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Heat waves are predicted to increase in frequency and intensity, endangering the productivity of agroecosystems including grapevines. In this work, we analyzed the effects of the long and intense heat wave (HW) occurring in July 2022 on the eco-physiological performance of a vineyard located in northern Italy (Caldaro, Province of Bolzano). The vineyard hosts two white grape cultivars (Sauvignon Blanc and Chardonnay on SO4 rootstock) with an average planting density of 6,500 vines ha-1 and is equipped with a drip irrigation system. The interrow alleys are covered by grasses or cover crops. Summer 2022 showed a continuous daily temperature increase from the second week of July and peaked in a heat wave lasting 8 days (DOYs 196-203) with maximal Tmax= 38 and average Tmax= 36°C. The HW period was then interrupted by summer storms and rain. The methodology included continuous monitoring of NEE (-NEP), Reco, GPP, ET and energy fluxes by eddy covariance method. Continuous measurements at the plant scale included sun-induced chlorophyll fluorescence, active chlorophyll fluorescence, and sap-flow rates. Periodically, leaf gas exchange, leaf water potential and chlorophyll content were also recorded. The period considered for the analysis stretched from July 1 to August 11.There was a general mild decreasing trend of GPP when the air temperature increased in July, reaching the minimum values during the HW (< 10 g C m-2d-1). Leaf photosynthesis also slightly decreased during the HW in both varieties down to values of 6-7 µmol CO2 m-2s-1. Interestingly, even during the HW, GPP always markedly increased the day after irrigation water was supplied (5 irrigation events were performed in July with a total of approximately 45 L/vine).Reco decreased when the air temperature increased, but a few days before the end of the HW it increased again, a process that was even more pronounced following the rainy period after the HW when negative values of NEP were recorded.Vineyard ET and vine transpiration did not show a clear response to the HW, while they both increased each time vines were irrigated, suggesting that most ET in that period derived from vine transpiration and not from the vineyard alleys. Interestingly, after the end of the HW, vineyard ET did not increase despite the increased soil moisture occurring also in the alleys. The energy partitioning was unaffected by the HW, with the Bowen ratio dropping below 0.4 after the irrigations. Continuous monitoring of active chlorophyll fluorescence showed a slight increase in NPQ parameter (measured only on Chardonnay leaves) in the last and hotter days of the HW, while Fv/Fm was not affected by the higher temperatures.In conclusion, despite the exceptional (for the cultivation area) intensity of the 2022 HW, the vines showed good tolerance to heat stress. We speculate that irrigation played an important role in the vines’ performance. The NEP, however, decreased during the HW and especially after its end due to soil-moisture triggered increasing Reco, which shifted the vineyard from sink to source of atmospheric CO2.

Forests cover roughly one third of the global land area and currently remove around one quarter of anthropogenic CO2 emissions, thus slowing down the increase in atmospheric CO2 concentrations. Forests account for approximately 90% of all terrestrial biomass, which corresponds to about 400 Gt C.However, the ratio of carbon uptake to release is a very fragile one and is determined by many factors such as water and nutrient availability, the amount and quality of light or stand age. In addition to the factors mentioned above, temperature is one of the most important factors that determine this ratio. Both fluxes determining the net ecosystem exchange (NEE), the gross uptake of CO2 by photosynthesis (GPP) and ecosystem respiration (Reco) are sensitive to temperature. Thus, we investigated the temperature sensitivity of NEE at a newly established pine forest field site in Austria at 960 m a.s.l. Applying an understory and an ecosystem scale eddy covariance system we were able to disentangle temperature effects on understory and tree crown CO2 exchange.We found a clear temperature optimum for CO2 uptake on ecosystem scale at around 20°C and a decrease in uptake on higher temperatures. This decrease was caused by (i) the understory turning from a slight sink for CO2 into a source of CO2 at higher temperatures, and (ii) a reduction of CO2 uptake in the tree canopy layer. Furthermore, we compared carbon flux data with continuous tree growth data from dendrometer measurements.

Forests play an important role in the exchange of greenhouse gases like CO2 and CH4 between the biosphere and the atmosphere. Although fluxes vary depending on biotic and abiotic factors like ecosystem composition and climatic conditions, forests generally are a sink for CO2. For CH4, forests with well-drained soils are considered a sink due to the oxidation of CH4 within those soils. However, recent study results indicate that CH4 emissions from trees may offset this soil CH4 sink, resulting in a lower sink strength or even source at the ecosystem level.To quantify the net CH4 ecosystem exchange of a temperate needleleaf forest, we started CH4 flux measurements above a Scots Pine forest at the ‘FAIR’ research site in Austria in August 2022 using the eddy covariance method. The results show very small fluxes with nighttime fluxes close to zero while fluxes during the day are more variable. Overall, the site is a very small sink for CH4 at most. This site is temporally snow-covered during winter. As snowmelt increases soil moisture, we hypothesize that snowmelt in spring may further reduce the ecosystem CH4 sink strength. Here, besides presenting flux results, we will also discuss methodical challenges of measuring very small fluxes.

In order to estimate the gross primary productivity (GPP) of terrestrial ecosystems from the canopy uptake of carbonyl sulfide (COS), leaf relative uptake rate (LRU) of COS with respect to carbon dioxide needs to be known a priori. Currently, the variability of LRU between plant species in different biomes of the world is poorly understood, making the choice of an appropriate LRU uncertain and hampering further progress towards developing COS as a tracer of GPP. Here we propose a novel approach for estimating light-saturated LRU based on plant optimality principles, validate it against in situ leaf gas exchange measurements and provide global monthly climatological estimates. The global vegetation season average simulated LRUs fall into the 95 % range of 0.68–1.58 and are thus lower than most other published global estimates. We advocate for these LRU estimates to be adopted by global modellers in order to test to what degree these are compatible with our current understanding of the sources and sinks in the global COS budget.

In search for a constraint on the gross primary productivity (GPP) on ecosystem level, the sulfur containing gas carbonyl sulfide (COS) shows great promise. COS takes a very similar route into the leaf as carbon dioxide (CO2), that is through the leaf boundary layer, the stomata and the mesophyll until COS is fully catalyzed in a one-way reaction by the enzyme carbonic anhydrase (CA), whereas CO2 reaches its endpoint at RuBisCO within the chloroplast stroma. For the calculation of the GPP based on COS fluxes, a beforehand knowledge about the ratio of the deposition velocities of COS to CO2, also called leaf relative uptake (LRU), is needed. Differences in the LRU between plants and under different environmental conditions might hinder a straightforward usage of this approach.To investigate the LRU and its dependencies on the involved conductances, we conducted eddy covariance measurements of COS, CO2 and H2O at an agricultural field in Ariis, Italy. At this field, 2 different varieties of soybean were planted in adjacent plots. One was the commercial variety Eiko, to which we will refer to as the green “wild type” (WT). The other was a chlorophyll deficient golden/yellow variety called MinnGold (MG). Due to a lack of rain, all plots were irrigated 2 and 3 weeks into our 4 week campaign.Despite having a reduced chlorophyll content, MG was able to match and even exceeded the rate of photosynthesis of the WT during our observation period. While the GPP was similar for both varieties during the first week, we observed a higher decline for WT in week 2 due to a naturally occurring drought. Even after the irrigation of both plots, the GPP of MG recovered faster. We also observed considerably higher COS uptake by MG during the whole campaign. The resulting LRU under high light conditions was also consequently higher for MG (1.41) than for WT (0.97).We calculated the aerodynamic, boundary layer, stomatal and internal conductance for both varieties and grouped the values into 4 phases: pre-drought, drought, rewetting and recovery. Based on these values and a linear perturbation analysis, we identified the internal conductance as the largest driver for the different LRUs between the two varieties.Our results indicate that the stomatal conductance is not the only controlling factor for the LRU and that the mesophyll conductance can’t be neglected. We also show, the LRU response to drought differs between plants, even at the level of varieties.

