A stochastic trajectory model was used to estimate scalar fluxfootprints in neutral stabilityfor canopies of varying leaf area distributions andleaf area indices. An analytical second-order closure ...model wasused to predict mean wind speed, second moments and the dissipationrate of turbulent kinetic energy within a forest canopy.The influence of source vertical profile on the flux footprint wasexamined. The fetch is longer for surface sourcesthan for sources at higher levels in the canopy. In order tomeasure all the flux components, and thus the total flux, with adesired accuracy, sources were located at the forest floor in thefootprint function estimation. The footprint functions werecalculated for five observation levels above the canopy top. Itwas found that at low observation heights both canopy density andcanopy structure affect the fetch. The higher abovethe canopy top the flux is measured, the more pronounced is the effectof the canopy structure. The forest fetch for flux measurements isstrongly dependent on the required accuracy: The 90% flux fetchis greater by a factor of two or more compared to the 75% fetch. Theupwind distance contributing 75% of flux is as large as 45 timesthe difference between canopy height and the observation heightabove the canopy top, being even larger for low observationlevels.PUBLICATION ABSTRACT
Vertical profiles of turbulence statistics were used together with horizontal and vertical profiles of CO
2 concentration to investigate the magnitude of advective fluxes at a sloping alpine site ...(Renon, BZ, Italy), characterised by typical mountain-valley breeze circulation. The analysis was performed in order to highlight and quantify the contribution of non-turbulent fluxes to the ecosystem carbon exchange under different atmospheric conditions. Both vertical and horizontal advection components consistently showed positive values (CO
2 emission) during the night, leading to an increase in the carbon loss from the ecosystem. These results are in contrast with previous experimental analyses on night-time advective fluxes performed in more homogeneous and flat sites, in which the positive vertical advection flux was generally balanced by negative horizontal advection. The occurrence of unbalanced advective components is attributed to the strong horizontal heterogeneities in canopy structure and source distribution. For the calculation of vertical advection, the hypothesis of linear decay of residual vertical velocities in the canopy space was tested against measured profiles of vertical velocities. Results show that this hypothesis does not introduce substantial errors in the estimates of vertical advection at this specific site. Finally, the night-time dependence of turbulent flux, advective fluxes and storage on friction velocity was analysed. While the eddy flux shows a strong dependence on
u
*
at low friction velocity values, the total flux is less dependent on this factor.
The net ecosystem carbon dioxide (CO sub(2)) 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 CO sub(2) 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 (BM sub(ag)). 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.
We present carbon stable isotope, δ13C, results from air and organic matter samples collected during 98 individual field campaigns across a network of Carboeuroflux forest sites in 2001 (14 sites) ...and 2002 (16 sites). Using these data, we tested the hypothesis that δ13C values derived from large‐scale atmospheric measurements and models, which are routinely used to partition carbon fluxes between land and ocean, and potentially between respiration and photosynthesis on land, are consistent with directly measured ecosystem‐scale δ13C values. In this framework, we also tested the potential of δ13C in canopy air and plant organic matter to record regional‐scale ecophysiological patterns.
Our network estimates for the mean δ13C of ecosystem respired CO2 and the related ‘discrimination’ of ecosystem respiration, δer and Δer, respectively, were −25.6±1.9‰ and 17.8 ±2.0‰ in 2001 and −26.6±1.5‰ and 19.0±1.6‰ in 2002. The results were in close agreement with δ13C values derived from regional‐scale atmospheric measurement programs for 2001, but less so in 2002, which had an unusual precipitation pattern. This suggests that regional‐scale atmospheric sampling programs generally capture ecosystem δ13C signals over Europe, but may be limited in capturing some of the interannual variations.
