The main focus of the DFG-funded Emmy Noether research group is to provide a detailed process understanding of carbon and water cycling in trees and forests exposed to varying stress scenarios, intensities and timing of stress. The main study object, Scots pine, one of the most widespread trees within Europe, has already shown significant decline in response to extreme drought events. We are particular interested in the underlying physiological responses including tree hydraulics, gas exchange, carbon allocation and growth during heat and drought stress and the ability to recover from stress. Based on experimental results and ecosystem observations we will improve a model framework (LandscapeDNDC) to assess pine forest responses to future climate extremes.

Tree recovery following stress release depends on critical stress impacts Ruehr et al (2019) Tree Physiology

As a consequence of climate change, the frequency of summer drought events is increasing worldwide. The combination of drought stress and high temperatures can cause lethal damage to trees. The dry summers in 2018 and 2019 have severely affected forests in large parts of Central Europe, and many million trees died, causing large financial losses to forest owners. To better understand the impacts of these extreme events on forests in Germany and Central Europe, we want to combine hyper-spectral satellite information with high-resolution imagery and forest monitoring data. This combined dataset will then be used to further investigate the environmental drivers that affect the spatial pattern of stress-induced forest damage. A refined spatial understanding of damage occurrence in relation to environmental factors is a prerequisite for model development and predictive analysis of forest health. This project is funded by the KIT Center for Disaster Management and Risk Reduction Technology . Birds-view of dead tree canopies identified using a deep-learning approach.

The interdisciplinary BioWaWi project aims to develop strategies for municipal water management that support biodiversity and ecosystem services. Our specific research within the project addresses water use and groundwater supply in representative ecosystems, and vulnerability of these ecosystems to changes in climate and water management. The LandscapeDNDC model will be initialized for different ecosystems within selected catchments, and site-specific water fluxes will be calculated for current and projected future climates. A specific objective is to evaluate how the performance and vulnerability of dominant tree species may change and how water and forest management could be adapted to meet ecosystem water demands.

We aim to evaluate microclimate and air pollution mitigation effects of urban trees in Karlsruhe, Germany under current and future climate conditions. A newly developed physiology-based model. will be applied which consider drought impacts on vegetation cooling as well as pollutant uptake by the plants. The work will potentially include the impact of more than 100.000 trees in Karlsruhe and surroundings and contributes to decision support about where, how many, and what species should be planted. This is a sub-project of the BMBF-funded research project Gruene Lunge.

The International Tree Mortality Network aims to bring together a multidisciplinary team of experts to quantify trends and causes of tree mortality globally with the aim to provide crucial information for forest managers and policymaking.

Climate benefits and feedbacks of semi-arid forests
In this joined DFG-funded project (2015-2019) with colleagues from the Weizmann Institute, we quantified responses of Aleppo pine trees to future, increasingly extreme conditions as predicted for the Negev region, where the Yatir forest, a large Aleppo pine plantation is growing. We focused on how the trees cope with heat waves and drought today and in the future under increased atmospheric carbon dioxide concentrations. We measured whole-tree gas exchange, emissions of volatile organic compounds and metabolites under controlled conditions. We found that heat and drought combined can kill seedlings due to overheating (Birami et al. 2018) and that dying seedlings could be identified at their unique VOC fingerprint (Birami et al. 2021). Further, we found no clear indications that elevated CO2 would mitigate hot drought stress responses in Aleppo pine seedlings (Birami et al 2020; Gattmann et al. 2020).

Stress-induced BVOC emissions
Biogenic volatile organic compounds (BVOCs) are emitted from plants and can be highly reactive altering atmospheric chemistry and air quality. In this Georg-Forster fellowship from the Humboldt Foundation to Dr. Elizabeth Gaona (2019-2021), emissions of volatile organic compounds from trees under heat and drought stress were investigated. In addition, the atmospheric oxidation of stress-related BVOC emissions was quantified in a cooperation with colleagues from the Forschungszentrum Juelich using the Saphir-PLUS chamber.