The Alps are experiencing large climatic and socio-economic changes. Climate change is leading to an above-average increase in temperatures and subsequent changes in the timing and duration of snow cover. In parallel, socio-economic changes are affecting land use in the Alpine region. Both, snow cover duration/timing and land use changes directly affect the surface albedo of this landscape and therefore the energy balance of this region. Globally, changes in surface albedo due to land use changes and changes in snow/ice cover affect surface albedo, and thus radiative forcing, in opposite directions.In this study, we investigated the impact of four different future land use scenarios, 12 future snow cover scenarios on the surface albedo in the alpine region of South Tyrol (Italy) in the year 2100 compared to conditions in 2010. Both, the individual effects of changes in land use and future snow cover patterns were investigated, as well as the interactive effects of these two processes.The hypothetical changes in albedo until 2100 associated with changes in land and/or snow cover were assessed by establishing a surface albedo model based on remotely sensed albedo (MODIS MCD43A1), snow cover data (MODIS MOD10A1), land cover data, as well as geographical information (ASTER ASTGTM). Potential future land covers were developed on the basis of likely socio-economic pathways and their spatial distribution was mapped. Snow cover scenarios for 2100 are based on EURO CORDEX RCP 2.6 and 8.5 climate scenarios.Snow cover was by far the most important predictor for albedo, followed by the occurrence of needle leaf forests using a regression tree algorithm. This algorithm exhibited excellent skill in modelling current albedo conditions based on the above-mentioned predictors.Likely future snow cover conditions lead to a decrease in average albedo, the magnitude of which depended on the chosen RCP and combination of global/regional climate model. Likely future land cover scenarios caused changes in spatially averaged albedo of the study domain in the same order of magnitude like the RCP 2.6 snow cover scenarios. Simulations with factorial combinations of land cover and snow cover scenarios showed the compounding effect of these two processes.

Solar energy absorbed by chlorophyll molecules of plants is either used for the carboxylation of CO2 (i.e. photosynthesis), dissipated (in a regulated and unregulated fashion) as heat or re-emitted at higher wavelengths as fluorescence. Proximal or remote sensing measurements of the so-called sun-induced chlorophyll fluorescence (SIF) are thus thought to offer a non-obtrusive approach for quantifying a key ecosystem carbon cycle component and metric of plant health, gross primary productivity (GPP). In contrast to SIF, which is a relatively new approach, active measurements of chlorophyll fluorescence using the pulse amplitude modulation (PAM) technology represent a well-established standard approach. Up to date, few studies have combined active and passive (i.e. SIF) chlorophyll fluorescence measurements and the link between active chlorophyll fluorescence metrics and SIF is thus poorly understood, hampering progress in the physiological interpretation of the SIF signal.The overarching goal of this study is to improve our physiological understanding of the SIF signal. To this end we jointly quantified ecosystem-scale passive (i.e. SIF) and leaf-scale active chlorophyll fluorescence. Measurements were conducted during the vegetation period (March to November 2021) at a mountain Scots Pine (Pinus sylvestris L.) forest close to the village of Obermieming (Austria). Two sets of automated PAM fluorometers (MoniPAM, Walz, Germany) were installed from a walk-up tower to measure active leaf-scale chlorophyll fluorescence (ChlF) on branches in the sunlit top of the canopy and in the more shaded sub-canopy zone of the forest. Additionally, SIF at the canopy scale was measured via a high-resolution spectrometer system (FloX, JB Hyperspectral Devices, Germany) and the eddy covariance method was used to determine net ecosystem CO2 exchange.Preliminary results show an increase of SIF yield (SIF/aPAR) with warming temperature throughout summer followed by a steady decrease starting in autumn. A major heatwave event in June resulted in a decrease of the SIF and PSII yields, leading to an increase in NPQ. An overall decrease of PSII efficiency for shaded leaves compared to sun-exposed ones was detected, with significant reductions throughout decreasingly colder days in spring and autumn for both groups. Shaded leaves responded with an overall higher investment into NPQ mechanisms. In contrast, an increased yield of fluorescence and constitutive thermal energy dissipation (f,D) in sun-exposed leaves could indicate higher heat dissipation for the exposed group.Further analyses will use active chlorophyll fluorescence metrics for studying seasonal variability in SIF and SIF yield and their relationship to ecosystem-scale GPP.

Partitioning the measured net ecosystem carbon dioxide (CO2) exchange into gross primary productivity (GPP) and ecosystem respiration remains a challenge, which is usually tackled by disentangling the net ecosystem CO2 exchange using various methods. A relatively new approach uses the trace gas carbonyl sulfide (COS) to estimate GPP. This is possible because of the very similar pathways CO2 and COS take into and within leaves, allowing researchers to use COS uptake as a proxy for the CO2 uptake in plants. In order to assess the viability of COS as a GPP proxy, COS sources and sinks in ecosystems have to be quantified. One of the biggest unknowns in this regard is the contribution of the soil.In our study we looked at the effects of live roots on the soil COS-exchange, a topic that has not yet been explored in the literature. While in the last couple of years different working groups measured soil samples in the lab, no study to date looked at the impact of live roots on the soil COS flux. We hypothesized that live roots will change the COS flux by changing microbial community composition and activity via root exudates. In order to investigate the root contribution of a live plant we had to build an experimental setup that would allow us to only measure the belowground plant parts and the soil, while at the same time keep the whole plant alive. The plants used in this study were young beech trees (~2 years) and the soil was commonly used potting soil, in order to ensure a mostly homogeneous substrate for the trees. The measurements were spread out over one year to cover the different phenological stages of the trees, from no leaves in winter to new and mature leaves in spring and summer, respectively, to senescent leaves in autumn. Growth lamps were used to supply the aboveground parts of the plants with light during the day.Most pots, with and without plants, emitted COS during the course of the experiment. COS and CO2 emissions increased in pots with roots compared to the control pots, but the increase in CO2 emissions was much stronger compared to the increase in the COS flux, which lead to consistently higher COS/CO2 emission ratios in the control pots, which contained potting soil only. A diurnal pattern was visible in all the measurements with the largest emissions for COS and CO2 occurring in the afternoon, when soil temperatures were the highest. Comparing the measurements over the whole experiment a clear difference in the COS/CO2 ratio could be observed between the measurements without leaves in February compared to the measurements with leaves in summer and autumn, indicating a dynamic effect of live roots on the soil COS exchange.

When it comes to monitoring the health status of ecosystems, satellite-based remote sensing approaches hit a sweet spot in terms of global spatial coverage and temporal resolution. Conventional optical remote sensing approaches, however, offer limited potential for the early detection of ecosystem stress, as changes in ecosystem structure and function often need to be substantial in order to be detectable from the reflectance in the visible and near-infrared range of the energy spectrum.Satellite-based remote sensing of sun-induced chlorophyll fluorescence (SIF) offers much greater potential to that end. Chlorophyll fluorescence is generated when solar energy absorbed by chlorophyll inside plant leaves is not used for photosynthesis or dissipated as heat, but is instead emitted at a slightly higher wavelength. Chlorophyll fluorescence thus results from fine-tuned changes in chlorophyll energy partitioning and SIF provides a highly sensitive optical signal, which allows the early detection of plant stress before symptoms become apparent in classical optical remote sensing indices.Our previous work, however, has shown that in order to correctly diagnose whether or not plants are exposed to stress, SIF needs to be quantified jointly with the energy that is dissipated as heat and that this process can be accurately quantified on the basis of reflectance changes around the green peak, exploited by the so-called photochemical reflectance index (PRI). SIF data have become available from a few satellite platforms during the past couple of years, however their spatio-temporal resolution and signal-to-noise ratio is still unsatisfactory. A major step forward in data quality is expected from the upcoming ESA Earth Explorer mission FLEX, scheduled to launch in mid-2023.The overarching goal of the AustroSIF project is to make present and future satellite-based sun-induced chlorophyll fluorescence measurements a sensitive and reliable means for the early detection of ecosystem stress by combining remotely sensed SIF and PRI. To that end we propose to simulate satellite measurements using ground-based, proximal sensing of active and passive chlorophyll fluorescence and hyperspectral reflectance. These measurements will be conducted in the field covering a wide range of ecosystems typical for Austria. Available satellite products will be used to test this approach at larger spatial and temporal scales. Process-based models will be used to disentangle the underlying drivers.This contribution will detail the project structure and approach and provide first, preliminary results obtained during the first project year.

