OPERATING INSTITUTE: Karlsruhe Institute of Technology, Institute for Meteorology and Climate Research, Atmospheric Environmental Research (IMK-IFU) -  Garmisch - Partenkirchen.

MAIN PURPOSE: Biosphere-atmosphere-hydrosphere exchange processes.



LOCALISATION: 48.30259237681369 11.082000732421875

FACILITIES: The Höglwald is a Norway spruce-dominated forest of about 370ha surrounded by farmland in the hilly land-scape of Southern Bavaria, approx. 70km north of the Alps and 40km west of Munich at 11° 5' E and 48°18' N. The Höglwald site (560m asl) has been operated by IMK-IFU perma-nently since 1993.
With more than a decade of data, Höglwald forest holds the world record for the longest detailed dataset on biosphere-atmosphere exchange of greenhouse gases.
The site is equipped with 2 fully automated, remotely controlled measuring and data acquisition systems in standard containers for continuous quantification of net exchange of trace gases (CO2, N2O, CH4, NOx) at the soil-atmosphere interface in high temporal resolution using static and dynamic chamber techniques. These use a tower (50m height) for quantification of net ecosystem exchange of CO2, H2O and sensible heat.
Within the ExpeER TA scheme, new chambers for measuring GHG emissions from tree stems have been developed, mainly N2O. They have been used in a field study and stem fluxes were compared to soil fluxes measured by the automatic measuring system.

CONTACT: R. KIESE (This email address is being protected from spambots. You need JavaScript enabled to view it.) - R. GASCHE (This email address is being protected from spambots. You need JavaScript enabled to view it.)

●  Luo, G.J., Bruggemann, N., Wolf, B., Gasche, R., Grote, R., Butterbach-Bahl K. (2012) Decadal variability of soil CO2, NO, N2O, and CH4 fluxes at the Hoglwald Forest, Germany. Biogeosciences: 9, 1741-1763. DOI: 10.5194/bg-9-1741-2012.
●  Gundersen, P., Christiansen, J. R., Alberti, G., Bruggemann, N., Castaldi, S., Gasche R., Kitzler, B., Klemedtsson, L., Lobo-do-Vale, R. Moldan, F., Rutting, T., Schleppi, P., Weslien, P., Zechmeister-Boltenstern, S. (2012) The response of methane and nitrous oxide fluxes to forest change in Europe. Biogeosciences: 9: 3999–4012. DOI: 10.5194/bg-9-3999-2012.
●  Van Oijen, M., Cameron, D. R., Butterbach-Bahl, K., Farahbakhshazad, N., Jansson, P. E., Kiese R., Rahn, K. H., Werner, C., Yeluripati, J. B. (2011) A Bayesian framework for model calibration, comparison and analysis: Application to four models for the biogeochemistry of a Norway spruce forest. Agricultural and forest meteorology: 151: 1609–1621. DOI: 10.1016/j.agrformet.2011.06.017.


TA PROJECTS: Climate Change impact on tree growth (CC-Imp)

ExpeER TA Site: Hesse, FRANCE; Achenkirch, AUSTRIA; Klausenleopoldsdorf, AUSTRIA; Zöbelboden, AUSTRIA; Hoglwald Forest, GERMANY; Eifel, GERMANY
TA User (visit): Pierluigi Bombi, CNR-IBAF, Italy (March, 2015 – 05 days)
Project Description: Recent Climate Change is recognized as one of the main threat to natural ecosystems (Walther et al., 2002; Parmesan and Yohe, 2003; Root et al., 2003). Climate Change influences several aspects of species biology, such as physiology (e.g. Valentini et al., 2000), phenology (e.g. Peñuelas and Filella, 2001), and distribution (e.g. Kelly and Goulden, 2008). Species range shifts are due to changes in habitat suitability across space, which drive species to move towards those areas where climate continue to fulfill the species niche (Wiens et al., 2009). Species distribution models are often used for predicting future range shifts under climate change scenarios and adapting conservation strategies (Guisan and Zimmermann, 2000).
Nevertheless, the validation of species distribution models is difficult in dynamic scenarios and their real applicability is still debated (e.g. Araújo et al., 2005). In addition, in order to plan effective measures of impact mitigation, an instrument for detecting the real effects of climate change on natural ecosystem is required. We hypothesize that the spatial pattern of dynamic response (as tree growth) to climate change can allow to disentangle multiple effects of different drivers (e.g. habitat alteration, pollution, climate change), providing a key for validating models and monitoring ecosystem dynamics. In particular, in sites close to the front border of a shifting range, the environmental conditions are becoming more suitable than in the past. Therefore, we expect that in these sites tree growth has speeded up during the last years. On the contrary, in sites close to the rear border, the environment is less suitable than in the past and we can expect that tree growth has slowed down during the last years. Similarly, sites far from both front and rear borders are as suitable as in the past and we can expect that tree growth is rather constant. The detection of this spatial pattern of growth trends can confirm the model outcomes and highlight the occurring impact.
The aim of this project is to test our hypothesis and to set up a method for identifying climate change effect on forest ecosystems. To do this, we will use three species (i.e. Abies alba, Picea abies, and Fagus sylvatica) with different distributions and environmental requirements. On the basis of their current distributions, we will predict their range shifts due to climate change and we will verify whether their growth is following our expected spatial pattern. If we will find the expected pattern of growth for the three species, we will obtain at the same time a strong field-based validation for our models and a ring of alarm for European forests impacted by climate change.

