Winter at the alpine timberline causes complex within-tree patterns of water potential and embolism in Picea abies.

Winter temperatures at the alpine timberline cause ice formation in the xylem of conifers blocking water uptake as well as water shifts within the axes system. This amplifies drought stress that, in combination with freeze-thaw events, causes embolism. This study focussed on within-tree patterns of water potential (psi) and embolism in Norway spruce [Picea abies (L.) Karst.]. At five sampling dates in midwinter, psi was determined at numerous positions in the crown of three trees, and at the end of March, the extent of embolism in representative sections of the axes system was analysed. Until 14 March, mean psi decreased to -3.77 +/- 0.11 MPa with less negative psi in exposed crown parts. On 30 March, psi was -1.60 +/- 0.06 MPa, while loss of conductivity reached up to 100%. Conductivity losses increased with exposition and were highest in the smallest tree. The observed complex within-tree patterns of psi and embolism were caused by ice blockages and differences in stress intensities within the xylem. High conductivity losses despite moderate psi in exposed crown parts indicated freeze-thaw events to be a major inducer of winter embolism. Tree size may play a critical role for winter water relations as trees profit from water stored in the stem and in crown parts below the snow cover.