Climate change is expected to increase the frequency and intensity of extreme weather events like heat waves (HW), with potential negative impacts on grapevine productivity and wine quality in several regions. High temperatures during heat waves are usually accompanied by decreases in soil moisture, but few studies have explored the single and combined effects of temperature and drought on grapevine physiology. Using fully controlled environmental chambers we simulated a 6-day heat wave. Each chamber was assigned to one temperature treatment (control or heat). Inside each chamber, four grapevine plants (cv. Sauvignon b. on SO4 rootstock) were placed on weighing lysimeters. Half of the plants in each chamber were well-watered (watered plants) and the other half were subjected to drought (dry plants). Radiation intensity and air temperature mimicked average summer conditions near Bolzano (Italy). In the control chambers, the maximum daily temperature (Tmax) was 30°C for the entire period. In the heat chambers, Tmax reached 40° C on day 6, then it decreased on day 12 to 30° C. Instantaneous leaf photosynthesis (Pn) as well as chlorophyll fluorescence parameters (Fv/Fm, Y(NPQ), qN, qP and qL) were measured manually in the late morning (10:00-12:00) and in the afternoon (15:00-17:00), together with stem water potential (SWP; 15:00-17:00). Plant transpiration (T), soil water potential and leaf temperature were continuously monitored throughout the experiment. During the HW, well-watered plants showed a marked reduction of Pn from the morning to the afternoon, which was not visible in the vines under control temperature. Drought significantly reduced Pn and, when combined with the heat stress, further decreased Pn with respect to the control temperature, both in the morning and in the afternoon. Daily T of watered plants during the HW was about 50% higher than T under the control temperature, while the HW had no effect on T of dry plants. Regardless of the chamber temperature, progressive drought caused stomatal closure, which in turn prevented stem water potential from reaching low levels (all SWP values were >-1.45 MPa). This, in turn, caused an increase of canopy temperature, which in the heated chambers reached a peak of around 45°C (up to 5°C warmer than in the watered plants). The heat stress decreased Fv/Fm both in watered and in dry plants, an effect that was stronger in the afternoon, but also occurred in the morning if the exposure to heat stress was prolonged. Other fluorescence parameters, like Y(NPQ) and qN, were affected by heat stress, while qP and qL were affected mainly by drought. At the end of the HW, chlorophyll fluorescence parameters recovered in watered plants, but not in the dry ones. Grapevines of the cv. Sauvignon b. were able to cope with a 6-day heat wave provided enough soil water was available to support the significantly enhanced transpiration rate. Drought had adverse effects on Pn regardless of the air temperature, but the drought-stressed plants that were also exposed to the heat wave experienced excess canopy temperatures. Leaf chlorophyll fluorescence proved to be a reliable indicator of heat stress.

Gross primary productivity (GPP), the CO2 uptake by means of photosynthesis, cannot be measured directly on ecosystem scale, but has to be inferred from proxies or models. One newly emerged proxy is the trace gas carbonyl sulfide (COS). COS diffuses into plant leaves in a fashion very similar to CO2, but is generally not emitted by plants. Laboratory studies on leaf level gas exchange have shown promising correlations between the leaf relative uptake (LRU) of COS to CO2 under controlled conditions. However, in situ measurements including daily to seasonal environmental changes are required, to test the applicability of COS as a tracer for GPP at larger temporal scales. To this end, we conducted concurrent ecosystem scale CO2 and COS flux measurements above an agriculturally managed temperate mountain grassland. We also determined the magnitude and variability of the soil COS exchange, which can affect the LRU on ecosystem level. The cutting and removal of the grass at the site had a major influence on the soil as well as the total exchange of COS. The grassland acted as a major sink for CO2 and COS during periods of high leaf area. The sink strength decreased after the cuts and the grassland turned into a net source for CO2 and COS on ecosystem level. The soil acted as a small sink for COS when the canopy was undisturbed, but also turned into a source after the cuts, which we linked to higher incident radiation hitting the soil surface. However, the soil contribution was not large enough to explain the COS emission on ecosystem level, hinting to an unknown COS source possibly related to dead plant matter degradation. Over the course of the season, we observed a concurrent decrease of CO2 and COS uptake on ecosystem level. With the exception of the short periods after the cuts, the LRU under high light conditions was rather stable and indicates a high correlation between the COS flux and GPP across the growing season.

The Innsbruck Atmospheric Observatory (IAO) aims to investigate atmospheric chemistry, micrometeorology, and mountain meteorology in a synergistic fashion within an urban setting. A new measurement supersite has been established in order to study processes affecting the exchange of momentum, energy, trace gases, and aerosols in an Alpine urban environment. Various long-term continuous measurements are augmented by frequent focused research campaigns with state-of-the-art instrumentation, linking different classes of data and addressing significant gaps in scientific data availability for urban environments. Current activities seek to address research objectives related to the urban heat island, trace gas emissions, the influence of foehn on air quality, and the atmospheric distribution of trace gases and aerosols in a mountainous city. We present initial results from long-term operations and first highlights from two intensive operational phases, showing that 1) the exchange of greenhouse gas emissions is dominated by anthropogenic activities and is driven by location-specific venting of street canyon air; 2) foehn events significantly perturb the photostationary state indicative for an extensive and rapid airmass exchange of the valley atmosphere; 3) the temporal distribution of pollutants is often decoupled from their emissions and primarily modulated by mountain boundary layer dynamics; 4) we can detect a large number of volatile chemical products in the urban atmosphere, which can be used to fingerprint anthropogenic emission sources; and 5) the first urban carbonyl sulfide (COS) flux measurements point toward anthropogenic emission sources.

The FLUXNET2015 dataset provides ecosystem-scale data on CO2, water, and energy exchange between the biosphere and the atmosphere, and other meteorological and biological measurements, from 212 sites around the globe (over 1500 site-years, up to and including year 2014). These sites, independently managed and operated, voluntarily contributed their data to create global datasets. Data were quality controlled and processed using uniform methods, to improve consistency and intercomparability across sites. The dataset is already being used in a number of applications, including ecophysiology studies, remote sensing studies, and development of ecosystem and Earth system models. FLUXNET2015 includes derived-data products, such as gap-flled time series, ecosystem respiration and photosynthetic uptake estimates, estimation of uncertainties, and metadata about the measurements, presented for the frst time in this paper. In addition, 206 of these sites are for the frst time distributed under a Creative Commons (CC-BY 4.0) license. This paper details this enhanced dataset and the processing methods, now made available as open-source codes, making the dataset more accessible, transparent, and reproducible., European Union (EU), United States Department of Energy (DOE)

Hydropower emits less carbon dioxide than fossil fuels but the lower albedo of hydropower reservoirs compared to terrestrial landscapes results in a positive radiative forcing, offsetting some of the negative radiative forcing of hydroelectricity generat ion. The cumulative effect of this lower albedo has not been quantified. Here we show, by quantifying the difference in remotely sensed albedo between globally distributed hydropower reservoirs and their surrounding landscape, that 19% of all investigated hydropower plants required 40 years or more for the negative radiative forcing from the fossil fuel displacement to offset the albedo effect. The length of these break-even times depends on the specific combination of climatic and environmental constraints, power plant design characteristics and country-specific electricity carbon intensities. We conclude that future hydropower plants need to minimize the albedo penalty to make a meaningful contribution towards limiting global warming. The darker surfaces of water reservoirs absorb more sunlight than ground surfaces, yet how much this reduces the climate benefit of hydroelectricity has not been investigated. Now, Wohlfahrt et al. demonstrate that this albedo penalty may be considerable for certain hydropower reservoirs.