Gradient measurements of NO concentration in soil pore space (GraMeNO)

TA User (visit): Ute Skiba, Natural Environment Research Council, Centre for Ecology and Hydrology - CEH Edinburgh, UK (February / March, 2015 – 40 days)
Project Description: Recently we have completed two TA projects investigating the importance of soil NO emissions during cold time periods and its contribution to the annual budget. The “Wintertime emission of Nitric Oxide from Soil (WTENOS, P64)” project provided evidence of regularly occurring winter NO emissions with some large emission peaks, confirming previous reported data by Laville et al. (2011) and Yao et al., (2010). The contribution of cold time NO emissions was significant and contributed ca. 29% to the annual NO emission budget for Höglwald forest during the period 1994-2010. In the second TA project “Relationships between soil N2O and NO Emission under Freeze-Thaw Events (REFTE; P77)” we have demonstrated strong positive relationships between NO and N2O during freeze-thaw events. Moreover we have indicated steady correlation of both gases (NO and N2O) with CO2 fluxes, which suggests a heterotrophic origin of both N-gases. However, occasionally there were strict time delays of thawing induced NO emission peaks occurring ca. 2 days after the N2O peak, usually following periods a prolonged subzero temperatures. The processes for this delay may be of physical rather than biochemical nature, due to a gradual accumulation of N2O during the frozen period, where most of produced NO was trapped and converted into N2O underneath the ice film (Maljanen et al., 2009; Jefferies et al., 2010; Yanai et al., 2011). The data showed that during melt all accumulated N2O was rapidly released and both gases could be production and released (without ice obstacle) driven mainly by temperature and soil moisture. The data demonstrated that the soil moisture content in the appropriate layers could have negative effects on NO emission under very wet conditions. Also atmospheric pressure may be important in soil gas release to the atmosphere. Despite the progress we achieved in the WTENOS and REFTE TAs we still have no direct data of NO concentrations in the soil pore space at different depths neither during the cold nor the rest of the year to explain emission rates. It is widely reported that the top few cm of a soil are responsible for NO emissions (e.g., Ludwig et al., 2001; Schreiber et al., 2012; Pilegaard, 2013; Medinets et al., 2015) and those emissions seem to be cumulative originated by various processes (Medinets et al., 2015). However the answer to a seemingly simple question “why is only shallow soil layer responsible for NO emissions?” has not yet been answered. Possible causes could be related to 1) a NO concentration gradient at the soil atmosphere interface; 2) easier NO release from surface than deeper soil layers, 3) O2 concentration dependence, 4) favorable conditions for biochemical processes, 5) cumulative effect of several/all listed factors.
To address the above points we will develop a high resolution system for in situ soil gradient NO concentration measurements and apply it at Höglwald Forest. The main objective is to develop and install an in situ high resolution system for gradient NO concentration monitoring; study the levels of NO concentrations in soil pore spaces at different depths during the cold periods in winter/early spring. In addition the main abiotic parameters, such as soil temperature and soil moisture will be measured in each layer. We are considering the possibility to monitor O2 concentration in the soil pore space air, provided the equipment can be made available. The date will determine which soil layers are responsible for NO production and consumption. Knowledge of the O2 and moisture levels would indicate the dominant microbial processes responsible for NO production within each layer. These data are important for model development and refinement, such as Landscape DNDC. The data generated in this TA project are novel and will result in a publication to be submitted to a high impact journal.

