Jutta Holst

Regional climate models predict an intensification of extreme heat waves in Central Europe. Against this background, the significance of human-biometeorologically orientated urban planning strategies is increas-ing by which the impairment of thermal comfort for people in cities in the future can be minimised. Such strategies require quantitative information on factors determining human thermal comfort within different urban quarters. With respect to these problems, the joint research project KLIMES funded by the German Federal Ministry of Education and Research was initiated. Its methodical approaches and objectives are pre-sented in this article. One part of KLIMES are experimental investigations on human thermal comfort within different urban street canyons, whose variable arrangement generally characterises urban quarters. The in-vestigations are conducted in Freiburg (SW Germany). The experimental design and the concept to analyse the measured data related to the objectives of K...

Das Meteorologische Institut der Universität Freiburg betreibt im Rahmen des Sonderfor-schungsbereiches (SFB) 433 „Buchendominierte Laubwälder unter dem Einfluss von Klima und Bewirtschaftung“ fünf mikrometeorologische Messstationen auf den gemeinsamen SFB- ...

The Nordic region was subjected to severe drought in 2018 with a particularly long-lasting and large soil water deficit in Denmark, Southern Sweden and Estonia. Here, we analyse the impact of the drought on carbon and water fluxes in 11 forest ecosystems of different composition: spruce, pine, mixed and deciduous. We assess the impact of drought on fluxes by estimating the difference (anomaly) between year 2018 and a reference year without drought. Unexpectedly, the evaporation was only slightly reduced during 2018 compared to the reference year at two sites while it increased or was nearly unchanged at all other sites. This occurred under a 40 to 60% reduction in mean surface conductance and the concurrent increase in evaporative demand due to the warm and dry weather. The anomaly in the net ecosystem productivity (NEP) was 93% explained by a multilinear regression with the anomaly in heterotrophic respiration and the relative precipitation deficit as independent variables. Most of...

The 2015 Paris Agreement encourages stakeholders to implement sustainable forest management policies to mitigate anthropogenic emissions of greenhouse gases (GHG). The net effects of forest management on the climate and the environment are, however, still not completely understood, partially as a result of a lack of long-term measurements of GHG fluxes in managed forests. During the period 2010–2013, we simultaneously measured carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O) fluxes using the flux-gradient technique at two clear-cut plots of different degrees of wetness, located in central Sweden. The measurements started approx. one year after clear-cutting, directly following soil scarification and planting. The study focused on robust inter-plot comparisons, spatial and temporal dynamics of GHG fluxes, and the determination of the global warming potential of a clear-cut boreal forest. The clear-cutting resulted in significant emissions of GHGs at both the wet and the dr...

<p>We present the Net Ecosystem Carbon Balance (NECB) of a Northern mire ecosystem for the period 2016-2019. The Mycklemossen peatland is located in the hemi-boreal region in the Southwestern part of Sweden and is classified as a fen with bog-like vegetation. The NECB was determined from eddy covariance (EC) measurements of carbon dioxide (CO<sub>2</sub>) and methane (CH<sub>4</sub>) and continuous water discharge measurements with biweekly measurements of dissolved organic carbon (DOC), dissolved inorganic carbon (DIC) and dissolved CH<sub>4</sub>. <br>We focus on the carbon dynamics of the Mycklemossen ecosystem during summer droughts and on its recovery during normal years. During 2016-2018, the annual precipitation was lower than the 30-year average while 2019 was a normal year in terms of weather conditions. 2018 sticks out as an extreme year with a severe drought and unusually high air temperature at Mycklemossen, as was the case in much of Northern and Central Europe.<br>The EC results indicate that Mycklemossen lost carbon during 2016-2018. While CH<sub>4</sub> emissions decreased, the mire became a strong source of CO<sub>2</sub> these years, especially 2018. There were also large losses of DOC during this period, which were further enhanced during 2019.</p>