Lakes receive large amounts of carbon (C) from the surrounding catchment and, together with the connecting streams, play an important and active role in the global C cycle. The received C can either be lost through the outflow and eventually transported to the ocean, or transformed and stored in sediments or outgassed to the atmosphere. Globally, lakes are estimated to emit 0.3 – 0.64 Pg C m-2 in form of CO2 annually. Although subalpine and alpine lakes were observed to be supersaturated with CO2, long-term measurements of lake-atmosphere CO2 exchange are sparse. Several methods to quantify water-atmosphere gas exchange exist, like chambers, eddy covariance (EC), mass-balance or gradient based methods including boundary layer models (BLM), each having its own advantages and disadvantages. However, quantifying CO2 exchange in aquatic ecosystems has often proved to be challenging. Here, both the BLM and the EC methods were used to estimate the air-water CO2 exchange of Lake Lunz, a small lake situated in complex mountainous topography of the Austrian Alps. The results indicated that the lake was a small source of CO2. Fluxes were affected by the thermo-topographic flow regime of the field site and its surroundings which drove the local wind pattern but also determined the local atmospheric CO2 concentration. During most nights, a significant increase in atmospheric CO2 was observed which decreased the differential CO2 concentration at the air-water interface and therefore led to decreased nocturnal CO2 efflux. This diurnal pattern, however, was obscured in the EC measurements, because the method itself highly depends on the local wind regime. Because lakes are an integral part of mountain ranges which are characterized by catchments with complex topography, our findings are most likely of broader impact.

Ongoing changes in climate (both in the means and the extremes) are increasingly challenging grapevine production in the province of South Tyrol (Italy). Here we ask the question whether sun-induced chlorophyll fluorescence (SIF) observed remotely from space can detect early warning signs of stress in grapevine and thus help guide mitigation measures.Chlorophyll fluorescence refers to light absorbed by chlorophyll molecules that is re-emitted in the red to far-red wavelength region. Previous research at leaf and canopy scale indicated that SIF correlates with the plant photosynthetic uptake of carbon dioxide as it competes for the same energy pool.To address this question, we use time series of two down-scaled SIF products (GOME-2 and OCO-2, 2007/14-2018) as well as the original OCO-2 data (2014-2019). As a benchmark, we use several vegetation indices related to canopy greenness, as well as a novel near-infrared radiation-based vegetation index (2000-2019). Meteorological data fields are used to explore possible weather-related causes for observed deviations in remote sensing data. Regional DOC grapevine census data (2000-2019) are used as a reference for the analyses.

Solar radiation absorbed by chlorophyll in plants is either used for photosynthesis, dissipated as heat or is re-emitted as fluorescence at a slightly higher wavelength. Sun induced chlorophyll fluorescence (SIF) has thus the potential to act as a sensitive indicator for early stress detection in ecosystems. SIF signals at the top of the canopy are however influenced by canopy structure and not only by plant physiology, due to light scattering and (re)absorption.In this study we present the first results of a mesocosm experiment on the effect of drought stress on chlorophyll fluorescence in two plant stands differing in their canopy structure. In total we investigated 24 plots of planophile (Trifolium repens) and erectophile (Lolium perenne) plant stands. 12 plots acted as a control plots, while the remaining 12 plots underwent a progressively intensifying drought stress treatment. During the course of the experiment regular measurements of SIF using a passive spectrometer system were conducted. Furthermore, active chlorophyll fluorescence measurements with a multiplexed field spectrometer system were used to derive the maximum PSII efficiency (Fvm) and non-photochemical quenching (NPQ). Ancillary measurements included meteorological, leaf physiological, soil water and canopy structure variables.The drought treatment led to a relatively stronger decrease in NDVI and a relatively higher increase in PRI in the Trifolium repens stand, which also experienced a more pronounced increase in NPQ, especially during hot days. For both stands surface temperatures were clearly higher in the treatment groups, with a larger effect in the Trifolium stand. SIF-yield (SIF/aPAR) did remain more or less constant or increased slightly for the control groups. In both stands it dropped towards the end of the experiment for both treatment groups at very low soil water content levels, about at the same time when the active chlorophyll fluorescence measurements started to indicate persistent reductions in Fvm. The relative decrease in SIF and SIF-yield for the treatment groups was not significantly different between the two stands.

Solar-induced chlorophyll fluorescence (SIF) has been shown as a promising approach for the estimation of gross primary productivity (GPP), but whether SIF is merely a function of canopy structure or also contains precious physiological information, is presently heavily discussed. In this study, the SIF-GPP relationship was quantified at a Pinus sylvestris forest (Mezyk, Poland) during a series of short-term cold spells throughout the spring awakening to investigate the potential of SIF as a proxy for GPP during this period characterized by cold stress. GPP was inferred from the net ecosystem CO2 exchange measured by the eddy covariance technique. Canopy-scale SIF was measured using a high-resolution spectrometer system and retrieved via spectral fitting (SFM) algorithms. Active leaf-scale chlorophyll fluorescence measurements were conducted on seven branches using an automated field-deployable fluorometer system. Our results demonstrate a clear difference in GPP and the utilization of chlorophyll-absorbed energy between cold spell and warm days. At short, sub-daily time scales, the correlation between SIF and GPP was minor, but increased significantly when observed over extended temporal periods, when SIF exhibited a seasonal pattern that was more closely aligned with the GPP. Furthermore, the strong relationship between non-photochemical quenching (NPQ) and the photochemical reflectance index (PRI) shows good potential to better estimate GPP when integrated in the SIF-GPP model, as the integration of PRI overall increased the relation between SIF and GPP.

Limiting global warming to less than 2°C relative to preindustrial times by the end of this century requires a rapid and long-lasting decarbonization. In contrast to the other major renewable energy sources, solar and wind, hydropower reservoirs allow storing energy and releasing it when required, a significant advantage for stabilizing electrical grids. The establishment of hydropower reservoirs typically involves a land-use change when formerly terrestrial ecosystems are inundated. One, hitherto overlooked, consequence of this land-use change is a decrease in surface albedo, as waterbodies are characterized by a lower albedo compared to most terrestrial ecosystems. The main objective of this study is to quantify the positive radiative forcing resulting from this albedo change and to oppose it with the negative radiative forcing resulting from the fossil fuel displacement by the hydropower electricity generation. To that end, we compiled, on the basis of publicly available datasets, a global database of hydropower reservoirs. The hypothetical change in albedo associated with their construction was assessed on the basis of the difference in remotely sensed albedo (MODIS MCD43A1) between the hydropower reservoir and the surrounding landscape. We then calculated the break-even point, that is the time required for the time-integrated negative radiative resulting from the fossil fuel displacement to offset the positive radiative forcing from the albedo difference. The major result from this study is that break-even times range from less than a year up to several years and even a few decades. The key metric governing these differences is the annual electricity generation to reservoir surface area ratio, low ratios resulting in unfavorably long break-even times. Additional influence factors having a modulating influence are latitude, governing the incident solar radiation, and the magnitude of the albedo difference. We conclude that the displacement of fossil fuels by hydropower wins over the albedo penalty in the long-term. In the short-term, and thus for contributing towards the goal of a rapid decarbonization, the albedo penalty may be dominating and needs to be considered in the design of hydropower plants.

