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Biogenic volatile organic compounds (BVOCs) are emitted into the atmosphere by plants and other living organisms and play a significant role in various plant functions, such as growth, reproduction, and defense. BVOCs are also an essential part of many chemical reactions in the atmosphere and contribute to the formation of ozone and secondary organic aerosols and affect the radiation balance.Investigating the atmospheric vertical profile concentrations of BVOCs has become an important focus for understanding these processes. There are various methods that can be used to study the atmospheric vertical profile, including towers, balloons, aircraft, and unmanned aerial vehicles (UAVs). Among these methods, UAVs offer greater flexibility for local air sampling by hovering over a target area and can reach altitudes of up to 1000m, making them ideal for permanent BVOCs monitoring that requires repeated measurements in a specific spatial and temporal domain. However, the source contribution area of the obtained BVOCs concentrations is often not identified, potentially leading to inaccurate conclusions about the exchange between the surface, vegetation, and atmosphere above the target area.In this study, the vertical profile of BVOCs concentrations was obtained at SMEAR Estonia (Station for Measuring Ecosystem Atmosphere Relations) to analyze the composition and distribution of these compounds in the near-surface layer. The vertical samples were collected in 2020-2021 using a commercially available pump equipped with cartridges filled with adsorbents and mounted on a UAV. The UAV was used to collect samples from heights between 0 m and 90 m.To address the issue of space-time representativeness related to the source signal, a footprint analysis was conducted. Micrometeorological data for four target areas were obtained from three SMEAR Estonia flux towers. The Flux Footprint Prediction model (Kljun et al., 2015) was used for the footprint calculation. We determined temporal and spatial changes in roughness length (z0) and zero-displacement height (zd) for each day when BVOCs measurements were carried out using meteorological and geospatial data on land cover types and corresponding canopy heights. Due to the presence of surface heterogeneity, z0 and zd varied significantly for each wind sector. Therefore, we ran the spin-up of the FFP model with updated input parameters at each step. For the measurement results interpretation, we also evaluated the representativeness of the obtained footprints over the target areas in the space-time domain and analysed the land cover composition and vegetation characteristics.In this work, we show how the source contribution area of BVOCs concentrations can vary in size and shape depending on atmospheric conditions, and spatial and temporal variation and thus have an effect on the obtained species composition of BVOCs. Based on the presented findings we discuss the potential implementation of this approach for similar research and its future development.

Ecosystem water use efficiency (WUE) is a key characteristic that describes the coupling of carbon and water exchange and can be used as an indicator of a forest's adaptability to varying climatic conditions. Mixed forests, characterized by the coexistence of two or more dominant tree species, may potentially exhibit higher productivity and greater resistance to extreme weather events due to possible niche differentiation among dominant species, leading to more efficient nutrient utilization. However, the increased productivity may also result in higher evapotranspiration demand, resulting in lower WUE compared to monospecific forests. In this study, we aim to assess the variation in WUE of mixed and monospecific boreal forests in response to different environmental factors using eddy-covariance measurements. The two study sites are represented by forest stands of similar age, growing under the same climatic conditions and located in close proximity (~10km distance) in Northern Sweden. The Rosinedalsheden site is a ~100-year-old monospecific pine (Pinus sylvestris) forest stand with sandy soils. The Svartberget site is a mixed ~110-year-old forest featuring pine (Pinus sylvestris, 61%), spruce (Picea abies, 34%), and birch (Betula sp., 5%) species, with soils dominated by till and sorted sediments. Our study spans a period of seven years (2014-2020) and covers a wide range of weather conditions, including the 2018 heatwave.

Estimations of forests’ carbon sequestration capacity relies on proper assessment of the eddy covariance measurement mast’s footprint. Harsh winter temperatures in Estonia lead to ice formation on 3D sonic anemometer sensor heads and thus induce measurement gaps in the data. To maximise data availability, we use a smart heating algorithm to minimise ice formation on the anemometer sensor heads. Here, we studied the temperature distribution of ice formation on the measurement instruments. Three major temperature ranges were found, between 0°C and −3°C, which is the most abundant temperature range for ice formation, and two temperature regions with peaks around −10°C and −20°C. Our algorithm to prevent ice formation led to very short median heating intervals of about 25 to 30 seconds.