Publications	Nadine Ruehr - Professor Nadine received her PhD from ETH Zurich with a focus on soil-plant-atmosphere interactions. She was awarded an Emmy Noether research group and became Professor of Climatic Ecophysiology at the Institute of Geography and Geoecology in 2022. She strives to understand mechanisms of tree and forest responses to climate change with the aim to increase forest resilience to extreme events and integrates physiological, biogeochemical and ecological methods and modelling approaches.
Publications	Rüdiger Grote - Senior Scientist Rüdiger holds a PhD in Forestry from Göttingen University and received his habilitation on ‘Ecosystem Modelling’ from TU Munich. He has developed several ecosystem models that describe forest development based on plant-atmosphere exchanges and is an internationally known expert in simulating the emission of biogenic volatile organic compounds. His particular interest is in describing tree physiological responses to drought and their impacts on resource uptake, allocation, and senescence.
Publications	Daniel Nadal Sala - Postdoctoral researcher Daniel studied Biology and received a PhD in Ecology from the University of Barcelona. His research interest is on plant responses to abiotic stress including drought and heatwaves at different temporal and spatial scales. He focuses on training simulation models with data and improving model descriptions of tree responses to drought and mechanistical descriptions of tree mortality.
Publications	Rocco Pace - Postdoctoral researcher Rocco studied Forestry and Environmental Sciences and received a PhD in Natural Sciences from the Technical University of Munich. His research interests are in urban forest modeling and tree ecophysiology, with a focus on assessing ecosystem services to improve air quality and microclimate. He is integrating morphological and physiological traits of trees into model parameterization to provide appropriate tools for supporting urban forest management and planning.
Publications	Peter Petrík - Postdoctoral researcher Peter studied Forestry and received a PhD in Forest Phytology from Technical University in Zvolen. His research revolves around forest ecosystems under climate change with a focus on abiotic stresses on tree physiology. He applies sapflow and eddy-covariance methods to analyse carbon and water fluxes of forest ecosystems, and uses a process-based model to to estimate forest water demand under changing climate and management scenarios.
	Franklin Alongi - PhD student Franklin studied Plant Biology at Montana State University, and is interested in resource allocation in response to environmental stress. During his PhD, Franklin conducts greenhouse experiments to explore carbon use and physiological mechanisms of stress response in a variety of European tree species.
	Timo Knüver - PhD student Timo studied Geoecology at the University of Tübingen and is pursuing his PhD at the University of Innsbruck. His research addresses the drought recovery potential of Norway spruce and European beech trees. He studies drought hydraulic responses in mature trees and applies experiments under controlled conditions.
	Selina Schwarz - PhD student Selina studied Geoecology at the Karlsruhe Institute of Technology. Her research addresses environmental challenges through the application of remote sensing techniques. In her PhD she studies the impacts of recent summer droughts on European forests. She applies remote sensing and machine learning approaches to detect the environmental drivers of diffuse tree mortality.
	Yanick Ziegler - PhD student Yanick studied Physics and Environmental Physics. He is interested in climate change responses of forests and investigates plant-water relations in the context of droughts. In his PhD, he is applying field and lab measurements as well as modelling approaches to determine the relationship between tree water status and growth processes.
	Andreas Gast - Engineer Andreas studied Environmental Engineering in Augsburg. He is responsible for all technical issues around our scientific greenhouse facility including measurement automation. He has in-depth experience in designing and building environmentally controlled plant chambers for automatic gas exchange measurements.
	Anna Sontheim - Technician Anna studied Chemistry at the University of Heidelberg. She is responsible for the laboratory and has profound expertise in chemical analytics. She further supports experimental work in the greenhouse and plant cultivation.

Dr. Ines Bamberger	Postdoc – now at BayCEER
Dr. Elizabeth Gaona	Postdoc – now in Cordoba
Benjamin Birami	PhD Student – now Climate Protection Manager
Marielle Gattmann	PhD Student – now Climate Change Manager
Romy Rehschuh	PhD Student – now at TU Dresden
Andrea Jakab	Lab technician – now in Switzerland
Dr. Andre Duarte	MSc Student – now at U of Guelph, Canada
Mohitul Hossain	MSc Student
Benedict Spaan	MSc Student
Angelika Pointner	MSc Student – now at DB Netz AG
Johanna Schnurr	BSc Student
Tamara Wittmann	BSc Student

