Embolien in Nadelbäumen an der alpinen Waldgrenze

Research poject P 13782 Embolism in conifers at the alpine timberline Helmut BAUER 28.06.1999 Water transport in land plants occurs under negative tension caused by transpiration and transmitted via continuous water columns to the soil (cohesion theory). Cavitation of these water columns and the resulting embolism lead to reduced hydraulic conductance by blockage of xylem conduits and thus may be a real threat to the survival of a tree. Cavitations are caused by air seeding from an adjacent air filled conduit or by freeze-thaw events. There is evidence that the dominance of conifers in boreal and alpine habitats is a result of their hydraulic properties. With respect to the climatic conditions of alpine winter the following hypothesis can be defined for conifers growing at the alpine timberline: I) Extreme climatic conditions in winter (frost-drought, freeze-thaw events) cause cavitation in the xylem of trees. II) The short alpine, growing season is further limited by the essential reactivation of water transport through refilling of embolised conduits and/or growth of new xylem in spring. III) Alpine conifers are adapted to cavitation stress: Woodanatomical properties decrease probability of cavitation, effective recovery mechanisms enable reactivation of water transport. According to this hypothesis field measurements on Picea abies and Pinus cembra trees (periodic measurements of degree of embolism, detailed analysis of critical weather situations, observation of recovery of water transport in spring), analysis of altitudinal transects as well as transfer experiments with potted plants are planned. Based on these investigations probability and mechanisms of cavitation events, altitude and genetic disposition dependent adaptations in vulnerability and dynamics and mechanisms of recovery will be analysed. The proposed project will improve our understanding of the survival of trees at the alpine timberline and enable a detailed causalanalytic interpretation of the significance of cavitation events in this fragile habitat.

In conifers growing at the Alpine timberline, excessive conductivity losses in the water transport system during winter were detected. These low conductivities were caused by entry of air bubbles into the wood leading to so- called embolism. In this project causes, extent and seasonal dynamics of embolism as well as adaptation mechanisms of trees growing at the alpine timberline were analysed. Furthermore, the pattern of embolism and hydraulic conductivity within trees were studied. Highest embolism rates of up to 100% were observed in twigs of Norway spruce. This was caused by winter- drought and by freeze-thaw events. Winter-drought occurs because the frozen soil and stem block water supply, while high wind speeds and intensive radiation cause water losses from needles. Freeze-thaw events are numerous at the timberline as twigs reach positive temperatures during sunny days and freeze at night. Drought as well as freeze-thaw events were already known to induce embolism, but it was the first time that freeze-thaw induced embolism in conifers was demonstrated. Embolism was found to be unequally distributed within trees based on differences in stress intensities as well as in the woods resistance to embolism. Even within year rings of twigs, different resistance to drought induced embolism was observed. Several adaptations to avoid or repair winter-embolism were observed. Stone pine, the Alpine tree species growing at highest altitudes, reduces water losses and therefore drought stress by an effective transpiration protection (also by decreasing the angle between needles and axis). Stone pine is also more resistant to freeze-thaw induced embolism than Norway spruce. For the latter an adapted vulnerability to drought-induced embolism at high altitudes was demonstrated, furthermore its leader shoot has an increased resistance. Nevertheless, these adaptations were not sufficient to avoid embolism in Norway spruce. On the other hand, this species was found to be able to refill its transport system. This fascinating refilling mechanism is the aim of a follow-up project. The presented study enabled the analysis of new aspects related to the fields of timberline research and plant water physiology. The exclusive occurrence of massive conductivity losses at the timberline indicated winter-embolism to be a relevant factor influencing the altitudinal limit of the life form "tree".