Gross Primary Productivity (GPP) is the core component of the terrestrial and global carbon cycle and Earth’s climate research. In this study, GPP estimation was performed with the Boreal Ecosystem Productivity Simulator (BEPS) model to check its performance for hemi-boreal forests on the example of the Soontaga area in Estonia. The model was run by using a combination of remote sensing (leaf area index (LAI), clumping index) and meteorological data inputs (air temperature, global radiation, air humidity, precipitation and wind speed). The results were validated against GPP derived from the available flux tower measurements. The spatial representativeness of the site was evaluated using multiple spatial thresholds (500 m–2 km), as well. We found that the BEPS model can track the GPP changes with the season and inter-annual variation very well in a coniferous hemi-boreal forest, given that good quality input data are provided.

Changes in the net carbon sink of boreal forests constitute a major source of uncertainty in the future global carbon budget and hence climate change projections1. The annual net ecosystem exchange of carbon dioxide (CO2) controlling the terrestrial carbon stock is the result of the small difference between respiratory CO2 release and the photosynthetic CO2 uptake by vegetation2. The boreal forest, and the boreal biome in general, is regarded as a persistent and even an increasing net carbon sink3,4. However, decreases in photosynthetic CO2 uptake and/or concurrent increases in respiratory CO2 release under a changing climate may turn boreal forests from a net sink to a net source of CO21. Here, we assessed the dynamics of boreal forest net CO2 sink-source strength and its components from 1981 to 2018. Our remote sensing approach - trained by net CO2 flux observations at eddy covariance sites across the circumpolar boreal forests - employs satellite-derived retrievals of snow melt timing, landscape freeze-thaw status, and yearly maximum estimates of the normalized difference vegetation index as a proxy for peak vegetation productivity. Our results for the period 1981-2018 suggest that the mean annual evergreen boreal forest CO2 sink was 0.4 Pg C y-1 (0.3 Pg C y-1 for Eurasia and 0.2 Pg C y-1 for North America). In contrast to earlier investigations3,4 our results do not indicate an increasing trend in the circumpolar evergreen boreal forest CO2 sink, as the increase in CO2 uptake is compensated by increasing respiratory releases with both components of CO2 sink-source strength showing considerable interannual variabilities.

Riparian forests are known as hot spots of nitrogen cycling in landscapes. Climate warming speeds up the cycle. Here we present results from a multi-annual high temporal-frequency study of soil, stem, and ecosystem (eddy covariance) fluxes of N2O from a typical riparian forest in Europe. Hot moments (extreme events of N2O emission) lasted a quarter of the study period but contributed more than half of soil fluxes. We demonstrate that high soil emissions of N2O do not escape the ecosystem but are processed in the canopy. Rapid water content change across intermediate soil moisture was a major determinant of elevated soil emissions in spring. The freeze-thaw period is another hot moment. However, according to the eddy covariance measurements, the riparian forest is a modest source of N2O. We propose photochemical reactions and dissolution in canopy-space water as reduction mechanisms.

Eddy-covariance (EC) is a widely used method for measuring ecosystem-scale fluxes of various gases. The sensor placement height is typically constrained by the canopy height and area of interest size. We studied the carbon dioxide fluxes over a hemiboreal mixed forest with two EC measurement systems located at 30 m and 70 m. The lower system NEE (NEE30) values were more positive (smaller sink or higher source) than the NEE of the higher one (NEE70), but this difference was prevalent in low light conditions and in May–November of all studied years. The nighttime and early morning difference (ΔNEE) increased with wind speed until ~2 m s-1 and friction velocity until ~0.35 m s-1 and linearly decreased after. ΔNEE was irregularly distributed over the wind direction sectors with high values overlapping the directions of South-East and South-West guy wire tunnels. Moreover, the shape of the NEE30 seasonal cycle was closer to that of a clear-cut area, and the difference between the systems increased with air temperature. The forest under study varied between a weak net sink and a strong net source on the annual scale. Directional heterogeneity correction shifted the annual NEE towards more negative values, but neither removed the difference between the systems nor changed the shape of the seasonal cycle. More studies are needed to assess the impact of clearcutting on the carbon accumulation under the measurement point.

