Remote/proximate sensing of radiation reflected from the earth's surface ... more Remote/proximate sensing of radiation reflected from the earth's surface allows monitoring the `status' of the planet earth with reasonable spatial and temporal coverage. Here we explore the use of vegetation indices (VIs) calculated from reflectance in broad spectral regions for inferring ecosystem carbon dioxide (CO2) fluxes. To this end we use one year of concurrent measurements of reflectance in the
The quantification of greenhouse gas (GHG) budgets on a global scale is an important step in asse... more The quantification of greenhouse gas (GHG) budgets on a global scale is an important step in assessing the effect of anthropogenic and biogenic controls on a future climate. In the past, measurements of CO2 fluxes were conducted over a wide array of ecosystems, leading to a better understanding of its exchange patterns on different time scales and more sophisticated models. However, only few studies quantified the fluxes of the other two major GHG, methane (CH4) and nitrous oxide (N2O), mainly due to expensive sensors and their time-consuming maintenance. In addition, early CH4 and N2O measurements mainly focused on ecosystems with presumably high emissions of CH4 (e.g. wetlands) or N2O (e.g. heavily fertilized crops). In recent years, devices for CH4 and N2O measurements became widely available and more studies are conducted over sites that exert small and often close-to-zero fluxes. Despite recent advances in sensor sensitivity and stability, the quantification of CH4 and N2O exch...
Methane (CH4) and nitrous oxide (N2O) are major greenhouse gases and therefore play an important ... more Methane (CH4) and nitrous oxide (N2O) are major greenhouse gases and therefore play an important role in the atmosphere due to their warming potential. The various processes and mechanisms associated with the release and uptake of both compounds need further investigation to reliably assess CH4 and N2O budgets in different ecosystems. The exchange of CO2, CH4 and N2O was measured over 2 years at a temperate mountain grassland managed as a hay meadow near the village Neustift in the Stubai Valley, Austria. The three wind components, the speed of sound and the CO2mole densities were acquired at a time resolution of 20 Hz and used to calculate true eddy covariance CO2fluxes. CH4and N2O concentration values were recorded at 2 Hz, resulting in a disjunct time series when compared to the 20 Hz wind data. Fluxes of both compounds were then calculated using the virtual disjunct eddy covariance method (vDEC). The net ecosystem exchange of CO2 is monitored since 2001, while the measurement of...
In near future air temperature is expected to increase during the growing season and growing cond... more In near future air temperature is expected to increase during the growing season and growing conditions could become more arid in many parts of the world. Since the air temperature and the photosynthetically active radiation are the main drivers for the emission of many biogenic volatile organic compounds (BVOC), this may be expected to lead to a change in the emission patterns. To investigate the effects of drought on BVOC-emissions from mountain grassland we analysed the emission patterns during a whole vegetation period from March 2011 to December 2011. BVOC-concentrations were measured at a height of 2.4 m above the ground by means of a proton-transfer-reaction mass spectrometer and then combined with 3D wind data from a sonic anemometer. Half-hourly net BVOC fluxes (composed of grass and soil emissions) were calculated using the disjunct eddy covariance method. As a result of a dry period in May 2011 the soil water content dropped below 0.1 m3/m3 and methanol emissions increase...
Although the biosphere is currently thought to be the main source for atmospheric volatile organi... more Although the biosphere is currently thought to be the main source for atmospheric volatile organic compounds (VOCs) longer time series of VOC measurements are barely available and the accuracy VOC modeling approaches still suffers from a lack of flux measurement data. However, long-term VOC flux data sets could provide a way to improve the quality of modeling approaches and make a first step towards a better VOC quantification. We used VOC flux data of methanol, acetone, acetaldehyde and the monoterpenes, which were measured by means of a proton transfer reaction - mass spectrometer PTR-MS utilising the disjunct eddy covariance method (vDEC) above an intensively managed mountain grassland in Stubai Valley (Austria), to compare the performance of four different gap filling approaches and to get complete annual time series of the VOCs for the years 2009 and 2011. Assuming a zero flux from the grassland during the winter period, when the meadow is usually covered by snow, the average c...