In 2001, but less so in 2002, there were discernable longitudinal and seasonal trends in δer. From west to east, across the network, there was a general enrichment in 13C (∼3‰ and ∼1‰ for the 2 years, respectively) consistent with increasing Gorczynski continentality index for warmer and drier conditions. In 2001 only, seasonal 13C enrichment between July and September, followed by depletion in November (from about −26.0‰ to −24.5‰ to −30.0‰), was also observed. In 2001, July and August δer values across the network were significantly related to average daytime vapor pressure deficit (VPD), relative humidity (RH), and, to a lesser degree, air temperature (Ta), but not significantly with monthly average precipitation (Pm). In contrast, in 2002 (a much wetter peak season), δer was significantly related with Ta, but not significantly with VPD and RH. The important role of plant physiological processes on δer in 2001 was emphasized by a relatively rapid turnover (between 1 and 6 days) of assimilated carbon inferred from time‐lag analyses of δer vs. meteorological parameters. However, this was not evident in 2002. These analyses also noted corresponding diurnal cycles of δer and meteorological parameters in 2001, indicating a rapid transmission of daytime meteorology, via physiological responses, to the δer signal during this season.
Organic matter δ13C results showed progressive 13C enrichment from leaves, through stems and roots to soil organic matter, which may be explained by 13C fractionation during respiration. This enrichment was species dependent and was prominent in angiosperms but not in gymnosperms. δ13C values of organic matter of any of the plant components did not well represent short‐term δer values during the seasonal cycle, and could not be used to partition ecosystem respiration into autotrophic and heterotrophic components.
Abstract
We present carbon stable isotope,
δ
13
C, results from air and organic matter samples collected during 98 individual field campaigns across a network of Carboeuroflux forest sites in 2001 ...(14 sites) and 2002 (16 sites). Using these data, we tested the hypothesis that
δ
13
C values derived from large‐scale atmospheric measurements and models, which are routinely used to partition carbon fluxes between land and ocean, and potentially between respiration and photosynthesis on land, are consistent with directly measured ecosystem‐scale
δ
13
C values. In this framework, we also tested the potential of
δ
13
C in canopy air and plant organic matter to record regional‐scale ecophysiological patterns.
Our network estimates for the mean
δ
13
C of ecosystem respired CO
2
and the related ‘discrimination’ of ecosystem respiration,
δ
er
and Δ
er
, respectively, were −25.6±1.9‰ and 17.8 ±2.0‰ in 2001 and −26.6±1.5‰ and 19.0±1.6‰ in 2002. The results were in close agreement with
δ
13
C values derived from regional‐scale atmospheric measurement programs for 2001, but less so in 2002, which had an unusual precipitation pattern. This suggests that regional‐scale atmospheric sampling programs generally capture ecosystem
δ
13
C signals over Europe, but may be limited in capturing some of the interannual variations.
In 2001, but less so in 2002, there were discernable longitudinal and seasonal trends in
δ
er
. From west to east, across the network, there was a general enrichment in
13
C (∼3‰ and ∼1‰ for the 2 years, respectively) consistent with increasing Gorczynski continentality index for warmer and drier conditions. In 2001 only, seasonal
13
C enrichment between July and September, followed by depletion in November (from about −26.0‰ to −24.5‰ to −30.0‰), was also observed. In 2001, July and August
δ
er
values across the network were significantly related to average daytime vapor pressure deficit (VPD), relative humidity (RH), and, to a lesser degree, air temperature (
T
a
), but not significantly with monthly average precipitation (
P
m
). In contrast, in 2002 (a much wetter peak season),
δ
er
was significantly related with
T
a
, but not significantly with VPD and RH. The important role of plant physiological processes on
δ
er
in 2001 was emphasized by a relatively rapid turnover (between 1 and 6 days) of assimilated carbon inferred from time‐lag analyses of
δ
er
vs. meteorological parameters. However, this was not evident in 2002. These analyses also noted corresponding diurnal cycles of
δ
er
and meteorological parameters in 2001, indicating a rapid transmission of daytime meteorology, via physiological responses, to the
δ
er
signal during this season.
Organic matter
δ
13
C results showed progressive
13
C enrichment from leaves, through stems and roots to soil organic matter, which may be explained by
13
C fractionation during respiration. This enrichment was species dependent and was prominent in angiosperms but not in gymnosperms.
δ
13
C values of organic matter of any of the plant components did not well represent short‐term
δ
er
values during the seasonal cycle, and could not be used to partition ecosystem respiration into autotrophic and heterotrophic components.