The SMEAR Estonia is an important step towards understanding how forest ecosystem and the atmosphere affect each other. The station provides long-term continuously measured eddy-covariance CO2 flux data. Parameters such as wind speed and direction are not controllable by human, but forest management methods are, thus the flux tower helps to assess how human activities affect forest ecosystem-atmosphere relationship as well as to assess natural processes. In this study, the footprint for years 2015–2020 was calculated with Kljun model according to wind speed and direction. Measurements were taken from 30 m and 70 m height. Data was obtained by continuous high frequency (10 Hz) measurements by the eddy-covariance method and averaged over half-hour intervals. Results showed that the footprint area measured from 30 m over six-year period differed only by 5%. From 70 m this difference was only 1.2% over the six-year period. Average area for both 30 m and 70 m FFP was 61,5 ha and 4029,7 ha respectively. The growing stock of the forest was affected by forest management, but in general it grew by 3,2% for 30 m FFP. The main tree species growing in the area of the footprint are Scots pine (Pinus sylvestris), Norway spruce (Picea abies) and Silver birch (Betula pendula) with some small amount of aspen and alder species. The dominant wind directions were ranging from west to south in 2015–2017 and in 2018–2020 from south-west to south-east. The footprint area is affected mainly by wind speed and direction, and by forest management activities like harvesting and clear-cutting. Such measurements help to understand how human activity and natural processes affect formation of the footprint.

Funding Information: This study was supported by the Ministry of Education and Science of Estonia (SF0180127s08 grant), the Estonian Research Council (IUT2-16, PRG-352, and MOBERC20), the Czech Science Foundation (17-18112Y), SustES - Adaptation strategies for sustainable ecosystem services and food security under adverse environmental conditions (CZ.02.1.01/0.0/0.0/16_019/0000797), the Ministry of Education, Youth and Sports of Czech Republic within the National Sustainability Program I (NPU I, grant number LO1415), the EU through the European Regional Development Fund (ENVIRON and EcolChange Centres of Excellence, Estonia, and MOBTP101 returning researcher grant by the Mobilitas Pluss programme), the European Social Fund (Doctoral School of Earth Sciences and Ecology). This work was also supported by Academy of Finland (294088, 288494), from the European Research Council (ERC) under the European Union?s Horizon 2020 research and innovation programme under grant agreement No [757695], and a Department of Energy (DOE) grant to JPM (DE-SC0008165). Funding Information: This study was supported by the Ministry of Education and Science of Estonia ( SF0180127s08 grant), the Estonian Research Council ( IUT2-16 , PRG-352 , and MOBERC20 ), the Czech Science Foundation ( 17-18112Y ), SustES - Adaptation strategies for sustainable ecosystem services and food security under adverse environmental conditions ( CZ.02.1.01/0.0/0.0/16_019/0000797 ), the Ministry of Education, Youth and Sports of Czech Republic within the National Sustainability Program I (NPU I, grant number LO1415 ), the EU through the European Regional Development Fund (ENVIRON and EcolChange Centres of Excellence, Estonia, and MOBTP101 returning researcher grant by the Mobilitas Pluss programme), the European Social Fund (Doctoral School of Earth Sciences and Ecology). This work was also supported by Academy of Finland ( 294088 , 288494 ), from the European Research Council (ERC) under the European Union‘s Horizon 2020 research and innovation programme under grant agreement No [ 757695 ], and a Department of Energy (DOE) grant to JPM ( DE-SC0008165 ). Publisher Copyright: © 2021 Elsevier B.V. The carbon (C) budgets of riparian forests are sensitive to climatic variability. Therefore, riparian forests are hot spots of C cycling in landscapes. Only a limited number of studies on continuous measurements of methane (CH4) fluxes from riparian forests is available. Here, we report continuous high-frequency soil and ecosystem (eddy-covariance; EC) measurements of CH4 fluxes with a quantum cascade laser absorption spectrometer for a 2.5-year period and measurements of CH4 fluxes from tree stems using manual chambers for a 1.5 year period from a temperate riparian Alnus incana forest. The results demonstrate that the riparian forest is a minor net annual sink of CH4 consuming 0.24 kg CH4-C ha−1 y−1. Soil water content is the most important determinant of soil, stem, and EC fluxes, followed by soil temperature. There were significant differences in CH4 fluxes between the wet and dry periods. During the wet period, 83% of CH4 was emitted from the tree stems while the ecosystem-level emission was equal to the sum of soil and stem emissions. During the dry period, CH4 was substantially consumed in the soil whereas stem emissions were very low. A significant difference between the EC fluxes and the sum of soil and stem fluxes during the dry period is most likely caused by emission from the canopy whereas at the ecosystem level the forest was a clear CH4 sink. Our results together with past measurements of CH4 fluxes in other riparian forests suggest that temperate riparian forests can be long-term CH4 sinks.