When using the eddy covariance (EC) method for measuring the ecosystem–atmosphere exchange of sen... more When using the eddy covariance (EC) method for measuring the ecosystem–atmosphere exchange of sensible and latent heat, it is not uncommon to find that these two energy fluxes fall short of the available energy by 20 to 30%. As the causes for the energy imbalance are still under discussion, it is currently not clear how the energy balance should be closed. The objective of this study was to use independent measurements of evapotranspiration (ET) for empirically devising how to best close the energy balance. To this end, the ET of a temperate mountain grassland was quantified during two measurement campaigns using both an open- and a closed-path EC system, lysimeters, and an approach scaling up leaf-level stomatal conductance to canopy-level transpiration. Our study showed that both EC systems underestimated ET measured independently by lysimeters and the upscaling approach. The best correspondence with independently measured ET was achieved by assigning the entire energy imbalance t...
When using the eddy covariance (EC) method for measuring the ecosystem-atmosphere exchange of sen... more When using the eddy covariance (EC) method for measuring the ecosystem-atmosphere exchange of sensible and latent heat, it is not uncommon to find that these two energy fluxes fall short of available energy by 20-30 %. As the causes for the energy imbalance are still under discussion, it is currently not clear how the energy balance should be closed. The objective of the present paper is to use independent measurements of evapotranspiration (ET) for empirically devising on how to best close the energy balance. To this end ET of a temperate mountain grassland was quantified during two measurement campaigns using both an open- and a closed-path EC system, lysimeters and an approach scaling up leaf-level stomatal conductance to canopy level transpiration. Our study showed that both EC systems underestimated ET measured independently by lysimeters and the up-scaling approach. Best correspondence to independently measured ET was achieved by assigning the entire energy imbalance to ET and...
All-wave net surface radiation is greatly needed in various scientific research and applications.... more All-wave net surface radiation is greatly needed in various scientific research and applications. Satellite data have been used to estimate incident shortwave radiation, but hardly to estimate all-wave net radiation due to the inference of clouds on longwave radiation. A practical solution is to estimate all-wave net radiation empirically from shortwave radiation and other ancillary information. Since existing models were developed using a limited number of ground observations, a comprehensive evaluation of these models using a global network of representative measurements is urgently required. In this study, we developed a new day-time net radiation estimation model and evaluated it against seven commonly used existing models using radiation measurements obtained from 326 sites around the world from 1991 to 2010. MERRA re-analysis products from which the meteorological data were derived and remotely sensed products during the same period were also used. Model evaluations were performed in both global mode (all data were used to fit the models) and conditional mode (the data were divided into four subsets based on the surface albedo and vegetation index, and the models were fitted separately). Besides, the factors (i.e., albedo, air temperature, and NDVI) that may impact the estimation of all-wave net radiation were also extensively explored. Based on these evaluations, the fitting RMSE of the new developed model was approximately 40.0 Wm −2 in the global mode and varied between 18.2 and 54.0 Wm −2 in the conditional mode. We found that it is better to use net shortwave radiation (including surface albedo) than the incident shortwave radiation nearly in all models. Overall, the new model performed better than other existing linear models.
ABSTRACT This paper presents an improved algorithm for the retrieval of Leaf Area Index (LAI) fro... more ABSTRACT This paper presents an improved algorithm for the retrieval of Leaf Area Index (LAI) from Moderate Resolution Imaging Spectroradiometer (MODIS) satellite imagery that has been specifically customized for mountain grasslands in the Alps. The main features of the proposed algorithm, which is based on the inversion of a radiative transfer model, are: i) a higher spatial resolution (250 m) with respect to the corresponding standard MODIS product and ii) tuning the model to the spectral characteristics of mountain grasslands.