<p>Arctic climate is warming twice as much as the global average, due to a number of climate system feedbacks, including albedo change due to retreating snow cover and sea ice, and the forest cover expansion across the open tundra. Northern ecosystems are known to emit trace gases (e.g., methane and volatile organic compounds, VOCs) to the atmosphere, from sources as diverse as soils, vegetation and lakes. These trace gas fluxes are likely to show a trend towards greater emissions with climate warming.</p><p>Here we report ecosystem-level VOC fluxes from Stordalen Mire, a subarctic peatland complex with a high fraction of open pond and lake surfaces, underlain by discontinuous permafrost and located in the Subarctic Sweden (68º20' N, 19º03' E).</p><p>In 2018, we deployed two online mass spectrometers (PTR-TOF-MS) to measure rapid fluctuations in VOC mixing ratios and to quantify ecosystem-level fluxes with the eddy covariance technique. One of the instruments obtained a growing-season-long dataset of biogenic emissions from palsa mire vegetation dominated by mosses (e.g., <em>Sphagnum</em> spp.), graminoids (such as <em>Eriophorum</em> spp. and <em>Carex</em> spp.), dwarf shrubs (e.g. Empetrum spp. and Betula nana) surrounding the ICOS Sweden Abisko-Stordalen long-term measurement station. The second instrument measured VOC fluxes during two contrasting periods (the peak and the end of the growing season) from a subarctic lake and its adjacent fen, permafrost-free, minerotrophic wetland with vegetation dominated by tall graminoids, mainly <em>Carex rostrata</em> and <em>Eriophorum angustifolium</em>.</p><p>At both sites, isoprene was the dominant VOC emitted by vegetation, showing clear diurnal patterns along the season and especially during the peak of the growing season in July. At the ICOS Sweden station, isoprene fluxes exceeded 2 nmol m<sup>-2</sup> s<sup>-1</sup> on several days in July, with a July monthly average midday emission of 1 nmol m<sup>-2</sup> s<sup>-1</sup>. The fen site showed average midday emissions of 2 nmol m<sup>-2</sup> s<sup>-1</sup> during the peak growing season. Other VOCs emitted by vegetation at both sites in July were, with decreasing magnitude, methanol, acetone, acetaldehyde and monoterpenes. In contrast, acetaldehyde and acetone were not emitted but…

<p>In 2018, North-Western Europe experienced very dry and warm summer. These conditions can have considerable effects on the functioning and greenhouse gas exchange of terrestrial ecosystems. Peat-forming wetlands, or mires, are a characteristic component of the North-European boreal landscape, and crucial for long-term carbon storage as well as for methane emission. We have analyzed the effect of the drought on greenhouse gas (GHG) exchange of five North European mire ecosystems in Sweden and Finland in 2018. The low precipitation and high summer temperatures in Fennoscandia led to a lowered water table in majority of the mires. This lowered both carbon dioxide (CO<sub>2</sub>) uptake and methane (CH<sub>4</sub>) emission during 2018, turning many of the mires from CO<sub>2</sub>sinks to sources during this year. The changes in methane emission and total GHG exchange, expressed as CO<sub>2</sub>equivalent, were significantly correlated with change in water table position. The calculated time-evolving radiative forcing due to the changes in GHG fluxes in 2018 showed that the drought-induced changes in GHG fluxes first resulted in a cooling effect lasting 15-50 years, due to the lowered CH<sub>4</sub>emission, which was followed by longer-term warming phase due to the lower CO<sub>2 </sub>uptake in 2018.</p><p> </p>

<p>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.    </p> <p>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.</p>

Based on human-biometeorological measurements at two different sites within a street canyon in the city of Freiburg (southwest Germany), the shading influence of the canopy of street trees on human thermal comfort was investigated for a typical Central European summer day ...

We developed a xylem water transport model for European beech (Fagus sylvatica L.) to scale up organ based processes to the tree level and to quantify their influence on the whole tree transpiration and the water uptake from the soil. To better consider the root-soil interaction a finite ...

Increasing heat will be a significant problem for Central European cities in the future. Shading devices are discussed as a method to mitigate heat stress on citizens. To analyze the physical processes, which are characteristic of shading in terms of urban human-biometeorology, experimental investigations on the thermal effects of shading by a building and shading by tree canopies were conducted in Freiburg (Southwest Germany) during typical Central European summer weather. Urban human-biometeorology stands for the variables air temperatureTa, mean radiant temperatureTmrt, and physiologically equivalent temperature PET, that is the human-biometeorological concept to assess the thermal environment which was applied. The measuring setup consists of specific human-biometeorological stations, which enable the direct or indirect determination ofTa,Tmrt, and PET. With respect to both shading devices, theTareduction did not exceed 2°C, while PET as a measure for human heat stress was lower...