Hemiboreal forests form a transitional belt between boreal and temperate forests in Eurasia, and due to long-term climate warming they could potentially expand in a northerly direction. However, carbon (C) exchange studies in this transitional forest type are scarce. In 2014–2015 we analyzed CO2 exchange in a hemiboreal mixed forest at SMEAR Estonia (Station for Measuring Ecosystem-Atmosphere Relations) using eddy covariance (EC), soil chamber technique and biometric methods. Employing two plots that differ in forest stand composition (mixed (MP) and coniferous (CP)) located within the footprint of the EC tower, we evaluated heterotrophic and total soil respiration (Rh and Rs, respectively). Measurements showed that C, nitrogen (N), and soil organic matter (SOM) contents in topsoil were significantly higher in the coniferous plot, and the total C stock until 20 cm of mineral soil was higher by a factor of two (i.e., 170.6 and 94.8 t C ha−1 for the CP and MP, respectively). Only in 2015 there was a significant difference in Rs between the plots, while Rh exhibited no detectable difference. The cumulative Rs during the growing season was similar for both plots (CP and MP) and years: 532 and 548 g m−2, respectively, in 2014; 636 and 592 g m−2, respectively, in 2015. The same applied to Rh: 418 and 430 g m−2, respectively, in 2014; 406 and 387 g m−2, respectively, in 2015. The forest as a whole was a C sink. In 2015, net ecosystem exchange (NEE) was -585.62 ± 45.16 g C m−2 yr−1 (the average and SD of two gap-filling methods: MDS and nonlinear regressions). Gross primary production (GPP), and ecosystem respiration (RE) were -1280.62 ± 53.36, and 696.00 ± 98.51 g C m−2 yr−1, respectively (the average and SD of two flux partitioning methods: nighttime data-based and daytime data-based). NEE and GPP suggested a greater similarity to temperate forests, while RE was closer to more boreal forest stands. The mean value of net ecosystem production (NEP) of both plots, estimated using forest stand biometric, litter and chamber data was similar to the annual NEE value: 6.3 and −5.9 t C ha−1 yr−1, respectively.

Clear-cutting is a conventional method of forest management which significantly changes carbon (C) cycling at the ecosystem level for a long time. Estimation of the interim period during which the ecosystem turns from a C source to a C sink is crucial for clarifying the environmental effects of management on forest C cycling. The current study provided new knowledge of C cycling in young pine stand and demonstrated the recovery of C sequestration of the forest ecosystem during the post harvesting period. We estimated the C balance in a 6-year-old Scots pine stand by using two different methods: carbon budgeting, for estimating annual net ecosystem production (NEP), and eddy covariance (EC), for estimating net ecosystem exchange (NEE). For C budgeting, the above- and belowground biomass production of the ecosystem, as well as the soil heterotrophic respiration efflux at the studied site was estimated. Annual NEE at the studied young forest ecosystem was 1.19 ± 0.36 t C ha−1, gross primary ecosystem production was 9.87 and total ecosystem respiration was 11.06 t C ha−1. Estimated NEE was in good accordance with the results of NEP (1.37 t C ha−1), which confirms the relevance of the C budgeting method. Increased annual woody biomass production is the main factor which induced the young Scots pine ecosystem to act as a C sink: annual C accumulation in tree biomass in a 6-year-old stand was 1.0 t C ha−1 but reached already 2.4 t C ha−1 in the following year. Assuming that the annual Rh flux is of the same magnitude in the subsequent years, the ecosystem will become a C sink already during a short period after clear-cut. Annual soil respiration (Rs) and heterotrophic soil respiration (Rh) were 6.0 and 4.2 t C ha−1, respectively and the Rh/Rs ratio was 0.70. However, at this stage also the understorey vegetation contributed essentially to NEP, making up 56% of the annual C uptake accumulated in the plants. The methane flux and the leached C flux were negligible, 0.004 and 0.015 t C ha−1 yr−1, respectively. Our results demonstrate that well regenerated young Scots pine stand on a former clear-cut area will be able to turn into a C sequestering ecosystem already before ten years after cutting.

