Acoufreeze - Monitoring von Kältestress mittels Wellenanalyse von Schallemissionen

Climate change will affect the sustainability of forest ecosystems, which will result in strong ecological and economical repercussions. Mean temperature is expected to increase during the next century, which will lead to a reduction in frequency but not in intensity of frost events. Under these conditions, frost damages are very likely to increase, because paradoxically, the ability to withstand a deep frost event acquired gradually falls under the influence of higher temperatures. A major challenge for research is to provide relevant and operational criteria in order to identify genotypes more resistant to climatic hazards. Winter freezing tolerance is one of the key factors limiting survival and distribution of plants in many ecosystems. Freezing survival of plants involves tolerance of living tissues and of the nonliving water transport system (xylem). For living cells, damage and subsequent death may occur when intracellular water freezes or when cells dehydrate due to extracellular freezing of the sap. Plant xylem can be affected by freeze-thaw induced embolism as embolised conduits do not contribute to water transport. Before onset of the vegetation period, water transport has to be restored in all species by the development of new functional conduits and/or the refilling through active mechanisms, whereby living cells (cambium or vessel-associated cells) play a key role. Frost stress monitoring can be achieved with the electrolyte leakage test (LT50) for cells lyses and with quantification of the percent loss in hydraulic conductivity (PLC) for winter embolism. These two methods are rather laborious and time consuming. Recently, the Austrian Partner developed a new method for conifers to quantify drought induced PLC by analysis of waveform features of Acoustic Emission (AE). We see a high potential in the AE method for the analysis of frost damage both, in laboratory resistance tests and in field measurements on intact plants. The proposed project will focus on the analysis of frost resistance and monitoring of frost stress in a selected set of economically important European tree species. The study will be based on the AE technique with real time detection and wave form analysis of AE events, whereby wave form features should allow discriminating damage events in living cells and non-living tissues and the pinpointing of periods of severe frost stress online. A portable prototype for AE monitoring developed by the industrial partner will be used for monitoring of frost stress in natura. Based on these new methodical possibilities, (i) resistance to frost damage of selected species and a characterisation of AE signals related to the species` anatomy, (ii) visualisation of ice formation and water flow in freezing stems and (iii) frost damage in living cells and the xylem of trees growing at the alpine timberline as well as resistance characteristics of alpine tree species will be analysed. The promising combination of experimental and field studies is based on the cooperation of three partners complementing one another: Partner 1 (INRA, France) has long term experience in analysis of plant frost resistance, partner 2 (University Innsbruck, Austria) is a specialist for winter stress in alpine trees and ultrasonic measurements on trees and the industrial partner 3 (Mistras Group SA, France) will be responsible for technical developments. The new methodical approach and acquired data set on frost effects will help to improve future forest monitoring and management strategies and to optimise selection of forest trees and cultivars with high frost tolerance. It will enable new insights into the mechanisms of frost damage and resistance.

Freezing tolerance is one of the key factors limiting the survival and distribution of plants. The project Acoufreeze enabled important new insights into the mechanisms of freezing damage in plant xylem and respective counter strategies of plants. In freezing plants, ice formation can lead to damage and subsequent death of living cells when intracellular water freezes or when cells dehydrate due to extracellular freezing of sap. Ice can also interrupt the transport of water in plant xylem, and freezing potentially induces embolism, i.e. gas bubbles blocking the hydraulic system. Ultrasonic emission analysis has been used to monitor the formation of drought induced embolism, while the origin of ultrasonic acoustic emissions (UAE) on freezing was unclear. The project Acoufreeze focussed on the underlying mechanism of freezing induced damage and UAEs. It was based on cooperation between INRA Clermont-Ferrand, France the Department of Botany, University Innsbruck, Austria, the Mistras company, France and the University of Natural Resources and Life Sciences, Vienna, Austria. Experiments indicated that temperatures during freezing and thus the water potential of ice determine bubble formation (i.e. cavitation) and spreading, while bubble expansion and the resulting hydraulic dysfunction are related to thawing. UAEs enabled to detect this freezethaw- induced embolism as well as damage of xylem parenchyma. UAEs also demonstrated cavitation events near the growing ice front, allowed to estimate ice formation velocities and to detect ice in xylem via analysis of UAE velocity and attenuation. The dynamics of water fluxes, ice formation and cavitation events were also monitored with X-ray microtomography, magnetic resonance imaging and microdendrometers. In field measurements, the formation and repair of winter-embolism in timberline trees was analysed. A comparison on 11 European tree species showed embolism formation to be counterbalanced by active repair processes, and in Norway spruce changes in aquaporin patterns and uptake of water via branches was observed. Additional studies enabled insights into drought stress related aspects, the hydraulics of dwarf shrubs and root hydraulics. The project also led to a new analysis method based on the time-dependent rate of UAEs, and a prototype system for ultrasonic measurements under harsh winter conditions was developed. Outcomes of the project are important for our understanding of freezing in plants and how it limits plant physiology and life. This is of especial relevance in the context of climate change and thus of broad socio-ecological and economic relevance. Knowledge on the vulnerability and avoidance/repair mechanisms of plants under changing temperature and precipitation patterns has implications towards future management strategies in forestry and agriculture.