ABSTRACT During the last decade several gas analysers became available that are capable of measuring methane concentration with high sampling frequency needed for eddy covariance measurements. These new gas analysers require less maintenance compared with the models used in the 1990's and they give more reliable estimates for the ecosystem scale methane fluxes. However, with different instrument types available now, their performance should be crosscompared and validated. A gas analyser intercomparison campaign was held at Cabauw measurement station in the Netherlands between 6th and 27th of June, 2012. The campaign was organized within the InGOS FP7 project. Cabauw is well-established site with a long history in greenhouse gas monitoring and the surrounding landscape is a considerable source of methane. In total eight methane gas analysers manufactured by Picarro Inc., Los Gatos Research, Aerodyne Research Inc. and LI-COR Inc. were used in the experiment. Tentative results show relatively good agreement between the eight methane flux estimates and they also agree with previous studies done at the site. Magnitude and variation of the flux estimates are similar. Cumulative methane emissions calculated from not gapfilled data during a 10 day episode agree within 10 %, values ranging from 190 mg(CH4) m-2 to 210 mg(CH4) m-2. Comparison of random errors of the measured methane fluxes did not reveal any big differences between the instruments. Some of the gas analysers measuring methane were also capable of measuring water vapour at the same time. This is a big asset during data processing, since effect of water vapour on methane concentration measurement can then be easily corrected without need of additional water vapour measurement. The presentation will discuss the intercomparison campaign setup, instrument performance and will provide recommendations for CH4-EC measurements.

Economic activities and everyday life may create weekly variations in concentrations of air pollutants in urban settings. The present study contributes to this experience on the example of two typical medium-sized towns in Central Europe, Szeged and Freiburg considering the following ...

... heat, shows the dominating significance of the radiation heat during the daylight hours on PET ... ties from the three-dimensional surroundings receiving the stationary site within the urban street canyon ... Despite their supposed simple form, real ur-ban street canyons have a certain ...

ABSTRACT This study deals with a current problem of urban human-biometeorology on the micro-scale, which becomes more important due to the future increase of severe summer heat in Central Europe. The impact of street design parameters on the thermal comfort of citizens is analysed in an experimental way for typical summer conditions in Central Europe. The investigation is focused on the behaviour of mean radiant temperature T(mrt) and physiologically equivalent temperature PET as the most important human-biometeorological variables for thermal comfort during these atmospheric conditions. To get quantitative results on how they depend on small-scale characteristics of urban street canyons, an investigation design is applied which is based on measurements of relevant meteorological variables-like air temperature T(a) - by specific humanbiometeorological measuring systems. They were conducted in selected street canyons within different urban quarters of Freiburg, the warmest city in Germany, from 2007-2009. T(mrt) and PET were calculated from the measured meteorological variables by well-tested approaches. The geometry of urban street canyons is characterised by (i) the sky view factor SVF determined from fish-eye photos, (ii) the ratio of building height H to street width W, (iii) the orientation to the sun, and (iv) the fraction of ctc (coverage by the street tree canopy). To eliminate the influence of slightly different weather conditions even on typical summer days, the results are not presented in form of absolute values for the human-biometeorological variables, but in form of ratios for the measured radiative flux densities and in form of differences for the measured and calculated temperatures. As the results for T(mrt) and PET should primarily quantify universal patterns of the impact of street design parameters on human thermal comfort, they are only presented as mean values for the period 10-16 CET. The main results obtained from different analyses are: (i) SVF for the southern half of the upper hemisphere (SVF90-270) is more suitable to characterise the sites with respect to the thermal perception of citizens than SVF for the whole upper hemisphere (SVF1-360), (ii) in contrast to wide E-W oriented street canyons, narrow E-W oriented street canyons have larger spatial differences in T(a), T(mrt) and PET, (iii) with respect to the orientation, these differences are larger in E-W than in N-S oriented street canyons, and (iv) an increase of ctc by 10 % leads to a decrease of T(a) by only 0.2 degrees C, but to a reduction of T(mrt) by 3.6 degrees C and of PET by 1.4 degrees C.

