Forests in a warmer world
Forests ecosystems cover about 30% of the global land area and are critical for climate regulation, land biodiversity, for providing fresh water, timber and non-timber products, and also for human well-being and security. The important role of forests for humankind is highlighted by the Sustainable Development Goals of the United Nations. As anthropogenic climate change is leading to unprecedented heating of the Earth’s surface, trees as long-lived and sessile organisms are challenged in adapting to these increasingly extreme conditions.
While a global trend in forest health has not been established yet, evidence is accumulating that tree death in response to hotter and drier conditions is increasing. Also, warming-induced changes of forest soil properties such as carbon storage capacity or the ability for water and nutrient retention are critical issues. This may have manifold consequences for humankind, including a possible acceleration of global warming due to declining carbon uptake rates combined with carbon losses from decaying forests and soils.
Yet, to date we are not able to fully evaluate these impacts, let alone to provide management recommendations in response to environmental change. To provide the necessary answers, we study forests along the soil-tree-atmosphere continuum and among others consider effects of land use change and forest management decisions.
Experiments: The underlying mechanisms
To derive a better understanding of the underlying mechanisms of forest responses to extreme events, we study the stress resilience of economically and ecologically important tree species in our research greenhouse facility. We address tree metabolic and hydraulic responses to elevated CO2, drought and heat stress, using highly-advanced online measurement systems and stable isotope techniques. A main focus is on non-reversible tipping-points and the ability of trees to recover and repair stress-induced damages.
Furthermore, we assess effects of different silvicultural management techniques and forest diversification on soil functions such as carbon uptake, water storage and nutrient supply in extreme weather experiments in the field and laboratory.
Observations: The bigger picture
To quantify climate change impacts on forests directly, we analyze time series of long-term measurements of soil and ecosystem gas exchange and apply remote sensing techniques. For instance, we use a deep-learning approach to detect dead trees from aerial pictures automatically. In addition, we are actively involved in international networks to address forest health at the global scale.
Models: Projections of forest responses
To provide guidance to forest managers and to assess climate change mitigation potentials, we develop and apply process-oriented simulation models. Model development preliminary includes a better representation of plant hydraulic processes, tree mortality and stress legacy. We also model forest-related land use change, and the underpinning behaviors of forest owners that affect land use decision-making in response to climate and socio-economic changes.
We apply three major modelling tools:
LandscapeDNDC is a model of high temporal resolution that is based on the description of physiological processes. It is typically applied at the ecosystem or stand scale. Forest development is represented based on plant cohorts, which are defined by species and average dimension. In forests, the interdependence of individual environmental condition, carbon gain, and changes of tree size and social status is thus directly addressed. Most of the major central European tree species have been successfully parameterized and evaluated.
LPJ-GUESS an advanced dynamic global vegetation model applied at larger spatial and temporal scales. The model can simulate structural, compositional and functional properties of ecosystems in the main climate zones of the Earth. With its sophisticated representation of growth dynamics and forest management, it has been successfully used to study the impacts of changes in forest dynamics on the land carbon sink.
CRAFTY is an agent-based model of land use change that has been applied in Europe, Sweden and the UK to explore the role of forest landowners in making decisions about forest land use and management. The model explores land use behavior and decision-making across a range of forest types and management strategies based on the supply and demand of a number of ecosystem services, including food and timber provision, carbon stocks and biodiversity.
Elevated CO2 does not improve stress resistance in a semi-arid pine species
We collected seeds in an Aleppo pine plantation close to the Negev desert and grew the plants from seeds either under ambient or highly elevated atmospheric CO2. Under optimal conditions the seedlings increased growth rates, but when drought and heat stress intensified the CO2 benefit vanished. At a threshold temperature of 35 °C both seedlings under ambient and elevated CO2 turned into a carbon source. Moreover, elevated CO2 did not enhance the ability of seedlings to survive drought stress. This indicates that increased atmospheric CO2 will rather not improve physiological stress resistance.