Remote/proximate sensing of radiation reflected from the earth's surface allows monitoring th... more Remote/proximate sensing of radiation reflected from the earth's surface allows monitoring the `status' of the planet earth with reasonable spatial and temporal coverage. Here we explore the use of vegetation indices (VIs) calculated from reflectance in broad spectral regions for inferring ecosystem carbon dioxide (CO2) fluxes. To this end we use one year of concurrent measurements of reflectance in the
The FLUXNET project [1] aims at quantifying the magnitude and controls on the CO2, H2O and energy... more The FLUXNET project [1] aims at quantifying the magnitude and controls on the CO2, H2O and energy exchange of terrestrial ecosystems. Ideally, the various biomes of the Earth would be sampled in proportion to their spatial extent - in reality, however, study site selection is usually based on other (more practical) criteria so that a bias exists towards certain biomes
Fluxes of CO2 and energy have been measured over several differently managed mountain grasslands ... more Fluxes of CO2 and energy have been measured over several differently managed mountain grasslands in the Austrian Alps over the past seven years. Here we discuss the challenges associated with the application of the eddy covariance method in complex mountainous terrain and show how these flux measurements can be validated using a multiple-constraints approach. It is shown that defensible eddy
Recent studies revealed convergent temperature sensitivity of ecosystem respiration (R e ) within... more Recent studies revealed convergent temperature sensitivity of ecosystem respiration (R e ) within aquatic ecosystems and between terrestrial and aquatic ecosystems. We do not know yet whether various terrestrial ecosystems have consistent or divergent temperature sensitivity. Here, we synthesized 163 eddy covariance flux sites across the world and examined the global variation of the apparent activation energy (Ea), which characterizes the apparent temperature sensitivity of R e , and its interannual variability (IAV) as well as their controlling factors.
Atmospheric Chemistry and Physics Discussions, 2015
ABSTRACT Methanol is the second most abundant volatile organic compound in the troposphere and pl... more ABSTRACT Methanol is the second most abundant volatile organic compound in the troposphere and plays a significant role in atmospheric chemistry. While there is consensus about the dominant role of living plants as the major source and the reaction with OH as the major sink of methanol, global methanol budgets diverge considerably in terms of source/sink estimates reflecting uncertainties in the approaches used to model, and the empirical data used to separately constrain these terms. Here we compiled micrometeorological methanol flux data from eight different study sites and reviewed the corresponding literature in order to provide a first cross-site synthesis of the terrestrial ecosystem-scale methanol exchange and present an independent data-driven view of the land-atmosphere methanol exchange. Our study shows that the controls of plant growth on the production, and thus the methanol emission magnitude, and stomatal conductance on the hourly methanol emission variability, established at the leaf level, hold across sites at the ecosystem-level. Unequivocal evidence for bi-directional methanol exchange at the ecosystem scale is presented. Deposition, which at some sites even exceeds methanol emissions, represents an emerging feature of ecosystem-scale measurements and is likely related to environmental factors favouring the formation of surface wetness. Methanol may adsorb to or dissolve in this surface water and eventually be chemically or biologically removed from it. Management activities in agriculture and forestry are shown to increase local methanol emission by orders of magnitude; they are however neglected at present in global budgets. While contemporary net land methanol budgets are overall consistent with the grand mean of the micrometeorological methanol flux measurements, we caution that the present approach of simulating methanol emission and deposition separately is prone to opposing systematic errors and does not allow taking full advantage of the rich information content of micrometeorological flux measurements.
When using the eddy covariance (EC) method for measuring the ecosystem-atmosphere exchange of sen... more When using the eddy covariance (EC) method for measuring the ecosystem-atmosphere exchange of sensible and latent heat, it is not uncommon to find that these two energy fluxes fall short of available energy by 20-30 %. As the causes for the energy imbalance are still under discussion, it is currently not clear how the energy balance should be closed. The objective of the present paper is to use independent measurements of evapotranspiration (ET) for empirically devising on how to best close the energy balance. To this end ET of a temperate mountain grassland was quantified during two measurement campaigns using both an open- and a closed-path EC system, lysimeters and an approach scaling up leaf-level stomatal conductance to canopy level transpiration. Our study showed that both EC systems underestimated ET measured independently by lysimeters and the up-scaling approach. Best correspondence to independently measured ET was achieved by assigning the entire energy imbalance to ET and...
