At the timberline in the Central Alps, climatic conditions during winter frequently cause excessive drought stress (frost drought, 'Frosttrocknis'), which we hypothesized to induce cavitation in trees. We investigated the extent of winter-embolism in Norway spruce (Picea abies (L.) Karst.) growing near the timberline and analysed adaptations in vulnerability and anatomy. We found conductivity losses of up to 100% at the highest elevation (2020 m) correlated with low water potentials down to - 4.0 MPa. Vulnerability thresholds (50% loss in conductivity) decreased from - 3.39 MPa at 800 m to - 3.88 MPa at 1600 m corresponding to a decrease in tracheid cross-sectional area as well as pit and pit pore diameters. These thresholds were lower than potentials measured in embolized twigs near the timberline at the sampling dates probably due to lower potentials and/or a role of freeze-thaw events earlier in winter. Data indicated refilling processes, which may be of particular relevance for trees at the timberline, since adaptations in drought-induced vulnerability failed to prevent winter-embolism.

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 Ϫ3.5 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 refillingwere observed already in winter. Stone pine did not exhibit an adapted vulnerability (50% loss of conductivity at Ϫ3.5 MPa) but avoided critical potentials (minimum Ϫ2.3 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.During the winter season, trees growing at the alpine timberline have to withstand conditions extremely unfavorable for plant water status. Water supply is permanently blocked because soil and stem are frozen, on the other hand, the shoot is exposed to water losses ("Frosttrocknis"; e.g. Michaelis, 1934;Pisek and Larcher, 1954;Larcher, 1972;Tranquillini, 1980) and to frequent freeze-thaw events (Gross et al., 1991).Drought and freeze-thaw cycles are known to induce the formation of gas bubbles in the water transport system of trees. This "embolism" interrupts the transmission of negative pressure to the soil and subsequently the flow of water through xylem conduits ("cohesion theory"; e.g. Boehm, 1893;Dixon and Joly, 1894;Richter, 1972;Jackson and Grace, 1994). Drought stress leads to high tensions in the water columns causing entry of air bubbles (air seeding) from adjacent air-filled conduits through the pits (e.g. Zimmermann, 1983;Tyree et al., 1994). Vulnerability analysis revealed species-specific water potential () thresholds for the onset of cavitation, whereby conifers were found to be very resistant due to their special pit anatomy (see e.g. Sperry and Tyree, 1990;Cochard, 1992;Jackson et al., 1995;Brodribb and Hill, 1999). Freeze-thaw events induce embolism because air is not soluble in ice-remaining gas bubbles can expand during thawing and lead to cavitation. However, this effect was reported to be of minor importance in conifers (e.g. Sucoff, 1969;Ro...

Freezing and thawing lead to xylem embolism when gas bubbles caused by ice formation expand during the thaw process. However, previous experimental studies indicated that conifers are resistant to freezing-induced embolism, unless xylem pressure becomes very negative during the freezing. In this study, we show that conifers experienced freezing-induced embolism when exposed to repeated freeze-thaw cycles and simultaneously to drought. Simulating conditions at the alpine timberline (128 days with freeze-thaw events and thawing rates of up to 9.5 K h(-1) in the xylem of exposed twigs during winter), young trees of Norway spruce [Picea abies (L.) Karst.] and stone pine (Pinus cembra L.) were exposed to 50 and 100 freeze-thaw cycles. This treatment caused a significant increase in embolism rates in drought-stressed samples. Upon 100 freeze-thaw cycles, vulnerability thresholds (50% loss of conductivity) were shifted 1.8 MPa (Norway spruce) and 0.8 MPa (stone pine) towards less negative water potentials. The results demonstrate that freeze-thaw cycles are a possible reason for winter-embolism in conifers observed in several field studies. Freezing-induced embolism may contribute to the altitudinal limits of conifers.

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We investigated to what extent south-exposed leaves (E-leaves) of the evergreen ivy (Hedera helix L.) growing in the shadow of two deciduous trees suffered from photoinhibition of photosynthesis when leaf-shedding started in autumn. Since air temperatures drop concomitantly with increase in light levels, changes in photosynthetic parameters (apparent quantum yield, Φ i and maximal photosynthetic capacity of O2 evolution, Pmax; chlorophyll-a fluorescence at room temperature) as well as pigment composition were compared with those in north-exposed leaves of the same clone (N-leaves; photosynthetic photon flux density PPFD< 100 μmol · m(-2) · s(-2)) and phenotypic sun leaves (S-leaves; PPFD up to 2000 μmol · m(-2) · s(-1)).In leaves exposed to drastic light changes during winter (E-leaves) strong photoinhibition of photosynthesis could be observed as soon as the incident PPFD increased in autumn. In contrast, in N-leaves the ratio of variable fluorescence to maximum fluorescence (FV/FMm) and Φ i did not decline appreciably prior to severe frosts (up to -12° C) in January. At this time, Φ i was reduced to a similar extent in all leaves, from about 0.073 μmol O2 · μmol(-1) photons before stress to about 0.020. Changes in Φ i were linearly correlated with changes in fv/fm (r = 0.955). The strong reduction in FV/FM on exposure to stress was caused by quenching in FM. The initial fluorescence (F0), however, was also quenched in all leaves. The diminished fluorescence yield was accompanied by an increase in zeaxanthin content. These effects indicate that winter stress in ivy primarily induces an increase in non-radiative energy-dissipation followed by "photoinhibitory damage" of PSII. Although a pronounced photooxidative bleaching of chloroplast pigments occurred in January (especially in E-leaves), photosynthetic parameters recovered completely in spring. Thus, the reduction in potential photosynthetic yield in winter may be up to three times greater in leaves subjected to increasing light levels than in leaves not exposed to a changing light environment.