In summer 2018, Northern Europe experienced an extreme summer drought in combination with unusually high temperatures, which had a substantial impact on agricultural yields as well as on forest growth conditions in various ways. An earlier study, using ICOS RI (Integrated Carbon Observation Research Infrastructure) stations and other forest ecosystem stations in the Nordic region, shows that the drought dramatically decreased NEP in the southern Scandinavian and Baltic region, almost nullifying the carbon sinks in some of the forests [1]. Such severe conditions during a single year could be expected to influence a forest over following several years. Reduced tree storage of carbohydrates leads to a changed carbon allocation pattern in spring that may affect both the woody growth and pests' resistance. It is thus important to reveal the impact of such climatic events over a more extended period. This study aims at assessing the carry-over effects of the extreme weather conditions on the carbon and water fluxes and the forest growth to the years after the event. The analysis is based on measurement from the stations shown to be significantly affected by the drought through reduced carbon fluxes in 2018: the spruce forests Hyltemossa and Skogaryd and the mixed forests Norunda, Svartberget, Soontaga and Rumperöd. The ecosystem carbon and water fluxes will, together with tree-ring width data, be used to assess the carbon and water exchange and growth recovery in the years after the extreme 2018 drought (2019 and 2020) by comparisons to earlier normal years and extreme events.[1] Lindroth, A., et al. (2020): Effects of drought and meteorological forcing on carbon and water fluxes in Nordic forests during the dry summer of 2018 Phil. Trans. R. Soc. B37520190516 https://doi.org/10.1098/rstb.2019.0516

Secondary organic aerosols (SOA) formed from biogenic volatile organic compounds (BVOCs) constitute a significant fraction of atmospheric particulate matter and have been recognized to affect significantly the climate and air quality. Many laboratory and field experiments have studied SOA particle formation and growth in the recent years. Most of them have focused on a few monoterpenes and isoprene. However, atmospheric SOA particulate mass yields and chemical composition result from a much more complex mixture of oxidation products originating from many BVOCs, including terpenes other than isoprene and monoterpenes. Thus, a large uncertainty still remains regarding the contribution of BVOCs to SOA. In particular, organic compounds formed from sesquiterpenes have not been thoroughly investigated, and their contribution to SOA remains poorly characterized. In this study, a Filter Inlet for Gases and Aerosols (FIGAERO) combined with a high-resolution time-of-flight chemical ionization mass spectrometer (CIMS), with iodide ionization, was used for the simultaneous measurement of gas and particle phase atmospheric SOA. The aim of the study was to evaluate the relative contribution of sesquiterpene oxidation products to SOA in a spring-time hemi-boreal forest environment. Our results revealed that monoterpene and sesquiterpene oxidation products were the main contributors to SOA particles. The chemical composition of SOA particles was compared for times when either monoterpene or sesquiterpene oxidation products were dominant and possible key oxidation products for SOA particle formation were identified. Surprisingly, sesquiterpene oxidation products were the predominant fraction in the particle phase at some periods, while their gas phase concentrations remained much lower than those of monoterpene products. This can be explained by quick and effective partitioning of sesquiterpene products into the particle phase or their efficient removal by dry deposition. The SOA particle volatility determined from measured thermograms increased when the concentration of sesquiterpene oxidation products in SOA particles was higher than that of monoterpenes. Overall, this study demonstrates the important role of sesquiterpenes in atmospheric chemistry and suggests that the contribution of their products to SOA particles is being underestimated in comparison to the most studied terpenes.