Responses of plants and trees to environmental change are studied in this high-tech greenhouse facility. The experimental results increase process understanding and provide input for developing model frameworks. The research facility consists of four individual compartments in which temperature, humidity, radiation, and atmospheric CO₂ concentrations can be easily manipulated. We have designed additional chamber systems to continuously measure CO₂ and H₂O exchange between plants and the atmosphere, including volatile organic compounds. Our latest development allows to determine the tree net carbon balance under changing conditions (right picture), stress-triggered tipping-points as well as carbon allocation using stable isotope techniques. These measurements are supplemented by other instrumentation including a leaf gas exchange system, ¹³C isotope laser, automated stem dendrometers, pressure chamber, cavicams, thermography, soil moisture as well as lab instrumentation to analyze carbohydrates or isotopes in plant samples.

Examples of controlled experiments. Shown are saplings of black locust in temperature-controlled pots (left) and our custom-designed tree chamber system (right).

Measurement results from field and greenhouse are used for the development of models that then serve to analyze the impact of environmental conditions on tree and forest dynamics. To bridge from tree physiological impacts to forest ecosystems we apply LandscapeDNDC a model system that includes modules of different complexity to describe soil decomposition processes, water fluxes and plant processes. We focus on the physiological-oriented plant module PSIM to represent tree or ecosystem related pools and fluxes such as photosynthesis, respiration, carbon and nitrogen allocation, as well as senescence. The model results in dynamic changes of pools (leaves, fine roots, living and dead wood) and dimensions (tree height, diameter, crown dimension, and rooting depth). Recently, we further developed PSIM to consider tree hydraulic processes. In particular, we address non-stomatal limitations of photosynthesis (NSL), the support of transpiration by internal water storage once stomata have closed and the loss of xylem and leaf tissue in relation to plant water potential.

Conceptual scheme of new hydraulic process description within PSIM to model drought stress impacts on tree physiology.

In order to evaluate ecosystem services provided by urban trees in cities such as Karlsruhe, we use the Tree4C model, a physiology-oriented tree model that runs based on individual tree structure (from tree inventories), environmental forcings and pollution data. The model addresses three different ecosystem services for individual trees: cleaning, cooling, and carbon sequestration. Cleaning refers to a species- and size-specific calculation of air pollutant deposition. Cooling is derived from tree transpiration and tree canopy shading. Tree carbon sequestration is based on photosynthetic CO₂ uptake and tree growth. The model is being further developed to consider damaging effects from stress, in particular drought, which can result in leaf senescence and tree mortality. An outstanding feature is that the model is able to quantify the ecosystem services of a large population of trees in cities like Munich or Karlsruhe based on the morphological and physiological characteristics of individual trees.

Zweifel R, Pappas C, Peters RL, Babst F, Balanzategui D, Basler D, Bastos A, Beloiu M, Buchmann N, Bose AK, Braun S, Damm A, D'Odorico P, Eitel JUH, Etzold S, Fonti P, Freund ER, Gessler A, Haeni M, Hoch G, Kahmen A, Körner C, Krejza J, Krumm F, Leuchner M, Leuschner C, Lukovic M, Martínez-Vilalta J, Matula R, Meesenburg H, Meir P, Plichta R, Poyatos R, Rohner B, Ruehr NK, Salomón RL, Scharnweber T, Schaub M, Steger DN, Steppe K, Still C, Stojanović M, Trotsiuk V, Vitasse Y, von Arx G, Wilmking M, Zahnd C, Sterck F (2023) Networking the forest infrastructure towards near real-time monitoring – A white paper Science of The Total Environment

Gattmann M, McAdam S, Birami B, Link R, Nadal-Sala D, Schuldt B, Yakir D, Ruehr NK (2023) Anatomical adjustments of the tree hydraulic pathway decrease canopy conductance under long-term elevated CO2 in Aleppo pine Plant Physiology

Rehschuh R, Rehschuh S, Gast A, Jakab A-L, Lehmann MM, Saurer M, Gessler A, Ruehr NK (2022) Tree allocation dynamics beyond heat and hot drought stress reveal changes in carbon storage, belowground translocation and growth. New Phytologist

Schuldt B, Ruehr NK (2022) Responses of European forests to global change-type droughts. Plant Biology