FLUxes of greenhouse gases from tree STems and the influence of TREE species composition (FLUSTTREE).

TA User (visit): Agustin Rubio, Universidad Politécnica de Madrid, SPAIN (June, 2012).
Project Description: The main objective of the project is to investigate stem GHG (main focus on N2O but also CH4) emissions in the managed Höglwald forest on beech (Fagus sylvatica) stands of different ages: 1-3 year: 15 years, 100 years.
We will take advantage of the intensive investigation in relation to soil-atmosphere GHG exchanges which has been taking place at the experimental site since approximately 15 years. Therefore, a particular focus will be given to the links between soil and stem GHG exchange and underlying processes. We hypothesize that higher GHG concentrations in the soil atmosphere will lead to higher GHG emissions by both soils and plants. That would mean that, in situations when high GHG emissions from the soil are measured - by conventional soil chambers -, total ecosystem fluxes might still be underestimated, if stem emissions are not taken into account.
Given that the assumed mechanism of the GHG emissions from stems is thought to be: (1) dilution in soil water, (2) uptake of soil water by the roots and transportation through the phloem (3) release by diffusion and transpiration, we hypothesize that the release of GHG from plant stems will decrease with increasing height of the plant organ or plant part. Finally, since photosynthetic activity determines nutrient flow across organ parts, we hypothesize that light will play a role in the N2O and CH4 emissions, being higher during day which may could cause a lack time between soil and plant emissions.
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Soil organic carbon changes under different forest regimes (CaDiF).

TA User (visit): Tomasso Chiti, Univeristy of Tuscia/DIBAF, Italy (September, 2013 – 10 days).
Project Description: Forest management can actively induce changes in tree species composition, which have subsequent implications on the greenhouse gas (GHG) balance of forest areas. Despite carbon (C) dynamics in soils can be slow and changes in the stock detectable only after 5-10 years from a disturbance, soils are significantly influenced by forest management and the type of vegetation cover. Different forest species can induce substantial changes in the C inputs via a different amount of litter depositions and fine roots dynamics, affecting directly the SOC stock in both organic horizon and mineral soil.
The main objective of the project is to determine the variations in the SOC stock as a result of a different forest management in beech (Fagus sylvatica) and Norway spruce (Picea abies) stands included in the Höglwald forest. Both stands were planted 12 years ago following a clear cut of the former Norway spruce forest. A mature spruce forest stand will be used as a control. We will take advantage of the historical data on SOC concentrations and stocks, available from the different investigations which were and are carried out in the study area. Consequently we will evaluate the variations occurred as a consequence of the different forest management determining the actual SOC concentrations and stocks in the different stands.
A special focus will be given to the link between the SOC stock and its distribution in pools of different stability/liability. A SOC fractionation based on the magnetic susceptibility of the soil will be done in this purpose. This will allow to assess the differences in the SOC pools distribution related to the forest species composition.
The Höglwald forest represents an unique situation where to assess the SOC dynamics in relation to forest species composition, having a strictly controlled, documented and investigation-oriented management as well as a solid record of historical information on the GHG fluxes and SOC stocks that are fundamental to understand the C cycle in forest ecosystems.

Wintertime Emission of Nitric Oxide from Soil (WTENOS).