The gross uptake of CO2 on ecosystem level (GPP) can’t be measured directly, but has to be inferred from models or proxies. One of the newly emerged constrains on GPP is the trace gas carbonyl sulfide (COS). COS enters the plant leaf through the stomata and diffuses through the intercellular space, the cell wall, the plasma membrane and the cytosol like CO2. Within the cytosol, it is then catalyzed by the enzyme carbonic anhydrase (CA) in a one-way reaction to H2S and CO2. Basically, this one way flux would make COS a very promising tracer for GPP on ecosystem level, but there is growing evidence that plants are also capable of emitting COS. Mosses and even vascular plants that are under high stress like drought and fungal infection, have been reported to emit COS. Furthermore, a winter wheat field, that showed a good correlation between the CO2 and COS ecosystem fluxes during the peak growing phase turned into a source for COS after going into senescence. This indicates that yet unknown COS emission processes likely related to plant degradation, could complicate the use of COS as a tracer for GPP.Since the majority of studies have focused on measuring COS ecosystem fluxes during peak growing times or on evergreen forests, we seek to quantify the relationship between the ecosystem-scale exchange of CO2 and COS of an ecosystem going into senescence.Between September and November 2019 we deployed our quantum cascade laser (Aerodyne Research Inc., MA, USA) at a beech forest in Leinefelde, Germany to conduct eddy covariance measurements for COS, CO2 and H2O. Our observations started when the beech forest was still green and in full leaf and ended when most of the trees had already shed their leaves. The ecosystem fluxes of COS and CO2 concurrently decreased over the course of our campaign up to the point when we could not observe a net uptake of CO2 anymore. We will further compare the GPP estimates resulting from classical flux partitioning and flux partitioning with the additional use of COS to determine if the model differences increase towards the end of the season.

Quantitative understanding and monitoring of gross primary productivity (GPP) and its response to environmental variables is critical for understanding the feedbacks of ecosystems to the changing climate and projecting the future climate state.Due to limitations of the eddy covariance (EC) method related to the restricted spatial coverage obtained with the method, as well as drawbacks of the so-called CO2 flux partitioning approaches, adding scale-appropriate extra-information on canopy physiological status and flux partitioning is crucial for constraining gross photosynthesis (GPP), also beyond the ecosystem scale.Here, we present the outcome of the H2020-MSCA-IF COSIF project aiming at investigating the potential of two novel GPP traces, i.e. carbonyl sulfide (COS) and sun-induced fluorescence (SIF) for inferring GPP.The major result of the presented study are three independent GPP data sets obtained with different methods of contrasting theoretical backgrounds (CO2 flux partitioning, COS and SIF) in a temperate mountain grassland in Neustift, Austria (AT-Neu). Moreover, the study compares empirical approaches with a process-based estimates obtained using the “Soil-Canopy Observation Photosynthesis and Energy fluxes” (SCOPE) model, updated with a soil and leaf COS exchange module. The obtained results foster the use of repeated hyperspectral remote sensing observations together with radiative transfer and biochemical models for carbon assimilation monitoring.

The potential of carbonyl sulfide (COS) flux measurements as an additional constraint for estimating the gross primary production (GPP) depends, among other preconditions, on our understanding of the soil COS exchange and its contribution to the overall net ecosystem COS flux. We conducted soil chamber measurements of COS, with transparent chambers, in three different ecosystems across Europe. The in situ measurements were followed by laboratory measurements of soil samples collected at the study sites. The soil samples were exposed to UV radiation to investigate the role of photo‐degradation for COS exchange. In situ and laboratory measurements revealed pronounced inter‐ and intra‐site variability of COS exchange. In situ COS fluxes were primarily governed by radiation in the savannah‐like grassland (SAV), soil temperature and intra‐site heterogeneity in the deciduous broadleaf forest (DBF) and soil water content (SWC) and intra‐site heterogeneity in the evergreen needleleaf forest (ENF). The soil of the ecosystem with the highest light intensity incident on the soil surface, SAV, was a net source for COS, while the soils of the other two ecosystems were COS sinks. UV radiation increased COS emissions and/or reduced COS uptake from all soil samples under laboratory conditions. The impact of UV on the COS flux differed between soil samples, with a tendency towards a stronger response of the COS flux to UV radiation exposure in samples with higher SOM content. Our results emphasize the importance of photo‐degradation for the soil COS flux and stress the substantial spatial variability of soil COS exchange in ecosystems.

This work aims to assess a diagnostic approach which links evapotranspiration (ET) to land surface temperature (LST) measured by thermal remote sensing in the Alps. We estimated gridded ET, from field (30 m) to regional (1 km) scales, and we performed a specific study on grassland ecosystems in the Alps in South Tyrol (Italy), to evaluate the model sensitivity to different types of land management. The energy balance model TSEB ALEXI (Two Source Energy Balance Atmosphere Land EXchange Inverse) was first applied to Meteosat satellite data. Then ET was estimated by the flux disaggregation procedure DisALEXI, driven by MODIS and Landsat LST retrievals, which has never been applied before in a mountain region. We evaluated the model against eddy-covariance (EC) measurements from established stations in the Alps, and analyzed the main limitations which affect the model performance in mountainous regions. The TSEB model, applied in plot-scale mode using tower-based meteorological and LST input data, performed well with errors in daytime (6–18 UTC+1) latent heat flux around 30–60 W m−2 in comparison with flux measurements corrected for the lack of closure in the energy balance. For landscape ET retrievals, while Landsat resolution (30 m) is preferable for capturing small-scale heterogeneity in landscape moisture conditions, and for direct comparison with tower fluxes, persistent cloud cover resulted in no clear Landsat scenes during the study period. MODIS-based retrievals at 1 km resolution are too coarse to resolve the flux tower footprint in this complex landscape, yielding discrepancies of 100 W m−2 in model-measurement comparisons. Still, MODIS DisALEXI partitioning of the energy budget was reasonable and enabled to detect evaporative stress at regional scale expressed as the ratio between actual and potential ET, fPET. We evaluated fPET in comparison with a crop stress index based on cumulative air temperature and precipitation at different stations in the study area, and investigated ability to capture differential responses between managed and unmanaged grasslands. Results show that in the Alps i) moderate resolution thermal data can be used to monitor evaporative stress at the regional scale; ii) the spatial-temporal evolution of ET can be characterized from MODIS and Landsat thermal data with limitations which are due to the low availability of clear-sky scenes and to the small-scale (∼10 m) changes in soil moisture, topography and canopy density, which control ET patterns in mountainous regions; iii) solar radiation and leaf area index are critical variables which strongly affect the accuracy of the modeled energy fluxes.

Significance The exchange of nonmethane volatile organic compounds (NMVOC) at the surface–atmosphere interface is a fundamental constraint and important boundary condition for atmospheric chemistry and its effects on climate. Anthropogenic emissions are thought to account for about half of the NMVOC flux into the atmosphere of the Northern Hemisphere, yet their budget is considerably uncertain due to the scarcity of appropriate top-down constraints. Here we present direct flux measurements of NMVOCs based on the eddy covariance technique, showing that the contribution of typical urban emission sources is comprised of a surprisingly large portion of oxygenated NMVOC. These results suggest that typical urban NMVOC emission sources could be significantly higher than currently projected in air chemistry and climate models., Atmospheric chemistry is fueled by a large annual influx of nonmethane volatile organic compounds (NMVOC). These compounds influence ozone formation, lead to secondary organic aerosol production, and play a significant role for the oxidizing capacity of the atmosphere. The anthropogenic NMVOC budget is considerably uncertain due to the diversity of urban emission sources. Here, we present comprehensive observations of urban NMVOC eddy covariance fluxes using a newly designed proton-transfer-reaction quadrupole interface time-of-flight mass spectrometer. We found emission fluxes of a surprisingly large pool of oxygenated NMVOCs (OVOCs) with an appreciable fraction of higher oxidized OVOCs that cannot be explained by known fast photochemical turnaround or current primary emission estimates. Measured OVOC/NMVOC bulk flux ratios are two to four times higher than inferred from aggregated anthropogenic emission inventories. Extrapolating these results would double the global anthropogenic NMVOC flux. In view of globally accelerating urbanization, our study highlights the need to reevaluate the influence of anthropogenic NMVOC on atmospheric chemistry, human health, and the climate system.