Petrík P, Grote R, Gömöry D, Kurjak D, Petek-Petrik A, Lamarque LJ, Sliacka Konôpková A, Mukarram M, Debta H, Fleischer P (2022) The role of provenance for the projected growth of European beech seedlings under climate change Forests

Pace R, Chiocchini F, Sarti M, Endreny TA, Calfapietra C, Ciolfi M (2022) Integrating Copernicus land cover data into the i-Tree Cool Air model to evaluate and map urban heat mitigation by tree cover European Journal of Remote Sensing

Hikino K, Danzberger J, Riedel VP, Hesse BD, Hafner BD, Gebhardt T, Rehschuh R, Ruehr NK, Brunn M, Bauerle TL, Landhäusser SM, Lehmann MM, Rötzer T, Pretzsch H, Buegger F, Weikl F, Pritsch K, Grams TEE (2022) Dynamics of initial C allocation after drought release in mature Norway spruce - Increased belowground allocation of current photoassimilates covers only half of the C used for fine-root growth. Global Change Biology

Mahnken M, Cailleret M, Collalti A, Trotta C, Biondo C, D’Andrea E, Dalmonech D, Marano G, Mäkelä A, Minunno F, Peltoniemi M, Trotsiuk V, Nadal-Sala D, Sabaté S, Vallet P, Aussenac R, Cameron DR, Bohn FJ, Grote R, Augustynczik ALD, Yousefpour R, Huber N, Bugmann H, Merganicova K, Merganic J, Valent P, Lasch-Born P, Hartig F, Vega del Valle ID, Volkholz J, Gutsch M, Matteucci G, Krejza J, Ibrom A, Meesenburg H, Rötzer T, van der Maaten-Theunissen M, van der Maaten E, Reyer CPO (2022) Accuracy, realism and general applicability of European forest models. Global Change Biology

Hornick T, Richter A, Harpole WS, Bastl M, Bohlmann S, Bonn A, Bumberger J, Dietrich P, Gemeinholzer B, Grote R, Heinold B, Keller A, Luttkus ML, Mäder P, Motivans Švara E, Passonneau S, Punyasena SW, Rakosy D, Richter R, Sickel W, Steffan-Dewenter I, Theodorou P, Treudler R, Werchan B, Werchan M, Wolke R, Dunker S (2022) An integrative environmental pollen diversity assessment and its importance for the Sustainable Development Goals. PLANTS, PEOPLE, PLANET

Havermann F, Ghirardo A, Schnitzler J-P, Nendel C, Hoffmann M, Kraus D, Grote R (2022) Modeling intra- and interannual variability of BVOC emissions from maize, oil-seed rape, and ryegrass. Journal of Advances in Modeling Earth Systems

Gaglio M, Pace R, Muresan AN, Grote R, Castaldelli G, Calfapietra C, Fano EA (2022) Species-specific efficiency in PM2.5 removal by urban trees: From leaf measurements to improved modeling estimates. Science of The Total Environment

Pretzsch H, del Río M, Grote R, Klemmt H-J, Ordóñez C, Oviedo FB (2022) Tracing drought effects from the tree to the stand growth in temperate and Mediterranean forests: insights and consequences for forest ecology and management. European Journal of Forest Research

Hikino K, Danzberger J, Riedel VP, Rehschuh R, Ruehr NK, Hesse BD, Lehmann MM, Buegger F, Weikl F, Pritsch K, Grams TEE (2022) High resilience of carbon transport in long-term drought stressed mature Norway spruce trees within two weeks after drought release. Global Change Biology

Ghirardo A, Blande JD, Ruehr NK, Balestrini R, Külheim C (2022) Editorial: Adaptation of Trees to Climate Change: Mechanisms Behind Physiological and Ecological Resilience and Vulnerability. Frontiers in Forests and Global Change

Brunn M, Hafner BD, Zwetsloot MJ, Weikl F, Pritsch K, Hikino K, Ruehr NK, Sayer EJ, Bauerle TL (2022) Carbon allocation to root exudates is maintained in mature temperate tree species under drought. New Phytologist

Rehschuh R, Ruehr NK (2021) Diverging responses of water and carbon relations during and after heat and hot drought stress in Pinus sylvestris. Tree Physiology

