Winter-embolism in conifers: effects and recovery

In a previous study (FWF P13782-BIO, "Ecological significance of winter-embolism in conifers (Picea abies L. Karst., Pinus cembra L.) at the alpine timberline") winter-embolism was found to be a typical and frequent phenomenon in conifers growing at the alpine timberline. Thereby, gas bubbles lead to blockage of the water transport system (xylem) caused by frost-drought and possibly by freeze-thaw events. Although trees at the timberline exhibit adaptations to prevent embolism, extreme conductivity losses were observed in several species. This indicates winter-embolism to be a relevant factor influencing tree life or even survival at the timberline ecotone. In the proposed project, effects of embolism and recovery processes will be studied: We expect impairments of water status, photosynthesis and growth caused by embolism and on the other hand, potent refilling mechanisms and formation of new xylem to reduce these negative effects. To prove this hypothesis, investigations under natural and experimentally manipulated conditions on the level of young trees, twigs and adult conifer species (Picea abies (L.) Karst, Pinus cembra L., Pinus mugo Turra) will be combined: Pressure collar experiments will enable to study effects as well as refilling processes under controlled conditions. It is planned to develop a cavitation chamber for artificial induction of embolism in stems of adult trees. Additional field experiments with manipulation of naturally embolised twigs (e.g. removing of bark) will give insights into conditions favourable for refilling. Furthermore, dynamics of refilling processes and xylem formation will be studied under natural conditions at the alpine timberline. The proposed project will complete our knowledge about the importance of winter-embolism in alpine tree species and allow an estimation of its relevance for the formation of timberline. Furthermore, investigated conifers may be ideal model plants to study refilling, an important but not yet understood process in vascular plants.

Trees at the alpine timberline are exposed to frost drought and frequent freeze-thaw events during winter. In this project, we demonstrated that these stress factors lead to extreme losses in hydraulic conductivity in the axes system of several conifers. We analysed the formation of embolism, its effects for tree life and repair mechanisms. According to classical theory, freeze-thaw events induce embolism because gas bubbles formed during freezing expand during the thawing process. In contrast, we observed ultrasonic activity, which indicates embolism formation, only during freezing. We hypothesised that the ice formation may induce embolism. This mechanism is of importance for trees at the timberline as up to 115 freeze-thaw cycles were counted in the wood of twigs during one winter season. The number of freeze-thaw events, the duration of ice blockages in the wood and transpirational water losses vary within trees leading to within-tree patterns in drought stress and embolism. We mapped these patterns and found tree size to be critical for tree water relations in late winter. Tall trees store a lot of water in their axes system, which can be distributed within the tree when ice blockages melt. In late winter, we also observed the onset of repair processes: all conifers which exhibited embolism rates during winter were able to refill their water transport system. This fascinating process occurred although no water uptake from the frozen soil was possible and the wood was still under drought stress. Field and laboratory experiments indicated that a water uptake over the twig surface supports refilling and that snow packs at the twigs may play a role. The repair of embolism is important as negative effects on sap flow, water status and stomatal conductance of artificially cavitated trees could be demonstrated. Without this repair mechanism, trees entering the vegetation period would not survive. High altitude forests have essential protective function in all alpine regions but our knowledge about aspects limiting tree life is still poor. Our project contributed to the understanding of this ecologically and socio- economically important system. In addition, the extreme conditions at the "model system timberline" enable the analysis of general aspects of plant water relations.