Impact of CO2 storage flux sampling uncertainty on net ecosystem exchange measured by eddy covariance

Complying with several assumption and simplifications, most of the carbon budget studies based on eddy covariance (EC) measurements quantify the net ecosystem exchange (NEE) by summing the flux obtained by EC ( FC ) and the storage flux ( SC ). SC is the rate of change of a scalar, CO 2 molar fraction in this case, within the control volume underneath the EC measurement level. It is given by the difference in the quasi-instantaneous profiles of concentration at the beginning and end of the EC averaging period, divided by the averaging period. The approaches used to estimate SC largely vary, from measurements based on a single sampling point usually located at the EC measurement height, to measurements based on profile sampling. Generally a single profile is used, although multiple profiles can be positioned within the control volume. Measurement accuracy reasonably increases with the spatial sampling intensity, however limited resources often prevent more elaborated measurement systems. In this study we use the experimental dataset collected during the ADVEX campaign in which turbulent and non-turbulent fluxes were measured in three forest sites by the simultaneous use of five towers/profiles. Our main objectives are to evaluate both the uncertainty of SC that derives from an insufficient sampling of CO 2 variability, and its impact on concurrent NEE estimates.Results show that different measurement methods may produce substantially different SC flux estimates which in some cases involve a significant underestimation of the actual SC at a half-hourly time scales. A proper measuring system, that uses a single vertical profile of which the CO 2 sampled at 3 points (the two closest to the ground and the one at the lower fringe of the canopy layer) is averaged with CO 2 sampled at a certain distance and at the same height, improves the horizontal representativeness and reduces this (proportional) bias to 2–10% in such ecosystems. While the effect of this error is minor on long term NEE estimates, it can produce significant uncertainty on half-hourly NEE fluxes

Influence of Spring and Autumn Phenological Transitions on Forest Ecosystem Productivity

We use eddy covariance measurements of net ecosystem productivity (NEP) from 21 FLUXNET sites (153 site-years of data) to investigate relationships between phenology and productivity (in terms of both NEP and gross ecosystem photosynthesis, GEP) in temperate and boreal forests. Results are used to evaluate the plausibility of four different conceptual models. Phenological indicators were derived from the eddy covariance time series, and from remote sensing and models. We examine spatial patterns (across sites) and temporal patterns (across years); an important conclusion is that it is likely that neither of these accurately represents how productivity will respond to future phenological shifts resulting from ongoing climate change. In spring and autumn, increased GEP resulting from an ¿extra¿ day tends to be offset by concurrent, but smaller, increases in ecosystem respiration, and thus the effect on NEP is still positive. Spring productivity anomalies appear to have carry-over effects that translate to productivity anomalies in the following autumn, but it is not clear that these result directly from phenological anomalies. Finally, the productivity of evergreen needleleaf forests is less sensitive to phenology than is productivity of deciduous broadleaf forests. This has implications for how climate change may drive shifts in competition within mixed-species stands.JRC.H.5-Land Resources Managemen

Ecophysiological Responses to Rainfall Variability in Grassland and Forests Along a Latitudinal Gradient in Italy

In the Mediterranean region, ecosystems are severely affected by climate variability. The Italian Peninsula is a hot spot for biodiversity thanks to its heterogeneous landscape and the Mediterranean, Continental, and Alpine climates hosting a broad range of plant functional types along a limited latitudinal range from 40′ to 46′ N. In this study we applied a comparative approach integrating descriptive statistics, time series analysis, and multivariate techniques to answer the following questions: (i) do the climatic variables affect Gross Primary Productivity (GPP), Reco, Water Use Efficiency (WUE), and ET to a similar extent among different sites? (ii) Does a common response pattern exist among ecosystems along a latitudinal gradient in Italy? And, finally (iii) do these ecosystems respond synchronically to meteorological conditions or does a delayed response exist? Six sites along a latitudinal, altitudinal, and vegetational gradient from semi-arid (southern Italy), to a mountainous Mediterranean site (central Italy), and sub-humid wet Alpine sites (northern Italy) were considered. For each site, carbon and water fluxes, and meteorological data collected during two hydrologically-contrasting years (i.e., a dry and a wet year) were analyzed. Principal Component Analysis (PCA) was adopted to identify temporal and spatial variations in GPP, Ecosystem Respiration (Reco), WUE, and Evapotranspiration (ET). The model outlined differences among Mediterranean semi-arid, Mediterranean mountainous, and Alpine sites in response to contrasting precipitation regimes. GPP, Reco, WUE, and ET increased up to 16, 19, 25, and 28%, respectively in semi-arid Mediterranean sites and up to 15, 32, 15, and 11%, respectively in Alpine sites in the wet year compared to the dry year. Air temperature was revealed to be one of the most important variables affecting GPP, Reco, WUE, and ET in all the study sites. While relative air humidity was more important in southern Mediterranean sites, global radiation was more significant in northern Italy. Our work suggests that a realistic prediction of the main responses of Italian forests under climate change should also take in account delayed responses due to acclimation to abiotic stress or changing environmental conditions