Gattmann M, Birami B, Nadal Sala D, Ruehr NK (2021) Dying by drying: Timing of physiological stress thresholds related to tree death is not significantly altered by highly elevated CO2. Plant, Cell & Environment 44 (2):356-370.
Birami B, Nägele T, Gattmann M, Preisler Y, Gast A, Arneth A, Ruehr NK (2020) Hot drought reduces the effects of elevated CO2 on tree water use efficiency and carbon metabolism. New Phytologist 226:1607-1621.
Drought-induced tree hydraulic impairment and recovery
Trees have several adaptive mechanisms to respond to water limitation. A prominent one in order to reduce water loss and dehydration is closure of stomata, little pores on the leaves where water evaporates and carbon dioxide is taken-up. If drought stress intensifies for instance due to co-occurring heat stress, vital processes can be critically harmed and trees could be damaged. Additionally, once drought is released, such impacts may result in a slow recovery or even the death of individuals. Integrating tree hydraulic processes and tissue damage into models thus provides an avenue to improve predictions of forest drought and stress legacy responses.
Ruehr, N., R. Grote, S. Mayr, and A. Arneth (2019), Beyond the extreme: recovery of carbon and water relations in woody plants following heat and drought stress, Tree Physiol., 39(8), 1285–1299.
Nadal-Sala D, Grote R, Birami B, Knüver T, Rehschuh R, Schwarz S, Ruehr NK (accepted) 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.
European beech forest diversification with coniferous trees increases soil carbon stocks and associated soil functioning
European beech forests provide important ecosystem services on marginal soils hardly suitable for agriculture, but these forests are particularly sensitive to drought events in a warming climate. Admixture of deep-rooting coniferous trees such as silver fir not only increases soil water availability in beech stands through hydraulic lift and redistribution of soil water. In addition, carbon sequestration is enhanced, leading to an improved soil greenhouse gas sink strength and key soil functions. Thus, beech forest diversification with coniferous trees enhances key ecosystem processes and could improve forest resilience to climate change.
Töchterle P, Yang F, Rehschuh S, Rehschuh Romy, Ruehr NK, Rennenberg H, Dannenmann M (2020) Hydraulic redistribution of water by sliver fir occurs under severe soil drought. Forests, 11(2),162. https://doi.org/10.3390/f11020162
Rehschuh S, Fuchs M, Tejedor J, Schäfler-Schmid A, Magh RK, Burzlaff T, Rennenberg H, Dannenmann M (2019) Admixing fir to European beech forests improves the soil GHG balance. Forests 10(3), 213.
Rehschuh S, Jonard M, Wiesmeier M, Rennenberg H, Dannenmann M (2021) Impact of European beech forest diversification on soil organic carbon and total nitrogen – a meta-analysis. Frontiers in Forests and Global Change.
Importance of management in forest adaptation to climate change
Modelling of forest land use change with the CRAFTY model shows that the impact of socio-economic change or variation in landowner behaviors cannot be neglected when addressing climate change impacts on forests. However, substantial differences were found in the competitiveness and coping ability of landowners implementing different management strategies through time. Generally, multi-objective management – for instance when forests are used for both timber production and recreation – was found to provide the best basis for adaptation. Across large regions, however, a combination of management strategies was better at meeting ecosystem service demands in the face of climate change.
Blanco, V., Holzhauer, S., Brown, C., Lagergren, F., Vulturius, G., Lindeskog, M. & Rounsevell, M.D.A. (2017). The effect of forest owner decision-making, climatic change and societal demands on land-use change and ecosystem service provision in Sweden. Ecosystem Services, 23, 174–208.
Blanco, V., Holzhauer, S., Brown, C., Vulturius, G. & Rounsevell, M.D.A. (2017). The importance of socio-ecological system dynamics in understanding adaptation to global change in the forestry sector. Journal of Environmental Management, 196, 36-47.
Head of Division
Links to external partners or other external projects:
- Center for Disaster Management and Risk Reduction Technology
- KIT Climate and Environment Center
- International Tree Mortality Network
- International Union of Forest Research Organizations
- Institute of Geography and Geoecology
- Swiss Forest Lab