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Metabolic theory and body size constraints on biomass production and decomposition suggest that differences in the intrinsic potential net ecosystem production ( NEPPOT) should be small among contrasting C3 grasslands and therefore unable to explain the wide range in the annual apparent net ecosystem production ( NEPAPP) reported by previous studies. We estimated NEPPOT for nine C3 grasslands under contrasting climate and management regimes using multiyear eddy covariance data. NEPPOT converged within a narrow range, suggesting little difference in the net carbon dioxide uptake capacity among C3 grasslands. Our results indicate a unique feature of C3 grasslands compared with other terrestrial ecosystems and suggest a state of stability in NEPPOT due to tightly coupled production and respiration processes. Consequently, the annual NEPAPP of C3 grasslands is primarily a function of seasonal and short-term environmental and management constraints, and therefore especially susceptible to changes in future climate patterns and associated adaptation of management practices. [ABSTRACT FROM AUTHOR]

Abstract: Seven years of continuous eddy covariance measurements at an alpine meadow were used to investigate the impacts of climate drivers and ecosystem responses on the inter-annual variability (IAV) of the net ecosystem exchange (NEE). The annual cumulative value of NEE was positive (source) in 2003, 2005 and 2009 (50, 15 and 112gm−2 respectively) and negative (sink) in 2004, 2006, 2007 and 2008 (29, 75, 110 and 28gm−2 respectively). The IAV of carbon dioxide fluxes builds up in two phenological phases: the onset of the growing season (triggered by snow melting) and the canopy re-growth after mowing. Respiratory fluxes during the non-growing season were observed to increase IAV, while growing season uptake dampened it. A novel approach was applied to factor out the two main sources of IAV: climate drivers’ variability and changes in the ecosystem responses to climate. Annual values of carbon dioxide fluxes were calculated assuming (a) variable climate and variable ecosystem response among years, (b) variable climate and constant ecosystem response and (c) constant climate and variable ecosystem response. The analysis of flux variances calculated under these three assumptions indicates the occurrence of an important negative feedback between climate and ecosystem responses. Due to this feedback, the observed IAV of NEE is lower than one would expect for a given climate variability, because of the counteracting changes in ecosystem responses. This alpine meadow therefore demonstrates the ability to acclimatise and to limit the IAV of carbon fluxes induced by climate variability. [Copyright &y& Elsevier]

The net ecosystem carbon dioxide (CO2) exchange (NEE) of nine European mountain grassland ecosystems was measured during 2002–2004 using the eddy covariance method. Overall, the availability of photosynthetically active radiation (PPFD) was the single most important abiotic influence factor for NEE. Its role changed markedly during the course of the season, PPFD being a better predictor for NEE during periods favorable for CO2 uptake, which was spring and autumn for the sites characterized by summer droughts (southern sites) and (peak) summer for the Alpine and northern study sites. This general pattern was interrupted by grassland management practices, that is, mowing and grazing, when the variability in NEE explained by PPFD decreased in concert with the amount of aboveground biomass (BMag). Temperature was the abiotic influence factor that explained most of the variability in ecosystem respiration at the Alpine and northern study sites, but not at the southern sites characterized by a pronounced summer drought, where soil water availability and the amount of aboveground biomass were more or equally important. The amount of assimilating plant area was the single most important biotic variable determining the maximum ecosystem carbon uptake potential, that is, the NEE at saturating PPFD. Good correspondence, in terms of the magnitude of NEE, was observed with many (semi-) natural grasslands around the world, but not with grasslands sown on fertile soils in lowland locations, which exhibited higher maximum carbon gains at lower respiratory costs. It is concluded that, through triggering rapid changes in the amount and area of the aboveground plant matter, the timing and frequency of land management practices is crucial for the short-term sensitivity of the NEE of the investigated mountain grassland ecosystems to climatic drivers. [ABSTRACT FROM AUTHOR]

Abstract: Eddy-covariance measurements, which were performed in the summer 2003 in an alpine grassland site, were used to determine the footprint of CO2 turbulent fluxes and to investigate its dependence on stability. The analysis is based on the spatial variability of carbon sinks generated by the difference in the time of the cutting of two concentric portions of the footprint, located outside and inside the fence surrounding the eddy tower, at approximately 30m distance. Due to this time difference, turbulent flux measurements were performed both in homogeneous (BEFORE and AFTER cutting) and heterogeneous (BETWEEN cuts) condition of spatial sink distribution. The analysis is based exclusively on daytime measurements. Half-hourly records ranked in two stability classes were used to calculate light response functions separately for three 10-day periods (BEFORE, BETWEEN and AFTER cutting). The contribution of the area inside the fence to the total flux was determined for different photosynthetic photon flux density (PPFD) values considering the BETWEEN cuts light response curve as a weighted average of the BEFORE and AFTER ones. The weight of the BEFORE cutting light response curve has been analytically determined and corresponds to the flux fraction which originates from inside the fence. The experimental estimates of the relative importance of the area inside the fence produced values ranging from 30%, during stable conditions, and up to 80% during unstable conditions. These values derived from observations were later compared with the predictions of three analytical footprint models. One of the models systematically underestimated experimental observations, while observations concurred with the other two, particularly in moderately unstable conditions. [Copyright &y& Elsevier]