We present carbon stable isotope, delta super(13)C, results from air and organic matter samples collected during 98 individual field campaigns across a network of Carboeuroflux forest sites in 2001 ...(14 sites) and 2002 (16 sites). Using these data, we tested the hypothesis that delta super(13)C values derived from large-scale atmospheric measurements and models, which are routinely used to partition carbon fluxes between land and ocean, and potentially between respiration and photosynthesis on land, are consistent with directly measured ecosystem-scale delta super(13)C values. In this framework, we also tested the potential of delta super(13)C in canopy air and plant organic matter to record regional-scale ecophysiological patterns. Our network estimates for the mean delta super(13)C of ecosystem respired CO sub(2) and the related 'discrimination' of ecosystem respiration, delta sub(er) and Delta sub(er), respectively, were -25.6 plus or minus 1.9ppt and 17.8 plus or minus 2.0ppt in 2001 and -26.6 plus or minus 1.5ppt and 19.0 plus or minus 1.6ppt in 2002. The results were in close agreement with delta super(13)C values derived from regional-scale atmospheric measurement programs for 2001, but less so in 2002, which had an unusual precipitation pattern. This suggests that regional-scale atmospheric sampling programs generally capture ecosystem delta super(13)C signals over Europe, but may be limited in capturing some of the interannual variations. In 2001, but less so in 2002, there were discernable longitudinal and seasonal trends in delta sub(er). From west to east, across the network, there was a general enrichment in super(13)C ( similar to 3ppt and similar to 1ppt for the 2 years, respectively) consistent with increasing Gorczynski continentality index for warmer and drier conditions. In 2001 only, seasonal super(13)C enrichment between July and September, followed by depletion in November (from about -26.0ppt to -24.5ppt to -30.0ppt), was also observed. In 2001, July and August delta sub(er) values across the network were significantly related to average daytime vapor pressure deficit (VPD), relative humidity (RH), and, to a lesser degree, air temperature (T sub(a)), but not significantly with monthly average precipitation (P sub(m)). In contrast, in 2002 (a much wetter peak season), delta sub(er) was significantly related with T sub(a), but not significantly with VPD and RH. The important role of plant physiological processes on delta sub(er) in 2001 was emphasized by a relatively rapid turnover (between 1 and 6 days) of assimilated carbon inferred from time-lag analyses of delta sub(er) vs. meteorological parameters. However, this was not evident in 2002. These analyses also noted corresponding diurnal cycles of delta sub(er) and meteorological parameters in 2001, indicating a rapid transmission of daytime meteorology, via physiological responses, to the delta sub(er) signal during this season. Organic matter delta super(13)C results showed progressive super(13)C enrichment from leaves, through stems and roots to soil organic matter, which may be explained by super(13)C fractionation during respiration. This enrichment was species dependent and was prominent in angiosperms but not in gymnosperms. delta super(13)C values of organic matter of any of the plant components did not well represent short-term delta sub(er) values during the seasonal cycle, and could not be used to partition ecosystem respiration into autotrophic and heterotrophic components.
Surface albedo is a key parameter in the Earth's energy balance since it affects the amount of solar radiation directly absorbed at the planet surface. Its variability in time and space can be ...globally retrieved through the use of remote sensing products. To evaluate and improve the quality of satellite retrievals, careful intercomparisons with in situ measurements of surface albedo are crucial. For this purpose we compared MODIS albedo retrievals with surface measurements taken at 53 FLUXNET sites that met strict conditions of land cover homogeneity. A good agreement between mean yearly values of satellite retrievals and in situ measurements was found (R(exp 2)= 0.82). The mismatch is correlated to the spatial heterogeneity of surface albedo, stressing the relevance of land cover homogeneity when comparing point to pixel data. When the seasonal patterns of MODIS albedo is considered for different plant functional types, the match with surface observation is extremely good at all forest sites. On the contrary, in non-forest sites satellite retrievals underestimate in situ measurements across the seasonal cycle. The mismatch observed at grasslands and croplands sites is likely due to the extreme fragmentation of these landscapes, as confirmed by geostatistical attributes derived from high resolution scenes.