Preisler Y, Hölttä T, Grünzweig JM, Oz I, Tatarinov F, Ruehr NK, Rotenberg E, Yakir D (2021) The importance of tree internal water storage under drought conditions. Tree Physiology

Nadal-Sala D, Grote R, Birami B, Knüver T, Rehschuh R, Schwarz S, Ruehr NK (2021) Leaf shedding and non-stomatal limitations of photosynthesis mitigate loss of hydraulic conductance in of Scots pine saplings during severe drought stress. Frontiers in Plant Science

Nadal-Sala D, Grote R, Birami B, Lintunen A, Mammarella I, Preisler Y, Rotenberg E, Salmon Y, Tatrinov F, Yakir D, Ruehr NK (2021) Assessing model performance via the most limiting environmental driver (MLED) in two differently stressed pine stands. Ecological Applications

Birami B, Bamberger I, Ghirardo A, Grote R, Arneth A, Gaona-Colman E, Nadal-Sala D, Ruehr NK (2021) Heatwave frequency and tree death alter stress-specific emissions of volatile organic compounds in Aleppo pine. Oecologia

Nadal-Sala D, Medlyn BE, Ruehr NK, Ellsworth D, Barton C, Gracia C, Tissue D, Tjoelker MG, Sabaté S (2021) Increasing aridity will not offset CO2 fertilization in fast-growing eucalypts with access to deep soil water. Global Change Biology

Gattmann M, Birami B, Nadal-Sala D, Ruehr NK (2020) Dying by drying: timing of physiological stress thresholds related to tree death is not significantly altered by highly elevated CO2 in Aleppo pine. Plant, Cell and Environment

Bond-Lamberty et al. (2020) COSORE: A community database for continuous soil respiration and other soil-atmosphere greenhouse gas flux data. Global Change Biology 26 : 7268-7283

Joseph J, Decai G, Backes B, Bloch C, Brunner I, Gleixner G, Haeni M, Hartmann H, Hoch Gn, Hug C, Kahmen A, Lehmann MM, Li M, Luster Jr, Peter M, Poll C, Rigling A, Rissanen KA, Ruehr NK, Saurer M, Schaub M, Schönbeck L, Stern B, Thomas FM, Werner RA, Werner W, Wohlgemuth T, Hagedorn F, Gessler A (2020) Rhizosphere activity in an old-growth forest reacts rapidly to changes in soil moisture and shapes whole-tree carbon allocation. Proceedings of the National Academy of Sciences

Rehschuh R, Cecilia A, Zuber M, Farago T, Baumbach T, Hartmann H, Jansen S, Mayr S, Ruehr NK (2020) Drought-induced xylem embolism limits the recovery of leaf gas exchange in Scots pine Plant Physiology

Birami B, Naegel T, Gattmann M, Preisler Y, Arneth A, Ruehr NK (2020). Hot drought reduces the effects of elevated CO2 on tree water-use-efficiency and carbon metabolism New Phytologist

Toechterle P, Yang F, Rehschuh S, Rehschuh R, Ruehr NK, Rennenberg H, Dannenmann M (2020). Hydraulic redistribution of water by silver fir occurs under severe soil drought Forests

Schuldt B, Buras A, Arend M, Vitasse Y, Beierkuhnlein C, Damm A, Gharun M, Grams TEE, Hauck M, Hajek P, Hartmann H, Hilbrunner E, Hoch G, Holloway-Phillips M, Koerner C, Larysch E, Luebbe T, Nelson DB, Rammig A, Rigling A, Rose L, Ruehr NK, Schumann K, Weiser F, Werner C, Wohlgemuth T, Zang CS, Kahmen A (2020). A first assessment of the impact of the extreme 2018 summer drought on Central European forests Basic and Applied Ecology

Pascual-Benito M, Nadal-Sala D, Tobella M, Balleste E, Garcia-Aljaro C, Sabate S, Sabater F, Marti E, Gracia CA, Blanch A, Lucena F. (2020) Modelling the seasonal impacts of a wastewater treatment plant on water quality in a Mediterranean stream using microbial indicators Journal of Environmental Management

Sperlich D, Nadal-Sala D, Gracia C, Kreuzwieser J, Hanewinkel M, Yousefpour R (2020) Gains or loss in forest productivity under climate change? The uncertainty of CO2-fertilization and climate effects Climate