The following authors were omitted from the original version of this Data Descriptor: Markus Reichstein and Nicolas Vuichard. Both contributed to the code development and N. Vuichard contributed to the processing of the ERA-Interim data downscaling. Furthermore, the contribution of the co-author Frank Tiedemann was re-evaluated relative to the colleague Corinna Rebmann, both working at the same sites, and based on this re-evaluation a substitution in the co-author list is implemented (with Rebmann replacing Tiedemann). Finally, two affiliations were listed incorrectly and are corrected here (entries 190 and 193). The author list and affiliations have been amended to address these omissions in both the HTML and PDF versions. © 2021, This is a U.S. government work and not under copyright protection in the U.S.; foreign copyright protection may apply.

Alpine ecosystems are, similar to arctic ecosystems, characterized by a very long snow season. Previous studies investigating arctic or alpine ecosystems have shown that winter CO2 effluxes can dominate the annual balance and that the timing and duration of the snow cover plays a crucial role for plant growth and phenology and might also influence the growing season ecosystem CO2 strength and dynamics. The objective of this study was to analyze seasonal and annual CO2 balances of a grassland site at an elevation of 2440 m a.s.l in the Swiss central Alps. We continuously measured the NEP using the eddy covariance method from June 2013 to October 2014, covering two growing seasons and one winter. We analyzed the influence of snow melt date on the CO2 exchange dynamics at this site, because snow melt differed about 24 days between the 2 years. To this end, we employed a process-based ecosystem carbon cycling model to disentangle the co-occurring effects of growing season length, environmental conditions during the growing season, and physiological/structural properties of the canopy on the ecosystem carbon balance. During the measurement period, the site was a net sink for CO2 although winter efflux contributed significantly to the total balance. The cumulative growing season NEP as well as mean and maximum daily CO2 uptake rates was lower during the year with the later snow melt, and the results indicated that the differences were mainly due to differing growing season lengths.

Automated transparent chambers have gained increasing popularity in recent years to continuously measure net CO(2) fluxes between low-statured canopies and the atmosphere. In this study, we carried out four field campaigns with chamber measurements in a variety of mountainous grasslands. A mathematic stationary point (or critical point, a point at which the derivative of a function is zero) in the CO(2) mixing ratio time series was found in a substantial fraction of the measurements at all the sites, which had a significant influence on the performances of the regression algorithms. The stationary point was probably due to condensed water on the inner wall of the chamber dome, which reduced the solar radiation and resulted in a reversal of the CO(2) mixing ratio time series in the chamber (so called Clouded-Glass Effect or CGE in this study). This effect may be the cause of the observed underestimation of daytime CO(2) fluxes when using common linear and exponential regression models on continuous automated chamber observations. In order to avoid biased flux estimation of daytime CO(2) fluxes, we introduced a linearly increasing term to the exponential function so as to compensate for the influence of the CGE, which gives acceptable model errors and improves the CO(2) flux estimation by 5 % for temperate mountainous grasslands. We conclude that exponential regression models should be favoured over linear models and recommend to account for the effects of CGE by either excluding ambiguous observations from the flux computations where stationary points can be identified in the CO(2) mixing ratio time series, or by adding a linearly increasing term to the exponential regression model.

The modification of the surface radiation and energy balance in urban areas causes the temperatures in these areas to exceed those of the surrounding countryside. It has thus been suggested that urban environments may serve as field laboratories for studying the effects of a warming climate on biota in a space-for-time substitution. Here we investigated changes in the timing of plant phenology and temperature across study sites that differed in the degree of urbanization using publicly available pan-European datasets for the period 1981-2010. We found a significant advancement in the phenological phases of leaf development, flowering and fruiting with higher degrees of urbanization, whereas a significant delay was observed for phenological phases of leaf senescence. In addition to these phenological changes, an increase in air temperature with higher degrees of urbanization was observed. This increase was largest during the periods of leaf development, flowering and fruiting and smallest during the period of leaf senescence. On the basis of these results, we show that the apparent temperature sensitivity of phenological phases to urban warming is either significantly dampened (leaf development, flowering and fruiting) or reversed (leaf senescence) compared with the temperature sensitivity inferred from temporal changes in phenology and temperature. We conclude that gradients in urbanization represent a poor analogue for the temporal changes in plant phenology, apparently owing to confounding factors associated with urbanization.

Global Change Biology, 24 (5), ISSN:1354-1013, ISSN:1365-2486

Plant phenological development is orchestrated through subtle changes in photoperiod, temperature, soil moisture and nutrient availability. Presently, the exact timing of plant development stages and their response to climate and management practices are crudely represented in land surface models. As visual observations of phenology are laborious, there is a need to supplement long-term observations with automated techniques such as those provided by digital repeat photography at high temporal and spatial resolution. We present the first synthesis from a growing observational network of digital cameras installed on towers across Europe above deciduous and evergreen forests, grasslands and croplands, where vegetation and atmosphere CO2 fluxes are measured continuously. Using colour indices from digital images and using piecewise regression analysis of time series, we explored whether key changes in canopy phenology could be detected automatically across different land use types in the network. The piecewise regression approach could capture the start and end of the growing season, in addition to identifying striking changes in colour signals caused by flowering and management practices such as mowing. Exploring the dates of green-up and senescence of deciduous forests extracted by the piecewise regression approach against dates estimated from visual observations, we found that these phenological events could be detected adequately (RMSE < 8 and 11 days for leaf out and leaf fall, respectively). We also investigated whether the seasonal patterns of red, green and blue colour fractions derived from digital images could be modelled mechanistically using the PROSAIL model parameterised with information of seasonal changes in canopy leaf area and leaf chlorophyll and carotenoid concentrations. From a model sensitivity analysis we found that variations in colour fractions, and in particular the late spring `green hump' observed repeatedly in deciduous broadleaf canopies across the network, are essentially dominated by changes in the respective pigment concentrations. Using the model we were able to explain why this spring maximum in green signal is often observed out of phase with the maximum period of canopy photosynthesis in ecosystems across Europe. Coupling such quasi-continuous digital records of canopy colours with co-located CO2 flux measurements will improve our understanding of how changes in growing season length are likely to shape the capacity of European ecosystems to sequester CO2 in the future.

In this paper we explore the skill of hyperspectral reflectance measurements and vegetation indices (VIs) derived from these in estimating carbon dioxide (CO2) fluxes of grasslands. Hyperspectral reflectance data, CO2 fluxes and biophysical parameters were measured at three grassland sites located in European mountain regions using standardized protocols. The relationships between CO2 fluxes, ecophysiological variables, traditional VIs and VIs derived using all two-band combinations of wavelengths available from the whole hyperspectral data space were analysed. We found that VIs derived from hyperspectral data generally explained a large fraction of the variability in the investigated dependent variables but differed in their ability to estimate midday and daily average CO2 fluxes and various derived ecophysiological parameters. Relationships between VIs and CO2 fluxes and ecophysiological parameters were site-specific, likely due to differences in soils, vegetation parameters and environmental conditions. Chlorophyll and water-content-related VIs explained the largest fraction of variability in most of the dependent variables. Band selection based on a combination of a genetic algorithm with random forests (GA–rF) confirmed that it is difficult to select a universal band region suitable across the investigated ecosystems. Our findings have major implications for upscaling terrestrial CO2 fluxes to larger regions and for remote- and proximal-sensing sampling and analysis strategies and call for more cross-site synthesis studies linking ground-based spectral reflectance with ecosystem-scale CO2 fluxes.