The (13)C isotopic signature (C stable isotope ratio; delta(13)C) of CO(2) respired from forest ecosystems and their particular compartments are known to be influenced by temporal changes in environmental conditions affecting C isotope fractionation during photosynthesis. Whereas most studies have assessed temporal variation in delta(13)C of ecosystem-respired CO(2) on a day-to-day scale, not much information is available on its diel dynamics. We investigated environmental and physiological controls over potential temporal changes in delta(13)C of respired CO(2) by following the short-term dynamics of the (13)C signature from newly assimilated organic matter pools in the needles, via phloem-transported organic matter in twigs and trunks, to trunk-, soil- and ecosystem-respired CO(2). We found a strong 24-h periodicity in delta(13)C of organic matter in leaf and twig phloem sap, which was strongly dampened as carbohydrates were transported down the trunk. Periodicity reappeared in th...

The carbon (δ(13)C) and oxygen (δ(18)O) stable isotope composition is widely used to obtain information on the linkages between environmental drivers and tree physiology over various time scales. The tree-ring archive can especially be exploited to reconstruct inter- and intra-annual variation of both climate and physiology. There is, however, a lack of information on the processes potentially affecting δ(13)C and δ(18)O on their way from assimilation in the leaf to the tree ring. As a consequence, the aim of this study was to trace the isotope signals in European beech (Fagus sylvatica L.) from leaf water (δ(18)O) and leaf assimilates (δ(13)C and δ(18)O) to tree-ring wood via phloem-transported compounds over a whole growing season. Phloem and leaf samples for δ(13)C and δ(18)O analyses as well as soil water, xylem water, leaf water and atmospheric water vapour samples for δ(18)O analysis were taken approximately every 2 weeks during the growing season of 2007. The δ(13)C and δ(18)...

Understanding ecosystem water fluxes has gained increasing attention, as climate scenarios predict a drier environment for many parts of the world. Evaporative enrichment of (18)O (Delta(18)O) of leaf water and subsequent enrichment of plant organic matter can be used to characterize environmental and physiological factors that control evaporation, based on a recently established mechanistic model. In a Pinus sylvestris forest, we measured the dynamics of oxygen isotopic composition (delta(18)O) every 6 h for 4 d in atmospheric water vapour, xylem sap, leaf water and water-soluble organic matter in current (N) and previous year (N-1) needles, phloem sap, together with leaf gas exchange for pooled N and N-1 needles, and relevant micrometeorological variables. Leaf water delta(18)O showed strong diel periodicity, while delta(18)O in atmospheric water vapour and in xylem sap showed little variation. The Delta(18)O was consistently lower for N than for N-1 needles, possibly related to p...

The air quality index LAQx (Long-term Air Quality Index), which has been recently developed to evaluate the long-term integral air quality related to well-being and health of people, was applied to analyse the evolution of air pollution from 1985 to 2005 at different urban and rural sites in SW Germany. The LAQx method including the air pollutants taken into account

The relationships between plant carbon resources, soil carbon and nitrogen content, and ectomycorrhizal fungal (EMF) diversity in a monospecific, old-growth beech (Fagus sylvatica) forest were investigated by manipulating carbon flux by girdling. We hypothesized that disruption of the carbon supply would not affect diversity and EMF species numbers if EM fungi can be supplied by plant internal carbohydrate resources or would result in selective disappearance of EMF taxa because of differences in carbon demand of different fungi. Tree carbohydrate status, root demography, EMF colonization, and EMF taxon abundance were measured repeatedly during 1 year after girdling. Girdling did not affect root colonization but decreased EMF species richness of an estimated 79 to 90 taxa to about 40 taxa. Cenococcum geophilum, Lactarius blennius, and Tomentella lapida were dominant, colonizing about 70% of the root tips, and remained unaffected by girdling. Mainly cryptic EMF species disappeared. Th...

... Page 2. 2 Anschrift der Autoren: Prof. Dr. Helmut Mayer, Dr. Dirk Schindler, Gerhard Fernbach und Dirk Redepenning, Meteorologisches Institut der Universität Freiburg Werderring 10, D-79085 Freiburg Phone: 0049/761/203-3590; Fax: 0049/761/203-3586 ...

We developed a xylem water transport model for European beech (Fagus sylvatica L.) to scale up organ based processes to the tree level and to quantify their influence on the whole tree transpiration and the water uptake from the soil. To better consider the root-soil interaction a finite ...

Based on human-biometeorological measurements at two different sites within a street canyon in the city of Freiburg (southwest Germany), the shading influence of the canopy of street trees on human thermal comfort was investigated for a typical Central European summer day ...