Riparian forests are known as hot spots of N cycling in landscapes and climate warming speeds up the cycle. Here we present results from the first multi-annual high temporal-frequency study of soil, stem and ecosystem (eddy covariance) fluxes of N2O from a typical riparian forest in Europe.Hot moments (extreme events of N2O emission) last a quarter of the study period but contribute more than a half of soil fluxes. For the first time we demonstrate that high soil emissions of N2O do not reach the ecosystem level. During the drought onset, soil N2O emission peaks at intermediate soil water content. Rapid water content change is the main determinant of the emissions. The freeze–thaw period is another hot moment. However, according to the eddy covariance measurements the riparian forest is a modest source of N2O. We propose photochemical reactions and dissolution in canopy-space water as consumption mechanisms.

Forests are important regulators of carbon dioxide fluxes, whereas overall greenhouse gas (GHG) budgets, in particular, nitrous oxide (N2O), are still largely unknown. No studies on ecosystem-level N2O budgets (soil and tree stem fluxes with eddy covariance (EC) measurements above the canopy) are found. Only a few examples are available on N2O emissions from tree stems. Nevertheless, estimation of the N2O and the full GHG balance in different forest ecosystems under various environmental conditions is essential to understand their impact on climate.During the period of August 2017 to December 2019, we measured the N2O budget of a 40-yr old hemiboreal grey alder (Alnus incana) forest stand on former agricultural land in Estonia considering fluxes from the soil, tree stems and whole ecosystem. Grey alder (Alnus incana) is a fast-growing tree species typically found in riparian zones, with great potential for short-rotation forestry. Their symbiotic dinitrogen (N2) fixation ability makes alders important for the regulation of nitrogen (N) cycle in forested areas.We measured the N2O budget considering fluxes from the soil surface (12 automated chambers; Picarro 2508), tree stems (60 manual sampling campaigns from 12 model trees with chambers at 0.1, 0.8 and 1.7 m; gas chromatographic analysis in lab) and whole ecosystem (EC technique: Aerodyne TILDAS). Simultaneously, soil water level, temperature and moisture were measured automatically, and composite soil samples were taken for physico-chemical analysis. Potential N2 flux in intact soil cores was measured in the lab using the He-O incubation method.Average N2O fluxes from the soil and tree stems varied from 1.2 to 3.0 and 0.01 to 0.03 kg N2O-N ha–1 yr–1, respectively, being the highest during the wet periods, peaking during the freezing-thawing, and being the lowest in dry periods. The average annual potential N2 flux in the soil was 140 kg N2 ha–1 yr–1 which made the average N2:N2O-N ratio in the soil about 60. According to the EC measurements, the forest was a net annual source of N2O (3.4 kg N2O ha–1). Thus, the main gaseous nitrogen flux in this forest was N2 emission. Our carbon (C) budget showed that the forest was a significant net annual C sink.Results of our long-term study underline the high N and C buffering capacity of riparian alder forests. For better understanding of C and nutrient budgets of riparian forests, we need long-term, high-frequency measurements of N2O fluxes from the soil and tree stems in combination with ecosystem-level EC measurements. The identification of microorganisms and biogeochemical pathways associated with N2O production and consumption is another future challenge.

The SMEAR Estonia station was established in 2012 as southernmost “Station for Ecosystem-Atmosphere Relations” in Northern Europe. The station provides continuous data since 2014 and has steadily increased the amount of measured variables. Measurements cover atmospheric gases, air ions and particulate matter, radiation and energy fluxes, forest ecosystem and soil related parameters. Located in the hemiboreal forest ecosystem at the southern edge of the boreal forest biome the forests are characterised by a mix between coniferous and broadleaved species. The SMEAR Estonia station’s location near to an old growth forest, which is the oldest Estonian forest nature reserve established in 1924, allows for comparisons of atmosphere-biosphere related processes between unmanaged and managed forests. The application of continuous multi-scale data allows us to see first trends of hemiboreal ecosystem-atmosphere interactions in relation to natural and man made disturbances and climatic drivers.Here, we report and present our available multi-scale data and research results. Our focus lies on the heterogeneity and the dynamics of atmosphere-biosphere exchange processes and feedbacks in the footprint of the SMEAR Estonia station.