Numerical Study of the Interplay between Thermo-topographic Slope Flow and Synoptic Flow on Canopy Transport Processes

Canopy flow resulting from interaction between thermo-topographic slope flow and large-scale synoptic flow is very complicated and has been poorly understood. We apply a Reynolds-averaged Navier-Stokes (RANS) turbulence model to investigate how the interactions between local flow and synoptic winds affect CO2 movement in the canopy layer at the Renon site in the Italian Alps. Since the RANS simulations are compared to the data measured by multiple-tower experiments conducted during CarboEurope-IP advection campaigns (ADVEX) at Renon, our study can be viewed as a case study of a relatively common wooded slope. The thermal condition in the canopy is directly related to the canopy morphology: the dense canopy at our site causes stronger cooling but limits vertical exchange of heat flux, resulting in weak temperature inversion in the deep canopy. Under conditions with no synoptic wind, local flow leads to CO2 build-up mainly at downslope locations and no recirculation is formed. Recirculation that holds high CO2 mole fraction in the canopy is developed only under the condition that local slope wind is enhanced by northerly synoptic winds. No recirculation forms when southerly synoptic wind direction is opposite to the local wind direction, in which case CO2 is quite well mixed. This numerical study approach brings to light a better understanding of the CO2 closure problem: the measured net ecosystem exchange of CO2 is more likely to be underestimated in local non-synoptic slope flow and local synoptic-enhanced slope flow regimes at Renon. However, small-scale heterogeneity in canopy structure, variability in the CO2 source from soil and higher-resolution and larger-scale topography still challenge the application of this numerical approach in the FLUXNET community

Understanding carbon sequestration, allocation, and ecosystem storage in a grassed vineyard

Understanding if and to which extent a crop can act as a carbon sink is the basis of the assessment of its sustainability in the climate change context. Grassed vineyards have been indicated in the recent past as potentially large carbon sinks, questioning the assumption that crops are in general carbon sources. To this end, we conducted a detailed study along a growing season in a grassed mountain vineyard with two varieties (Chardonnay and Sauvignon blanc) to quantify the overall carbon stock of the system and to attribute the carbon fluxes to the specific components of the carbon cycle of the agroecosystem, including vines organs (shoots, fruits, roots), grasses (shoots and roots) and soil. We combined eddy covariance, soil respiration, biometric measurements, and soil analysis. Our findings determined the studied vineyard to be a moderate carbon sink. We found a gross primary production (2409 ± 35 g C m-2) much larger than previous data for vineyards, but the NEP (246 ± 54 g C m-2) of the growing season was on the lower end of previous reports. Based on similar above-ground net primary production values for the grapevines and herbaceous vegetation, we confirmed that the grassed alleys play an important role in overall carbon accumulation. We also observed that the soil represents by far the largest carbon storage, being the carbon retained by vegetation at harvest time only 7.3% of the total. The overall carbon stored in the vineyard (152.1 ± 7.1 t C ha-1) was less than that of forests and some orchards primarily due to the lower amount of plant biomass. Permanent grassland sites generally contained much higher amounts of carbon in the topsoil, indicating that there are vineyard characteristics or management practices which limit long term storage in this pool. Further studies are needed to unravel the relative contribution of the grapevines and grasses to overall gross primary productivity and carbon storage potential, especially in the context of different management decisions and the increasing frequency of drought events in similar mountain environments.Fil: Callesen Torben, Oliver. University of Bozen-Bolzano; ItaliaFil: Gonzalez, Carina Veronica. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mendoza. Instituto de Biología Agrícola de Mendoza. Universidad Nacional de Cuyo. Facultad de Ciencias Agrarias. Instituto de Biología Agrícola de Mendoza; ArgentinaFil: Bastos Campos, Flavio. University of Bozen-Bolzano; ItaliaFil: Zanotelli, Damiano. University of Bozen-Bolzano; ItaliaFil: Tagliavini, Massimo. University of Bozen-Bolzano; ItaliaFil: Montagnani. Leonardo. University of Bozen-Bolzano; Itali