Stomata are the major pathway by which plants exert control on the exchange of trace gases and wa... more Stomata are the major pathway by which plants exert control on the exchange of trace gases and water vapour with the aerial environment and thus provide a key link between the functioning of terrestrial ecosystems and the state and composition of the atmosphere. Understanding the nature of this control, i.e. how stomatal conductance differs between plant species and ecosystems and
Grassland ecosystems in a wider sense cover up to 40% of the global terrestrial surface (White et... more Grassland ecosystems in a wider sense cover up to 40% of the global terrestrial surface (White et al. 2000). Knowledge about the exchange of the major greenhouse gases (GHG's, carbon dioxide -CO 2 , methane -CH4 and nitrous oxide -N2O) remains still limited for grasslands, while other ecosystems such as forests and peatlands (particularly systems storing large amounts of carbon) have been investigated more intensively. Here, we give an overview of the current state of GHG measurements in the alpine region of Europe (Switzerland, Austria) and the associated challenges in deriving annual GHG budgets as well as determining the abiotic and biotic variables driving the fluxes of CO 2 , CH4 and N2O. In particular the importance of management activities, that is fertilization and removal of above-ground biomass through harvesting and grazing, besides climate, the challenges when trying to measure small fluxes of CH4 and N2O using chamber or micrometeorological methods and the need of including winter emissions in annual balances will be stressed.
At the vast majority of FLUXNET sites, the net ecosystem carbon dioxide (CO 2 ) exchange is evalu... more At the vast majority of FLUXNET sites, the net ecosystem carbon dioxide (CO 2 ) exchange is evaluated by means of the eddy covariance technique using a set of instruments on a single tower. This approach is assumed valid over flat, horizontally homogeneous terrain and stationary conditions. However, in complex terrain and during nighttime with low turbulent mixing and stable stratification, this assumption is likely to be violated resulting in an underestimation of the net CO 2 exchange. The daily net exchange of CO 2 is composed of net uptake during daytime, when plant photosynthesis exceeds respiration, and net emission of CO 2 during nighttime due to ecosystem respiration. The unaccounted presence of advective CO 2 fluxes thus may potentially bias net daily and longer-term flux measurements towards too much net uptake. We report on an advection experiment conducted during summer 2012 at a subalpine grassland site in Northern Italy (2160 m asl). The main flux tower at the site is on flat/slightly sloping terrain, however the surrounding area consists of a heterogeneous microtopography with slopes at various angles and expositions. Advection was quantified at several locations in the area along the main wind direction using measurements of horizontal and vertical CO 2 gradients and wind speed. Three flux towers were used to quantify the spatial variability in vertical eddy fluxes. An independent estimate of nighttime ecosystem respiration was obtained with a set of automated ecosystem chambers. Our main findings are that (i) despite the complex terrain, advection (both horizontal and vertical) plays a negligible role for the net ecosystem CO 2 exchange during daytime conditions, (ii) during nighttime, advection represents an appreciable and not negligible fraction (∼50%) of the total net ecosystem CO 2 exchange, and (iii) in this kind of complex terrain spatial variability in vertical eddy fluxes is large during nighttime with some spots catching only a small portion (∼40%) of the total ecosystem CO 2 efflux. Finally, the relationships between advection and micrometeorological factors such as wind speed, friction velocity (u*) and the stability parameter (z/L) are presented. Moreover, post-hoc approaches applied to correct for the fraction of the net ecosystem CO 2 exchange carried by advection are discussed.
