest systems is a necessary step towards characterizing carbon exchange for timberlands on large scales. [ABSTRACT FROM AUTHOR]

In this paper, we present and discuss the annual net ecosystem exchange (NEE) results from 5 years (1997–2001) of continuous eddy covariance measurements of CO2 flux above a mixed temperate forest. The forest was a 70-year-old coniferous (Scots pine)—deciduous mixture, with slow growth rate and a leaf area index (LAI) of about 3, and was part of the European CARBOEUROFLUX research network. Effects of the data pre-treatment and the gap filling method on annual NEE estimates were analyzed. The u*-correction increased the annual NEE by +61 g C m−2 per year on average. The maximum difference in annual NEE estimates from different gap filling methods amounted up to 130 g C m−2 per year in a year with a large gap in the CO2 flux series. The estimated average annual NEE over the 5 years was + 110 g C m−2 per year (ranging from −9 to 255 g C m−2 per year) when using the most defensible gap filling strategy. We also analyzed the inter-annual variability of carbon balance, which was found to be mainly dependent on the length of the growing season and on the annual temperature. The observation that this forest acted as a CO2 source contrasts with previous results from most other temperate forests. [Copyright &y& Elsevier]

Replying to: A. De Schrijver et al. 451, 10.1038/nature06578; W. de Vries et al. 451, 10.1038/nature06579 (2008)Nitrogen (N) deposition alters ecosystem function in several ways, with important effects on N leaching and water quality, as well as on interspecific competition and biodiversity. These changes have been attributed to ecosystem N saturation, defined as the alleviation of N limitations on rates of biological function. After an initial fertilization effect, N saturation has also been suggested to reduce plant function and growth, eventually leading to forest dieback. Although our observation of a substantial positive effect of N deposition on forest carbon (C) sequestration does not imply the absence of nitrate losses or other negative effects, as rightly stressed by De Schrijver et al., the sustained response observed demonstrates that the fear of a generalized forest decline in response to N fertilization could be overstated, at least within the rather broad N deposition range explored in our analysis. The nature of the observed response of forest C sequestration to N deposition, however, has been questioned outright by de Vries et al., who suggested that it could be an artefact resulting from the covariation between N deposition and other environmental variables. The arguments proposed against an overwhelming N effect, however, do not seem to stand up to close scrutiny. [ABSTRACT FROM AUTHOR]