Trugman AT, Anderegg LDL, Wolfe BT, Birami B, Ruehr NK, Detto M, Bartlett MK, Anderegg WRL (2019). Climate and plant trait strategies determine tree carbon allocation to leaves and mediate future forest productivity. Global Change Biology 25(10): 3395-3405

Ruehr NK, Grote R, Mayr S, Arneth A (2019). Beyond the extreme: Recovery of carbon and water relations in woody plants following heat and drought stress. Tree Physiology 39(8): 1285-1299

Etzold S, Zieminska K, Rohner B, Bottero A, Bose AK, Ruehr NK, Zingg A, Rigling A (2019). One century of forest monitoring data in Switzerland reveals species- and site-specific trends of climate-induced tree mortality. Frontiers in Plant Science 10(307)

Zeeman M, Shupe H, Baessler C, Ruehr NK (2019). Productivity and vegetation structure of three differently managed temperate grasslands. Agriculture, Ecosystems and Environment 270:129-148

Birami B, Gattmann M, Heyer AG, Grote R, Arneth A, Ruehr NK (2018). Heat waves alter carbon allocation and increase mortality of Aleppo pine under dry conditions. Frontiers in Forests and Global Change doi.org/10.3389/ffgc.2018.00008

Klein T, Zeppel MJB, Anderegg WRL, Bloemen J, De Kauwe MG, Hudson P, Ruehr NK, Powell TL, von Arx G, Nardini A (2018). Xylem embolism refilling and resilience against drought-induced mortality in woody plants: processes and trade-offs. Ecological Research. 33:839-855. doi.org/10.1007/s11284-018-1588-y

Brauner K, Birami B, Brauner HA, Heyer AG (2018). Diurnal periodicity of assimilate transport shapes resource allocation and whole-plant carbon balance. The Plant Journal.doi:10.1111/tpj.13898

Hartmann H*, Moura CF*, Anderegg WRL*, Ruehr NK*, Salmon Y*, Allen CD, Arndt SK, Breshears DD, Davi H, Galbraith D, Ruthrof KX, Wunder J, Adams HD, Bloemen J, Cailleret M, Cobb R, Gessler A, Grams TEE, Jansen S, Kautz M, Lloret F, and O'Brien M (2018). Research frontiers for improving our understanding of drought-induced tree and forest mortality. New Phytologist, 218(1):15-28. doi:10.1111/nph.15048
*Contributed as first authors in equal parts to the manuscript.

Hartmann H, Schuldt B, Sanders TGM, Macinnis-Ng C, Boehmer HJ, Allen CD, Bolte A, Crowther TW, Hansen MC, Medlyn BE, Ruehr NK, and Anderegg WRL (2018). Monitoring global tree mortality patterns and trends. Report from the VW symposium - Crossing scales and disciplines to identify global trends of tree mortality as indicators of forest health. New Phytologist 217(3):984-987. doi:10.1111/nph.14988

Bamberger I, Ruehr NK, Schmitt M, Gast A, Wohlfahrt G, and Arneth A (2017). Isoprene emission and photosynthesis during heat waves and drought in black locust. Biogeosciences 14 (15):3649-3667 doi:10.5194/bg-14-3649-2017

Rehschuh R, Mette T, Menzel A, and Buras A (2017). Soil properties affect the drought susceptibility of Norway spruce. Dendrochronologia, 45:81-89. doi:10.1016/j.dendro.2017.07.003

Duarte AG, Katata G, Hoshika Y, Hossain M, Kreuzwieser J, Arneth A, Ruehr NK (2016). Immediate and potential long-term effects of consecutive heat waves on the photosynthetic performance and water balance in Douglas-fir. Journal of Plant Physiology, 205:57-66 doi:10.1016/j.jplph.2016.08.012

Ruehr NK, Gast A, Weber C, Daub B, Arneth A (2016). Water availability as dominant control of heat stress responses in two contrasting tree species. Tree Physiology, 36(2):164-178 doi:10.1093/treephys/tpv102

In vivo visualization of drought-induced embolisms blocking water transport in pine saplings measured at the TOPO-TOMO beamline of the KIT-ANKA synchrotron.