Remote/proximate sensing of radiation reflected from the earth's surface ... more Remote/proximate sensing of radiation reflected from the earth's surface allows monitoring the `status' of the planet earth with reasonable spatial and temporal coverage. Here we explore the use of vegetation indices (VIs) calculated from reflectance in broad spectral regions for inferring ecosystem carbon dioxide (CO2) fluxes. To this end we use one year of concurrent measurements of reflectance in the
The quantification of greenhouse gas (GHG) budgets on a global scale is an important step in asse... more The quantification of greenhouse gas (GHG) budgets on a global scale is an important step in assessing the effect of anthropogenic and biogenic controls on a future climate. In the past, measurements of CO2 fluxes were conducted over a wide array of ecosystems, leading to a better understanding of its exchange patterns on different time scales and more sophisticated models. However, only few studies quantified the fluxes of the other two major GHG, methane (CH4) and nitrous oxide (N2O), mainly due to expensive sensors and their time-consuming maintenance. In addition, early CH4 and N2O measurements mainly focused on ecosystems with presumably high emissions of CH4 (e.g. wetlands) or N2O (e.g. heavily fertilized crops). In recent years, devices for CH4 and N2O measurements became widely available and more studies are conducted over sites that exert small and often close-to-zero fluxes. Despite recent advances in sensor sensitivity and stability, the quantification of CH4 and N2O exch...
Methane (CH4) and nitrous oxide (N2O) are major greenhouse gases and therefore play an important ... more Methane (CH4) and nitrous oxide (N2O) are major greenhouse gases and therefore play an important role in the atmosphere due to their warming potential. The various processes and mechanisms associated with the release and uptake of both compounds need further investigation to reliably assess CH4 and N2O budgets in different ecosystems. The exchange of CO2, CH4 and N2O was measured over 2 years at a temperate mountain grassland managed as a hay meadow near the village Neustift in the Stubai Valley, Austria. The three wind components, the speed of sound and the CO2mole densities were acquired at a time resolution of 20 Hz and used to calculate true eddy covariance CO2fluxes. CH4and N2O concentration values were recorded at 2 Hz, resulting in a disjunct time series when compared to the 20 Hz wind data. Fluxes of both compounds were then calculated using the virtual disjunct eddy covariance method (vDEC). The net ecosystem exchange of CO2 is monitored since 2001, while the measurement of...
In near future air temperature is expected to increase during the growing season and growing cond... more In near future air temperature is expected to increase during the growing season and growing conditions could become more arid in many parts of the world. Since the air temperature and the photosynthetically active radiation are the main drivers for the emission of many biogenic volatile organic compounds (BVOC), this may be expected to lead to a change in the emission patterns. To investigate the effects of drought on BVOC-emissions from mountain grassland we analysed the emission patterns during a whole vegetation period from March 2011 to December 2011. BVOC-concentrations were measured at a height of 2.4 m above the ground by means of a proton-transfer-reaction mass spectrometer and then combined with 3D wind data from a sonic anemometer. Half-hourly net BVOC fluxes (composed of grass and soil emissions) were calculated using the disjunct eddy covariance method. As a result of a dry period in May 2011 the soil water content dropped below 0.1 m3/m3 and methanol emissions increase...
Although the biosphere is currently thought to be the main source for atmospheric volatile organi... more Although the biosphere is currently thought to be the main source for atmospheric volatile organic compounds (VOCs) longer time series of VOC measurements are barely available and the accuracy VOC modeling approaches still suffers from a lack of flux measurement data. However, long-term VOC flux data sets could provide a way to improve the quality of modeling approaches and make a first step towards a better VOC quantification. We used VOC flux data of methanol, acetone, acetaldehyde and the monoterpenes, which were measured by means of a proton transfer reaction - mass spectrometer PTR-MS utilising the disjunct eddy covariance method (vDEC) above an intensively managed mountain grassland in Stubai Valley (Austria), to compare the performance of four different gap filling approaches and to get complete annual time series of the VOCs for the years 2009 and 2011. Assuming a zero flux from the grassland during the winter period, when the meadow is usually covered by snow, the average c...