Eddy-covariance (EC) method is widely used to calculate fluxes of different gases from different ecosystems. One of the assumptions is that the footprint of eddy tower is homogeneous in terms of plant species composition, height, age, soil properties, etc. In reality that is usually not the case: European forests are mostly managed and thus have compartments of tree species. This is even more so in the hemiboreal zone that is characterized by higher heterogeneity and tree species diversity. This results in the possibility for the individual features to have stronger influence on the eddy measurements. Two identical EC systems (LI-7200 gas analyser + Metek uStar Class A anemometer) were placed at 30m (EC30) and 70m (EC70) height on an atmospheric tower of SMEAR Estonia (Station for Measurements of Ecosystem Atmosphere Relations) above a 20m high forest to measure CO2 fluxes. The footprints are represented by compartments of Scots pine (Pinus sylvestris), Norway spruce (Picea abies) and Birch trees (Betula pendula and Betula pubescens) and clear-cut areas.According to the EC30 flux data, the mixed hemiboreal forest ecosystem was a source of CO2 (505 gC m-2 in 2015; 333 gC m-2 in 2016; 276 gC m-2 in 2017; 603 gC m-2 in 2018), while according to EC70 data it was a minor sink in some of the years (-47 gC m-2 in 2015; 10 gC m-2 in 2016; -142 gC m-2 in 2017; 151 gC m-2 in 2018).Both the ecosystem respiration (ER) and the gross primary production (GPP) were bigger when estimated from EC30 than in EC70 for all the years:GPP EC30: 1738 gC m-2 in 2015; 1669 gC m-2 in 2016; 1892 gC m-2 in 2017; 1654 gC m-2 in 2018;GPP EC70: 1242 gC m-2 in 2015; 1192 gC m-2 in 2016; 1215 gC m-2 in 2017; 988 gC m-2 in 2018;ER EC30: 2057 gC m-2 in 2015; 1999 gC m-2 in 2016; 1992 gC m-2 in 2017; 2070 gC m-2 in 2018;ER EC70: 1088 gC m-2 in 2015; 1120 gC m-2 in 2016; 1019 gC m-2 in 2017; 1021 gC m-2 in 2018; All the 4 study years (2015-2018) showed similar difference patterns between the two heights: higher EC30 nighttime NEE values and similar daytime NEE values throughout the season. The peak difference between the two systems was in the end of August - middle of September for all the years.

In summer 2018, Northern Europe experienced an extreme summer drought in combination with unusually high temperatures, which had a substantial impact on agricultural yields as well as on forest growth conditions in various ways. An ongoing study, using ICOS and other forest ecosystem stations in the Nordic region, shows that the drought dramatically decreased NEP in the southern Scandinavian and Baltic region, almost nullifying the carbon sinks in some of the forests. However, some of the forests that not were exposed to the most extreme drought actually increased their NEP because of the high evaporative demand. Such severe conditions during a single year could be expected to influence a forest over several following years. Reduced tree storage of carbohydrates leads to a changed carbon allocation pattern in spring that may affect both the woody growth and the resistance to pests. It is thus important to reveal the impact of such climatic events over a longer period. This study aims at assessing the carry-over effects of the extreme weather conditions on the carbon fluxes and the forest growth to the year after the event, 2019. The base of the analysis will be eddy covariance data combined with tree ring time series from measurement stations that has been shown to be significantly affected by the drought through reduced carbon fluxes: the spruce forests Hyltemossa and Skogaryd and the mixed forests Norunda, Svartberget, Soontaga and Rumperöd. The eddy covariance and tree ring data will be used to assess the forest ecosystem carbon fluxes and growth recovery in 2019 by comparisons to earlier normal years and extreme events.