Urban areas are known to have a local climate different from that of surrounding rural landscapes and the temperature difference constitutes an urban heat island (UHI). Despite being described as ‘heating islands’, cities are not isolated from their environment. But only few studies respect the UHI as a relative measure and consider both the drivers of the temperature level of the urban area and of the surrounding landscape in their UHI assessments. A series of 124 Landsat satellite images spanning four decades were used to derive land surface temperatures (LST) of 77 urban areas (between 20 and 1600 ha) and their surroundings in the Province of South Tyrol, northern Italy. These data were used to calculate urban–rural LST differences (ΔLST UHI ) and NDVI differences (ΔNDVI) for quantifying the urban heat island effect. ΔLST UHI were recorded between 1.0 K and 8.1 K, with a mean ΔLST UHI of 4.2 K. The results show pronounced UHI effects already for small cities of less than 1 km 2 , but suggest that the urban size is of only minor importance for the ΔLST UHI magnitude in case of urban areas smaller than about 10 km 2 . Urban–rural land surface temperature difference (ΔLST UHI ) was identified to be primarily driven by ΔNDVI, together with solar irradiance and land use. Threshold values for the occurrence of ΔLST UHI extremes are provided. Implications for landscape and urban planning are presented that underpin the crucial importance of considering the surrounding landscape for quantitative assessments of UHI intensity.

The trace gas carbonyl sulphide (COS) has lately received growing interest in the eddy covariance (EC) community due to its potential to serve as an independent approach for constraining gross primary production and canopy stomatal conductance. Thanks to recent developments of fast-response high-precision trace gas analysers (e.g. quantum cascade laser absorption spectrometers (QCLAS)), a handful of EC COS flux measurements have been published since 2013. To date, however, a thorough methodological characterisation of QCLAS with regard to the requirements of the EC technique and the necessary processing steps has not been conducted. The objective of this study is to provide for the first time a rigorous analysis of the most widely used QCLAS model for making defensible EC COS flux measurements. Data were collected from May to October 2015 at a temperate mountain grassland in Tyrol, Austria. Analysis of the Allan variance of high-frequency concentration measurements revealed laser drift to occur under field conditions after an averaging time of around 50 s. We thus explored the use of two high-pass filtering approaches (linear detrending and recursive filtering) as opposed to block averaging for covariance computation. Spectral analysis revealed considerable noise at higher frequencies, appearing to influence the high-frequency region of co-spectra (and therefore covariances). By applying a finite-impulse response filter we removed the noise-affected spectral region. The effects of this digital high- and low-pass filtering, and additional low-pass filtering due to the eddy covariance system design, were corrected for using a site-specific reference model co-spectrum and a series of transfer functions. An independent validation of these post-processing steps was achieved by comparison of the CO2 and H2O flux measurements obtained with the QCLAS against those obtained with a closed-path infrared gas analyser. While the validation showed good correspondence and minor statistical differences between the three different high-pass filtering approaches – the benefits of high pass filtering clearly emerged as a reduction of the random flux uncertainty and a higher fraction of data passing the applied QA/QC criterions. We conclude that the most widely used QCLAS can be used to make defensible COS flux measurements provided the appropriate corrections are applied.

Nitrogen oxide (NOx) pollution is emerging as a primary environmental concern across Europe. While some large European metropolitan areas are already in breach of EU safety limits for NO2, this phenomenon does not seem to be only restricted to large industrialized areas anymore. Many smaller scale populated agglomerations including their surrounding rural areas are seeing frequent NO2 concentration violations. The question of a quantitative understanding of different NOx emission sources is therefore of immanent relevance for climate and air chemistry models as well as air pollution management and health. Here we report simultaneous eddy covariance flux measurements of NOx, CO2, CO and non methane volatile organic compound tracers in a city that might be considered representative for Central Europe and the greater Alpine region. Our data show that NOx fluxes are largely at variance with modelled emission projections, suggesting an appreciable underestimation of the traffic related atmospheric NOx input in Europe, comparable to the weekend-weekday effect, which locally changes ozone production rates by 40%.

Above- and belowground processes in plants are tightly coupled via carbon and water fluxes through the soil-plant-atmosphere system. The oxygen isotopic composition of atmospheric CO₂ and water vapour (H₂Ov) provides a valuable tool for investigating the transport and cycling of carbon and water within this system. However, detailed studies on the coupling between ecosystem components and environmental drivers are sparse. Therefore, we conducted a H2 (18)O-labelling experiment to investigate the effect of drought on the speed of the link between below- and aboveground processes and its subsequent effect on C(18)OO released by leaves and soils. A custom-made chamber system, separating shoot from soil compartments, allowed separate measurements of shoot- and soil-related processes under controlled conditions. Gas exchange of oxygen stable isotopes in CO₂ and H₂Ov served as the main tool of investigation and was monitored in real time on Fagus sylvatica saplings using laser spectroscopy. H₂(18)O-labelling showed that drought caused a slower transport of water molecules from soil to shoot, which was indicated by its direct derivation from independently measured concentrations and (18)O/(16)O ratios of CO₂ and H₂Ov, respectively. Furthermore, drought reduced the (18)O equilibrium between H₂O and CO₂ at the shoot level, resulting in less-enriched C(18)OO fluxes from leaf to atmosphere compared with control plants. Compared with the shoot, (18)O equilibrium was not instantaneous in the soil and no drought effect was apparent.

Journal of Experimental Botany, 65 (6), ISSN:1460-2431, ISSN:0022-0957

Field measurements of photosynthetic carbon isotope discrimination ((13)Δ) of Fagus sylvatica, conducted with branch bags and laser spectrometry, revealed a high variability of (13)Δ, both on diurnal and day-to-day timescales. We tested the prediction capability of three versions of a commonly used model for (13)Δ [called here comprehensive ((13)(Δcomp)), simplified ((13) Δsimple) and revised ((13)(Δrevised)) versions]. A Bayesian approach was used to calibrate major model parameters. Constrained estimates were found for the fractionation during CO(2) fixation in (13)(Δcomp), but not in (13)(Δsimple), and partially for the mesophyll conductance for CO(2)(gi). No constrained estimates were found for fractionations during mitochondrial and photorespiration, and for a diurnally variable apparent fractionation between current assimilates and mitochondrial respiration, specific to (13)(Δrevised). A quantification of parameter estimation uncertainties and interdependencies further helped explore model structure and behaviour. We found that (13)(Δcomp) usually outperformed (13)(Δsimple) because of the explicit consideration of gi and the photorespiratory fractionation in (13)(Δcomp) that enabled a better description of the large observed diurnal variation (≈9‰) of (13)Δ. Flux-weighted daily means of (13)Δ were also better predicted with (13)(Δcomp) than with (13)(Δsimple).