When using the eddy covariance (EC) method for measuring the ecosystem–atmosphere exchange of sen... more When using the eddy covariance (EC) method for measuring the ecosystem–atmosphere exchange of sensible and latent heat, it is not uncommon to find that these two energy fluxes fall short of the available energy by 20 to 30%. As the causes for the energy imbalance are still under discussion, it is currently not clear how the energy balance should be closed. The objective of this study was to use independent measurements of evapotranspiration (ET) for empirically devising how to best close the energy balance. To this end, the ET of a temperate mountain grassland was quantified during two measurement campaigns using both an open- and a closed-path EC system, lysimeters, and an approach scaling up leaf-level stomatal conductance to canopy-level transpiration. Our study showed that both EC systems underestimated ET measured independently by lysimeters and the upscaling approach. The best correspondence with independently measured ET was achieved by assigning the entire energy imbalance t...
When using the eddy covariance (EC) method for measuring the ecosystem-atmosphere exchange of sen... more When using the eddy covariance (EC) method for measuring the ecosystem-atmosphere exchange of sensible and latent heat, it is not uncommon to find that these two energy fluxes fall short of available energy by 20-30 %. As the causes for the energy imbalance are still under discussion, it is currently not clear how the energy balance should be closed. The objective of the present paper is to use independent measurements of evapotranspiration (ET) for empirically devising on how to best close the energy balance. To this end ET of a temperate mountain grassland was quantified during two measurement campaigns using both an open- and a closed-path EC system, lysimeters and an approach scaling up leaf-level stomatal conductance to canopy level transpiration. Our study showed that both EC systems underestimated ET measured independently by lysimeters and the up-scaling approach. Best correspondence to independently measured ET was achieved by assigning the entire energy imbalance to ET and...
All-wave net surface radiation is greatly needed in various scientific research and applications.... more All-wave net surface radiation is greatly needed in various scientific research and applications. Satellite data have been used to estimate incident shortwave radiation, but hardly to estimate all-wave net radiation due to the inference of clouds on longwave radiation. A practical solution is to estimate all-wave net radiation empirically from shortwave radiation and other ancillary information. Since existing models were developed using a limited number of ground observations, a comprehensive evaluation of these models using a global network of representative measurements is urgently required. In this study, we developed a new day-time net radiation estimation model and evaluated it against seven commonly used existing models using radiation measurements obtained from 326 sites around the world from 1991 to 2010. MERRA re-analysis products from which the meteorological data were derived and remotely sensed products during the same period were also used. Model evaluations were performed in both global mode (all data were used to fit the models) and conditional mode (the data were divided into four subsets based on the surface albedo and vegetation index, and the models were fitted separately). Besides, the factors (i.e., albedo, air temperature, and NDVI) that may impact the estimation of all-wave net radiation were also extensively explored. Based on these evaluations, the fitting RMSE of the new developed model was approximately 40.0 Wm −2 in the global mode and varied between 18.2 and 54.0 Wm −2 in the conditional mode. We found that it is better to use net shortwave radiation (including surface albedo) than the incident shortwave radiation nearly in all models. Overall, the new model performed better than other existing linear models.
ABSTRACT This paper presents an improved algorithm for the retrieval of Leaf Area Index (LAI) fro... more ABSTRACT This paper presents an improved algorithm for the retrieval of Leaf Area Index (LAI) from Moderate Resolution Imaging Spectroradiometer (MODIS) satellite imagery that has been specifically customized for mountain grasslands in the Alps. The main features of the proposed algorithm, which is based on the inversion of a radiative transfer model, are: i) a higher spatial resolution (250 m) with respect to the corresponding standard MODIS product and ii) tuning the model to the spectral characteristics of mountain grasslands.
