Acoufollow – freezing induced xylem dysfunction and repair

Freezing stress is the main factor limiting plant distribution at high latitude and elevation. Climate change will not only lead to changes in precipitation patterns and earlier snow melt but also to more climate extremes, which may cause increased winter drought and frost stress in woody plants. Frost has been shown to affect living and dead wood tissues and to impair their hydraulic function by induction of embolism. Embolism (air bubbles enclosed in the wood tissue) blocks water transport and thus impairs the water supply to the crown. Plants have to cope with such hydraulic limitation by repairing dysfunctional wood or forming new wood tissue. Based on the project Acoufreeze, which focused on the processes and dynamics during ice formation in the wood, the project Acoufollow aims at a better understanding of how plants overcome wood dysfunction following freezing stress. The repair of freezing induced embolism will be studied in six contrasting model species (acer, larch, pine, juniper, mountain rose, mountain ash) both in controlled temperature chamber experiments and in the field. At high elevation field sites, monitoring under natural conditions and in snow manipulation experiments are planned. Analyses are based on ultrasonic acoustic emission and stem diameter measurements, wood core measurements and various complementary methods (wood pressure probes, infrared thermography, hydraulics, micrometeorology etc.) . Here, the use of ultrasonic and stem diameter systems will be most challenging with respect to harsh winter conditions. Experiments, numerical simulations and field monitoring are expected to unravel the complex spatio-temporal dynamics of wood pressure during freeze-thaw cycles and embolism recovery. Monitoring and snow manipulation approaches will enable to analyse embolism formation and repair under highly constrained environmental conditions and facing various snow depth situations. In consequence, relevant climate parameters will be linked to multi-decadal growth responses of study species to enable projections of future critical conditions for wood dysfunction and repair. The project is based on a cooperation between UMR PIAF (INRA-Université Clermont Auvergne, Clermont-Ferrand, France), Geolab (CNRS-Université Clermont Auvergne, Clermont-Ferrand, France), the Department of Botany (University of Innsbruck, Austria) and Department of Botany (BOKU University, University of Natural Resources and Life Sciences, Vienna, Austria). The close cooperation of involved partners, which contribute expertises in different methodical and scientific aspects, will enable new insights into the underlying processes during and after freezing of wood and its relevance for plants under changing climate. Results will help to better understand freezing in study species and its relevance to plant life at high elevation, but also improve our knowledge of freezing tolerance and resilience of plants in general.

Frost stress is a major factor limiting the distribution of plants at high latitudes and elevations. Climate change will lead not only to changes in precipitation patterns and earlier snowmelt, but also to more climate extremes, which may lead to increased winter drought and frost stress in woody plants. In wood, frost can affect living and dead tissues and impair their hydraulic function by inducing embolism. Embolism (air bubbles trapped in the wood tissue) blocks water transport and thus impairs the water supply to the crown, forcing plants to cope with this hydraulic limitation by repairing dysfunctional wood or forming new wood tissue. The project "Acoufollow", a collaboration of the Department of Botany, University of Innsbruck, Austria and UMR PIAF as well as Geolab, Clermont-Ferrand, France, investigated how plants can overcome these wood dysfunctions after frost stress. The repair of frost-induced embolism was studied in six broadleaved and conifer species (Acer, Larch, Pine, Juniper, Mountain Rose, Mountain Ash), both in controlled temperature chamber experiments and in field studies, including monitoring under natural conditions and snow manipulation experiments. The analyses were based on measurements of ultrasonic acoustic emissions, stem diameter variations, wood anatomy, hydraulic analyses and various complementary methods (vitality assessment, wood pressure probes, micrometeorology etc.). Experiments at the alpine tree line showed relevant damage to plants in the absence of a protective snow cover. Affected trees showed drought stress, conductivity losses, cell damage, reduced growth and, in some cases, even dieback. In contrast, snow removal in spring did not affect the plants. A comparison of juvenile trees and shrubs revealed that shrub species were more affected by snow cover reductions. This is probably due to differences in the hydraulics of shrubs and trees, as shown in a literature review. Recovery from winter damage depended on the degree of conductivity loss and cell damage in the wood for all species. Further experiments by the French cooperation partners demonstrated pressure changes in the wood during ice formation and the effects on water reservoirs after the frost period. These measurements also allowed the development of a model for freezing processes in the wood. Furthermore, dendroecological measurements were carried out on Pinus uncinata and Rhododendron at sites in France and Tirol to analyse the effects of extreme frost events. Studies showed that frost is a relevant stress factor for woody plants and that climate change is causing an increase in stress intensity. Observed species-specific responses are therefore key to our understanding of plant frost tolerance and resilience and thus to estimating future developments under changing climate conditions.