TA User (visit): Ute Skiba, Natural Environment Research Council, Centre for Ecology and Hydrology - CEH Edinburgh, UK (March/April, 2014 – 30 days).
Project Description: Rewetting of dry soils and thawing of frozen soils have similar short-term effect increasing the soil water availability that lead to rehydrate cells, soil nutrient mobilization and stimulating of metabolic activity of dormant or senescent microbial community (Kieft et al., 1987; Schimel and Clein, 1996; Kemmitt et al., 2008; Kim et al., 2012). Whereas the magnitude of such effects could depend on soil properties, vegetation type and climate condition (Balser and Firestone, 2005; Vargas et al., 2010). It is well known that during/after rewetting periods high NO emission peaks have been observed for different ecosystems (Guenzi et al., 1994; Hutchinson and Brams, 1992; Wu et al., 2010; McCalley and Sparks, 2008) and could contribute up to 80% of annual emission. However data regarding thawing effect on NO flux is still limited (Kim et al., 2012) mainly due to lack of continuous data sets covering whole year including winter period and due to complexity of field measurements under negative temperature and snow cover. At present time only sporadically, on an occasional basis, published data, regarding some evidence of NO emission during freezing/thawing periods could be found. Clear evidence of NO flux rising following thawing has been reported from cropland in France (Laville et al., 2011) and from laboratory incubation of four types of soils in Mongolia (Yao et al., 2010). Recently occasionally wintertime NO peaks were observed in Southern Ukraine during ECLAIRE project measurements (Medinets et al., unpublished data) as well as the same pattern was found after screening of Höglwald forest stand data (Kiese, pers. comm.). The main objective of this project is to investigate retrospectively the evidence, frequency, magnitude and driving factors of wintertime NO peaks as well as a significance for annual budget, analysing NO emission, soil environment and climatic long-term dataset of Höglwald site, which is the only unique research-monitoring station with continuous data set (including wintertime periods) covering more than 15 years of intensive investigations regarding soil-atmosphere exchange. Furthermore based on that knowledge we plan to develop and potentially conduct targeted laboratory/field experiments for further improvement of process understanding driving peak NO winter emissions across different ecosystem types. We suggest that despite of freeze-thaw events usually occur occasionally the contribution to annual NO flux magnitude could be significant and winter period of investigation should not be neglected.

Relationships between soil N2O and NO Emission under Freeze-Thawing Events (REFTE).

TA User (visit): Ute Skiba, Natural Environment Research Council, Centre for Ecology and Hydrology - CEH Edinburgh, UK. July 2014 (1st stage) and September 2014 (2nd stage).
Project Description: Recently we have completed the TA project “Wintertime emission of Nitric Oxide from Soil” (WTENOS, P64) and confirmed the evidence (Laville et al., 2011; Yao et al., 2010) and importance of cold time NO emission in forest and arable sites. From our first assessment of 16 years of high resolution wintertime soil NO efflux data we know that contribution of cold time NO emission is significant with a contribution of up to 29% to the annual NO emission budget. Although wintertime soil temperature explained a larger part of the variation in NO efflux, peaks in NO efflux occurred during freeze-thaw events and very likely after snowmelt. Due to current lack in snow data, the latter however remains speculative. We further found evidence that NO and N2O emission patterns overlap during distinct cold season periods. Surprisingly, up to date there are no published data regarding this challenge and focuses on the joint dependence of N2O and NO emissions during freeze-thaw events. This is mainly due to lack of continuous data sets which include winter periods and due to the complexity of field measurements at sub-zero temperatures and snow cover as well as simultaneously measurements of N2O and NO trace gases. It is well known that thawing frozen soils increases the availability of soil water content, thereby rehydrating cells, mobilizing soil nutrients and triggering metabolic activity of dormant or senescent microbial communities (Kieft et al., 1987; Schimel and Clein, 1996; Kemmitt et al., 2008; Kim et al., 2012). The magnitude of such effects seems to depend on the duration of the thaw period, rate of temperature increase, soil properties and climatic conditions (Balser and Firestone, 2005; Vargas et al., 2010) and thereby affect processes involved in the consumption and production of N trace gases such as NO and N2O. Therefore, during winter, we assume to find correlations but maybe also offsets between N2O and NO peak emissions, triggered by freeze/thaw or snowmelt.
In the proposed project we will use the recently generated high resolution dataset on NO fluxes in combination with those available N2O raw data for selective representative years, which have to be re-proceeded in high resolution during that project. The main objective is to investigate retrospectively the evidence, frequency, magnitude and driving factors of cold time N2O emission peaks and find relationships and determine offset with NO emission peaks in order to get an idea of possible interactions between of NO and N2O production and consumption during winter. We will proceed, refine, combine and analyse N2O and NO emissions, soil environmental and climatic long-term datasets from the Höglwald site, which is the only and thus unique research-monitoring station with continuous N2O and NO flux data sets covering more than 16 years. Furthermore, we plan to further validate already analysed data, especially focusing on temperature and soil moisture data. We will compare and gapfill our temperature and rainfall data and add snow data from nearby research stations or weather stations. Our first TA project “Wintertime Emission of Nitric Oxide from Soil” enabled us to generate a basic dataset of high resolution NO fluxes. By analyzing the improved climate data and the new long-term high frequency soil NO and N2O efflux data as proposed in this follow up project, we are very confident to come up with important findings which can be published in a high impact journal. The project will deliver also highly relevant data to further develop and refine the process descriptions of LandscapeDNDC for simulation of freeze-thaw driven NO emissions.