Remote/proximate sensing of radiation reflected from the earth's surface allows monitoring th... more Remote/proximate sensing of radiation reflected from the earth's surface allows monitoring the `status' of the planet earth with reasonable spatial and temporal coverage. Here we explore the use of vegetation indices (VIs) calculated from reflectance in broad spectral regions for inferring ecosystem carbon dioxide (CO2) fluxes. To this end we use one year of concurrent measurements of reflectance in the
The FLUXNET project [1] aims at quantifying the magnitude and controls on the CO2, H2O and energy... more The FLUXNET project [1] aims at quantifying the magnitude and controls on the CO2, H2O and energy exchange of terrestrial ecosystems. Ideally, the various biomes of the Earth would be sampled in proportion to their spatial extent - in reality, however, study site selection is usually based on other (more practical) criteria so that a bias exists towards certain biomes
Fluxes of CO2 and energy have been measured over several differently managed mountain grasslands ... more Fluxes of CO2 and energy have been measured over several differently managed mountain grasslands in the Austrian Alps over the past seven years. Here we discuss the challenges associated with the application of the eddy covariance method in complex mountainous terrain and show how these flux measurements can be validated using a multiple-constraints approach. It is shown that defensible eddy
Recent studies revealed convergent temperature sensitivity of ecosystem respiration (R e ) within... more Recent studies revealed convergent temperature sensitivity of ecosystem respiration (R e ) within aquatic ecosystems and between terrestrial and aquatic ecosystems. We do not know yet whether various terrestrial ecosystems have consistent or divergent temperature sensitivity. Here, we synthesized 163 eddy covariance flux sites across the world and examined the global variation of the apparent activation energy (Ea), which characterizes the apparent temperature sensitivity of R e , and its interannual variability (IAV) as well as their controlling factors.
Atmospheric Chemistry and Physics Discussions, 2015
ABSTRACT Methanol is the second most abundant volatile organic compound in the troposphere and pl... more ABSTRACT Methanol is the second most abundant volatile organic compound in the troposphere and plays a significant role in atmospheric chemistry. While there is consensus about the dominant role of living plants as the major source and the reaction with OH as the major sink of methanol, global methanol budgets diverge considerably in terms of source/sink estimates reflecting uncertainties in the approaches used to model, and the empirical data used to separately constrain these terms. Here we compiled micrometeorological methanol flux data from eight different study sites and reviewed the corresponding literature in order to provide a first cross-site synthesis of the terrestrial ecosystem-scale methanol exchange and present an independent data-driven view of the land-atmosphere methanol exchange. Our study shows that the controls of plant growth on the production, and thus the methanol emission magnitude, and stomatal conductance on the hourly methanol emission variability, established at the leaf level, hold across sites at the ecosystem-level. Unequivocal evidence for bi-directional methanol exchange at the ecosystem scale is presented. Deposition, which at some sites even exceeds methanol emissions, represents an emerging feature of ecosystem-scale measurements and is likely related to environmental factors favouring the formation of surface wetness. Methanol may adsorb to or dissolve in this surface water and eventually be chemically or biologically removed from it. Management activities in agriculture and forestry are shown to increase local methanol emission by orders of magnitude; they are however neglected at present in global budgets. While contemporary net land methanol budgets are overall consistent with the grand mean of the micrometeorological methanol flux measurements, we caution that the present approach of simulating methanol emission and deposition separately is prone to opposing systematic errors and does not allow taking full advantage of the rich information content of micrometeorological flux measurements.
When using the eddy covariance (EC) method for measuring the ecosystem-atmosphere exchange of sen... more When using the eddy covariance (EC) method for measuring the ecosystem-atmosphere exchange of sensible and latent heat, it is not uncommon to find that these two energy fluxes fall short of available energy by 20-30 %. As the causes for the energy imbalance are still under discussion, it is currently not clear how the energy balance should be closed. The objective of the present paper is to use independent measurements of evapotranspiration (ET) for empirically devising on how to best close the energy balance. To this end ET of a temperate mountain grassland was quantified during two measurement campaigns using both an open- and a closed-path EC system, lysimeters and an approach scaling up leaf-level stomatal conductance to canopy level transpiration. Our study showed that both EC systems underestimated ET measured independently by lysimeters and the up-scaling approach. Best correspondence to independently measured ET was achieved by assigning the entire energy imbalance to ET and...