Global parameterization and validation of a two-leaf light use efficiency model for predicting gross primary production across FLUXNET sites:TL-LUE Parameterization and Validation

Light use efficiency (LUE) models are widely used to simulate gross primary production (GPP). However, the treatment of the plant canopy as a big leaf by these models can introduce large uncertainties in simulated GPP. Recently, a two-leaf light use efficiency (TL-LUE) model was developed to simulate GPP separately for sunlit and shaded leaves and has been shown to outperform the big-leaf MOD17 model at six FLUX sites in China. In this study we investigated the performance of the TL-LUE model for a wider range of biomes. For this we optimized the parameters and tested the TL-LUE model using data from 98 FLUXNET sites which are distributed across the globe. The results showed that the TL-LUE model performed in general better than the MOD17 model in simulating 8 day GPP. Optimized maximum light use efficiency of shaded leaves (εmsh) was 2.63 to 4.59 times that of sunlit leaves (εmsu). Generally, the relationships of εmsh and εmsu with εmax were well described by linear equations, indicating the existence of general patterns across biomes. GPP simulated by the TL-LUE model was much less sensitive to biases in the photosynthetically active radiation (PAR) input than the MOD17 model. The results of this study suggest that the proposed TL-LUE model has the potential for simulating regional and global GPP of terrestrial ecosystems, and it is more robust with regard to usual biases in input data than existing approaches which neglect the bimodal within-canopy distribution of PAR

What eddy-covariance measurements tell us about prior land flux errors in CO2-flux inversion schemes

0.2 after 200 km). Separating out the plant functional types did not increase the spatial correlations, except for the deciduous broad-leaved forests. Using the statistics of the flux measurements as a proxy for the statistics of the prior flux errors was shown not to be a viable approach. A statistical model allowed us to upscale the site-level flux error statistics to the coarser spatial and temporal resolutions used in regional or global models. This approach allowed us to quantify how aggregation reduces error variances, while increasing correlations. As an example, for a typical inversion of grid point (300 km × 300 km) monthly fluxes, we found that the prior flux error follows an approximate e-folding correlation length of 500 km only, with correlations from one month to the next as large as 0.6

Evapotranspiration Dynamics and Partitioning in a Grassed Vineyard: Ecophysiological and Computational Modeling Approaches

Plenty of information on evapotranspiration (ET) dynamics and partitioning into nonbiological (evaporation, E) and biological (transpiration, T) components is available in literature. However, in agro-ecosystems where more than one vegetation group is found, like intercropping or grassed orchards and vineyards, it is of great use to understand the contribution to T due to the single plant type or group of plants. We deployed empirical and modeling methods to study the ecosystem evapotranspiration (ETEC) components in a grassed vineyard in Caldaro (Italy) aiming to assess (a) which process, E or T, had greater influence on ETEC dynamics; (b) which component among grapevines and understorey portion dominated the ETEC; and (c) how rainfall influences ETEC components. A top-down approach combined the eddy covariance method to estimate ETEC, and the Transpiration Estimation Algorithm method to partition it. A bottom-up approach integrated the understorey evapotranspiration (ETu) with modeled vines transpiration (Tv((mod))). Measured and modeled fluxes showed high daily variability, consistently with meteorological conditions (vapor pressure deficit, Rn and Tair). The mean daily ETEC integrals were 3.45 and 3.40 mm d(-1) (2021 and 2022), being T-EC (estimated transpiration fraction of ETEC) the higher contributor (T-EC/ETEC of 0.77 and 0.79, same years). From the bottom-up approach, ETu assessed during ground flux chamber campaigns (0.74-1.65 mm d(-1)) was lower than Tv((mod)). A high agreement (R-2 = 0.85) was found between the eddy covariance ET hourly values and ET by summing Tv((mod)) and ETu. We concluded that the T process represented major fluxes in the agroecosystem during the warm season. Furthermore, the bottom-up approach indicated the vines as primary contributors to ecosystem T, particularly noticeable after rainfall, as the understorey T fraction (Tu) increased when the system became drier. This study helps disentangling grapevine contribution to evapotranspiration from adjacent herbaceous vegetation in a vineyard, and emphasizes the dominance of biologically mediated transpiration influenced by meteorological conditions. This novel combination of approaches not only enhances understanding of Mediterranean viticulture but also illuminates broader applications in sparsely vegetated environments, such as agroforestry systems and orchards, advancing ecological management practices