Winter at the alpine timberline. Why does embolism occur in Norway spruce but not in stone pine?

Conifers growing at the alpine timberline are exposed to frost drought and freeze-thaw cycles during winter-stress factors known to induce embolism in tree xylem. The two dominant species of the European Central Alps timberline were studied: Norway spruce (Picea abies [L.] Karst) and stone pine (Pinus cembra), which usually reaches higher altitudes. We hypothesized to find embolism only at the timberline and to observe less embolism in stone pine than in Norway spruce due to avoidance mechanisms. Seasonal courses of embolism and water potential were studied at 1,700 and 2,100 m during two winter seasons and correlated to vulnerability (to drought-induced embolism), leaf conductance, and micrometeorological data. Embolism was observed only at the timberline and only in Norway spruce (up to 49.2% loss of conductivity). Conductivity losses corresponded to low water potentials (down to -3.5 MPa) but also to the number of freeze-thaw events indicating both stress factors to contribute to embolism induction. Decreasing embolism rates-probably due to refilling-were observed already in winter. Stone pine did not exhibit an adapted vulnerability (50% loss of conductivity at -3.5 MPa) but avoided critical potentials (minimum -2.3 MPa): Cuticulare conductance was 3.5-fold lower than in Norway spruce, and angles between needles and axes were found to decrease in dehydrating branches. The extent of conductivity losses in Norway spruce and the spectrum of avoidance and recovery mechanisms in both species indicates winter embolism to be relevant for tree line formation.