The understanding of biophysical mechanisms influencing the spatial and temporal distribution of CO2 flux is important for predicting the response of forest ecosystem on any environmental changes. It has been shown, that the most important controlling factors responsible for CO2 flux fluctuation from the forest soils are soil moisture, temperature and the type of the forest stand. In our work, we present three years of soil CO2 flux measurements in the hemiboreal forest that is characterized by high spatial heterogeneity of vegetation and soil. Three sample plots that represent the main common tree species (Pinus sylvestris, Picea abies and Betula sp) were chosen to assess the influence of tree species composition on the soil CO2 flux. The chosen sample plots have clear microtopographical structure with depressions, elevations and flat zones. The data were collected from three sample plots according to forest floor microtopography using manual closed dynamic chamber equipped with IRGA sensor (The Vaisala GMP343 probe), humidity and temperature sensors (Vaisala HMP155). Obvious temporal resolution limitation of manual chamber method is compensated by higher spatial coverage.Previous research has indicated that one of the major sources of uncertainties in the flux estimation is the choice of the model for flux calculation. We compared the commonly used models (linear, exponential and HMR) using two available R packages: “gasflux” and “flux” packages. Additionally, we developed the algorithm that allows for automatically choosing the best model based on widely used criteria (MAE, RAE, AIC, RMSE).The results showed that in most of the cases linear and exponential models performed better. The comparison of sample plot showed that the biggest influence of microtopography was in the birch forest but the moisture had a bigger effect in the pine forest stand.

In our recent study in Global Change Biology (Li et al., ), we examined the relationship between solar-induced chlorophyll fluorescence (SIF) measured from the Orbiting Carbon Observatory-2 (OCO-2) and gross primary productivity (GPP) derived from eddy covariance flux towers across the globe, and we discovered that there is a nearly universal relationship between SIF and GPP across a wide variety of biomes. This finding reveals the tremendous potential of SIF for accurately mapping terrestrial photosynthesis globally.

Solar-induced chlorophyll fluorescence (SIF) has been increasingly used as a proxy for terrestrial gross primary productivity (GPP). Previous work mainly evaluated the relationship between satellite-observed SIF and gridded GPP products both based on coarse spatial resolutions. Finer resolution SIF (1.3 km × 2.25 km) measured from the Orbiting Carbon Observatory-2 (OCO-2) provides the first opportunity to examine the SIF-GPP relationship at the ecosystem scale using flux tower GPP data. However, it remains unclear how strong the relationship is for each biome and whether a robust, universal relationship exists across a variety of biomes. Here we conducted the first global analysis of the relationship between OCO-2 SIF and tower GPP for a total of 64 flux sites across the globe encompassing eight major biomes. OCO-2 SIF showed strong correlations with tower GPP at both midday and daily timescales, with the strongest relationship observed for daily SIF at the 757 nm (R2 = 0.72, p

Plants can respond to climate change by either migrating, adapting to the new conditions or going extinct. Relict plant species of limited distribution can be especially vulnerable as they are usually composed of small and isolated populations, which may reduce their ability to cope with rapidly changing environmental conditions. The aim of this study was to assess the vulnerability of Cneorum tricoccon L. (Cneoraceae), a Mediterranean relict shrub of limited distribution, to a future drier climate. We evaluated population differentiation in functional traits related to drought tolerance across seven representative populations of the species’ range. We measured morphological and physiological traits in both the field and the greenhouse under three water availability levels. Large phenotypic differences among populations were found under field conditions. All populations responded plastically to simulated drought, but they differed in mean trait values as well as in the slope of the phenotypic response. Particularly, dry-edge populations exhibited multiple functional traits that favored drought tolerance, such as more sclerophyllous leaves, strong stomatal control but high photosynthetic rates, which increases water use efficiency (iWUE), and an enhanced ability to accumulate sugars as osmolytes. Although drought decreased RGR in all populations, this reduction was smaller for populations from the dry edge. Our results suggest that dry-edge populations of this relict species are well adapted to drought, which could potentially mitigate the species’ extinction risk under drier scenarios. Dry-edge populations not only have a great conservation value but can also change expectations from current species’ distribution models., This study was supported by a JAE-predoc fellowship from the LINCGlobal. Funding was provided by the Spanish Ministry for Innovation and Science with the grants Consolider Montes (CSD2008_00040), VULGLO (CGL2010-22180-C03-03), PATSIMON (CGL2010-18759) and REMEDINAL III (CM-S2013/MAE-2719)