Stomata are the major pathway by which plants exert control on the exchange of trace gases and wa... more Stomata are the major pathway by which plants exert control on the exchange of trace gases and water vapour with the aerial environment and thus provide a key link between the functioning of terrestrial ecosystems and the state and composition of the atmosphere. Understanding the nature of this control, i.e. how stomatal conductance differs between plant species and ecosystems and
Grassland ecosystems in a wider sense cover up to 40% of the global terrestrial surface (White et... more Grassland ecosystems in a wider sense cover up to 40% of the global terrestrial surface (White et al. 2000). Knowledge about the exchange of the major greenhouse gases (GHG's, carbon dioxide -CO 2 , methane -CH4 and nitrous oxide -N2O) remains still limited for grasslands, while other ecosystems such as forests and peatlands (particularly systems storing large amounts of carbon) have been investigated more intensively. Here, we give an overview of the current state of GHG measurements in the alpine region of Europe (Switzerland, Austria) and the associated challenges in deriving annual GHG budgets as well as determining the abiotic and biotic variables driving the fluxes of CO 2 , CH4 and N2O. In particular the importance of management activities, that is fertilization and removal of above-ground biomass through harvesting and grazing, besides climate, the challenges when trying to measure small fluxes of CH4 and N2O using chamber or micrometeorological methods and the need of including winter emissions in annual balances will be stressed.
At the vast majority of FLUXNET sites, the net ecosystem carbon dioxide (CO 2 ) exchange is evalu... more At the vast majority of FLUXNET sites, the net ecosystem carbon dioxide (CO 2 ) exchange is evaluated by means of the eddy covariance technique using a set of instruments on a single tower. This approach is assumed valid over flat, horizontally homogeneous terrain and stationary conditions. However, in complex terrain and during nighttime with low turbulent mixing and stable stratification, this assumption is likely to be violated resulting in an underestimation of the net CO 2 exchange. The daily net exchange of CO 2 is composed of net uptake during daytime, when plant photosynthesis exceeds respiration, and net emission of CO 2 during nighttime due to ecosystem respiration. The unaccounted presence of advective CO 2 fluxes thus may potentially bias net daily and longer-term flux measurements towards too much net uptake. We report on an advection experiment conducted during summer 2012 at a subalpine grassland site in Northern Italy (2160 m asl). The main flux tower at the site is on flat/slightly sloping terrain, however the surrounding area consists of a heterogeneous microtopography with slopes at various angles and expositions. Advection was quantified at several locations in the area along the main wind direction using measurements of horizontal and vertical CO 2 gradients and wind speed. Three flux towers were used to quantify the spatial variability in vertical eddy fluxes. An independent estimate of nighttime ecosystem respiration was obtained with a set of automated ecosystem chambers. Our main findings are that (i) despite the complex terrain, advection (both horizontal and vertical) plays a negligible role for the net ecosystem CO 2 exchange during daytime conditions, (ii) during nighttime, advection represents an appreciable and not negligible fraction (∼50%) of the total net ecosystem CO 2 exchange, and (iii) in this kind of complex terrain spatial variability in vertical eddy fluxes is large during nighttime with some spots catching only a small portion (∼40%) of the total ecosystem CO 2 efflux. Finally, the relationships between advection and micrometeorological factors such as wind speed, friction velocity (u*) and the stability parameter (z/L) are presented. Moreover, post-hoc approaches applied to correct for the fraction of the net ecosystem CO 2 exchange carried by advection are discussed.
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Papers by Georg Wohlfahrt