Thermal infrared (TIR) cameras perfectly bridge the gap between (i) on-site measurements of land surface temperature (LST) providing high temporal resolution at the cost of low spatial coverage and (ii) remotely sensed data from satellites that provide high spatial coverage at relatively low spatio-temporal resolution. While LST data from satellite (LSTsat) and airborne platforms are routinely corrected for atmospheric effects, such corrections are barely applied for LST from ground-based TIR imagery (using TIR cameras; LSTcam). We show the consequences of neglecting atmospheric effects on LSTcam of different vegetated surfaces at landscape scale. We compare LST measured from different platforms, focusing on the comparison of LST data from on-site radiometry (LSTosr) and LSTcam using a commercially available TIR camera in the region of Bozen/Bolzano (Italy). Given a digital elevation model and measured vertical air temperature profiles, we developed a multiple linear regression model to correct LSTcam data for atmospheric influences. We could show the distinct effect of atmospheric conditions and related radiative processes along the measurement path on LSTcam, proving the necessity to correct LSTcam data on landscape scale, despite their relatively low measurement distances compared to remotely sensed data. Corrected LSTcam data revealed the dampening effect of the atmosphere, especially at high temperature differences between the atmosphere and the vegetated surface. Not correcting for these effects leads to erroneous LST estimates, in particular to an underestimation of the heterogeneity in LST, both in time and space. In the most pronounced case, we found a temperature range extension of almost 10 K.

This study analyzes the impact of droughts, compared with average climatic conditions, on the supporting ecosystem service water provision in sub‐watersheds in managed alpine grasslands in two climatically different regions of the Alps, Lautaret (French Alps) and Stubai (Austrian Alps). Soil moisture was modelled in the range of 0–0.3 m. At both sites, current patterns showed that the mean seasonal soil moisture was (1) near field capacity for grasslands with low management intensity and (2) below field capacity for grasslands with higher land‐use intensity. Soil moisture was significantly reduced by drought at both sites, with lower reductions at the drier Lautaret site. At the sub‐watershed scale, soil moisture spatial heterogeneity was reduced by drought. Under drought conditions, the evapotranspiration to precipitation ratios at Stubai was slightly higher than those at Lautaret, indicating a dominant ‘water spending’ strategy of plant communities. Regarding catchment water balance, deep seepage was reduced by drought at Stubai more strongly than at Lautaret. Hence, the observed ‘water spending’ strategy at Stubai might have negative consequences for downstream water users. Assessing the water provision service for alpine grasslands provided evidence that, under drought conditions, evapotranspiration was influenced not only by abiotic factors but also by the water‐use strategy of established vegetation. These results highlight the importance of ‘water‐use’ strategies in existing plant communities as predictors of the impacts of drought on water provision services and related ecosystem services at both the field and catchment scale. © 2015 The Authors. Ecohydrology published by John Wiley & Sons, Ltd.

Climate change is expected to affect the Alps by increasing the frequency and intensity of summer drought events with negative impacts on ecosystem water resources. The response of CO 2 and H 2 O exchange of a mountain grassland to natural fluctuations of soil water content was evaluated during 2001–2009. In addition, the physiological performance of individual mountain forb and graminoid plant species under progressive soil water shortage was explored in a laboratory drought experiment. During the 9-year study period the natural occurrence of moderately to extremely dry periods did not lead to substantial reductions in net ecosystem CO 2 exchange and evapotranspiration. Laboratory drought experiments confirmed that all the surveyed grassland plant species were insensitive to progressive soil drying until very low soil water contents ( 3 m −3 ) were reached after several days of drought. In field conditions, such a low threshold was never reached. Re-watering after a short-term drought event (5 ± 1 days) resulted in a fast and complete recovery of the leaf CO 2 and H 2 O gas exchange of the investigated plant species. We conclude that the present-day frequency and intensity of dry periods does not substantially affect the functioning of the investigated grassland ecosystem. During dry periods the observed “water spending” strategy employed by the investigated mountain grassland species is expected to provide a cooling feedback on climate warming, but may have negative consequences for down-stream water users.

Recent (13) CO(2) canopy pulse chase labeling studies revealed that photosynthesis influences the carbon isotopic composition of soil respired CO(2) (δ(13) C(SR)) even on a diel timescale. However, the driving mechanisms underlying these short-term responses remain unclear, in particular under drought conditions. The gas exchange of CO(2) isotopes of canopy and soil was monitored in drought/nondrought-stressed beech (Fagus sylvatica) saplings after (13) CO(2) canopy pulse labeling. A combined canopy/soil chamber system with gas-tight separated soil and canopy compartments was coupled to a laser spectrometer measuring mixing ratios and isotopic composition of CO(2) in air at high temporal resolution. The measured δ(13) C(SR) signal was then explained and substantiated by a mechanistic carbon allocation model. Leaf metabolism had a strong imprint on diel cycles in control plants, as a result of an alternating substrate supply switching between sugar and transient starch. By contrast, diel cycles in drought-stressed plants were determined by the relative contributions of autotrophic and heterotrophic respiration throughout the day. Drought reduced the speed of the link between photosynthesis and soil respiration by a factor of c. 2.5, depending on the photosynthetic rate. Drought slows the coupling between photosynthesis and soil respiration and alters the underlying mechanism causing diel variations of δ(13) C(SR).

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 to ET and by adjusting the ET according to the average energy balance ratio during the first and second measurement campaigns, respectively. Due to a large spatial variability in ET during the first measurement campaign and given large differences in spatial scale between the EC and the independent methods, we are more confident with the comparison of approaches during the second measurement campaign and thus recommend forcing energy balance closure by adjusting for the average energy balance ratio.

The Penman–Monteith combination equation, which is most frequently used to derive the surface conductance to water vapour (Gs), implicitly assumes the energy balance to be closed. Any energy imbalance (positive or negative) will thus affect the calculated Gs. Using eddy covariance energy flux data from a temperate grassland and a desert shrub ecosystem we explored five possible approaches of closing the energy imbalance and show that calculated Gs may differ considerably between these five approaches depending on the relative magnitudes of sensible and latent heat fluxes, and the magnitude and sign of the energy imbalance. Based on our limited understanding of the nature of the energy imbalance, we tend to favour an approach which preserves the Bowen-ratio and closes the energy balance on a larger time scale.

Pre-rain-event soil moisture (preSM) plays a crucial role when evaluating runoff formation during heavy rainfall. Using sprinkling experiments and numerical modeling, this study investigates the impact of preSM on runoff formation at the small hillslope scale (≤100 m2). Sprinkling experiments were conducted on three sites in the Austrian Alps and observed hydrological behavior was simulated by using the hillslope model HILLFLOW. For each site, runoff was modeled with different preSM scenarios, covering the whole soil moisture range between permanent wilting point and saturation. Depending on the dominant runoff processes, the results showed impacts of preSM on both the maximum runoff and the total runoff. A predominant threshold was observed near field capacity (35 and 32% by volume). In general, the results clearly showed that the consideration of preSM is a prerequisite to reproduce runoff formation and total runoff amounts accurately. Field capacity turned out to be a good indicator (threshold...

Carbon dioxide, latent and sensible energy fluxes were measured by means of the eddy covariance method above a mountain meadow situated on a steep slope in the Stubai Valley/Austria, based on the hypothesis that, due to the low canopy height, measurements can be made in the shallow equilibrium layer where the wind field exhibits characteristics akin to level terrain. In order to test the validity of this hypothesis and to identify effects of complex terrain in the turbulence measurements, data were subjected to a rigorous testing procedure using a series of quality control measures established for surface layer flows. The resulting high-quality data set comprised 36 % of the original observations, the substantial reduction being mainly due to a change in surface roughness and associated fetch limitations in the wind sector dominating during nighttime and transition periods. The validity of the high-quality data set was further assessed by two independent tests: i) a comparison with the net ecosystem carbon dioxide exchange measured by means of ecosystem chambers and ii) the ability of the eddy covariance measurements to close the energy balance. The net ecosystem CO2 exchange measured by the eddy covariance method agreed reasonably with ecosystem chamber measurements. The assessment of the energy balance closure showed that there was no significant difference in the correspondence between the meadow on the slope and another one situated on flat ground at the bottom of the Stubai Valley, available energy being underestimated by 28 and 29 %, respectively. We thus conclude that, appropriate quality control provided, the eddy covariance measurements made above a mountain meadow on a steep slope are of similar quality as compared to flat terrain.