Mayr Stefan, Hacke Uwe, Schmid Peter, Schwienbacher Franziska, Gruber Andreas
Institut für Botank, University Innsbruck, Sternwartestr. 15, A-6020 Innsbruck, Austria.
Ecology. 2006 Dec;87(12):3175-85. doi: 10.1890/0012-9658(2006)87[3175:fdicat]2.0.co;2.
Drought stress can cause xylem embolism in trees when the water potential (psi) in the xylem falls below specific vulnerability thresholds. At the alpine timberline, frost drought is known to cause excessive winter embolism unless xylem vulnerability or transpiration is sufficiently reduced to avoid critical psi. We compared annual courses of psi and embolism in Picea abies, Pinus cembra, Pinus mugo, Larix decidua, and Juniperus communis growing at the timberline vs. low altitude. In addition, vulnerability properties and related anatomical parameters as well as wood density (D(t)) and wall reinforcement (wall thickness related to conduit diameter) were studied. This allowed an estimate of stress intensities as well as a detection of adaptations that reduce embolism formation. At the alpine timberline, psi was lowest during winter with corresponding embolism rates of up to 100% in three of the conifers studied. Only Pinus cembra and Larix decidua avoided winter embolism due to moderate psi. Minor embolism was observed at low altitude where the water potentials of all species remained within a narrow range throughout the year. Within species, differences in psi50 (psi at 50% loss of conductivity) at high vs. low altitude were less than 1 MPa. In Picea abies and Pinus cembra, psi50 was more negative at the timberline while, in the other conifer species, psi50 was more negative at low altitude. Juniperus communis exhibited the lowest (-6.4 +/- 0.04 MPa; mean +/- SE) and Pinus mugo the highest psi50 (-3.34 +/- 0.03 MPa). In some cases, D(t) and tracheid wall reinforcement were higher than in previously established relationships of these parameters with psi50, possibly because of mechanical demands associated with the specific growing conditions. Conifers growing at the alpine timberline were exposed to higher drought stress intensities than individuals at low altitude. Frost drought during winter caused high embolism rates which were probably amplified by freeze-thaw stress. Although frost drought had a large effect on plant water transport, adaptations in hydraulic safety and related anatomical parameters were observed in only a few of the conifer species studied.
当木质部水势（ψ）降至特定脆弱性阈值以下时，干旱胁迫会导致树木木质部栓塞。在高山林线，已知霜旱会导致过度的冬季栓塞，除非木质部脆弱性或蒸腾作用充分降低以避免临界ψ。我们比较了生长在林线与低海拔地区的欧洲云杉、西伯利亚松、高山松、欧洲落叶松和刺柏的ψ和栓塞的年度变化过程。此外，还研究了脆弱性特性及相关解剖参数，以及木材密度（D(t)）和细胞壁强化（与导管直径相关的壁厚）。这使得能够估计胁迫强度，并检测出减少栓塞形成的适应性特征。在高山林线，冬季ψ最低，在所研究的三种针叶树中，相应的栓塞率高达100%。只有西伯利亚松和欧洲落叶松由于适度的ψ而避免了冬季栓塞。在低海拔地区观察到轻微栓塞，所有物种的水势全年都保持在狭窄范围内。在物种内部，高海拔与低海拔地区的ψ50（导水率损失50%时的ψ）差异小于1 MPa。在欧洲云杉和西伯利亚松中，林线处的ψ50更负，而在其他针叶树种中，低海拔处的ψ50更负。刺柏的ψ50最低（-6.4±0.04 MPa；平均值±标准误差），高山松的ψ50最高（-3.34±0.03 MPa）。在某些情况下，D(t)和管胞壁强化高于这些参数与ψ50先前确立的关系，这可能是由于与特定生长条件相关的机械需求。生长在高山林线的针叶树比低海拔地区的个体面临更高的干旱胁迫强度。冬季的霜旱导致高栓塞率，这可能因冻融胁迫而加剧。尽管霜旱对植物水分运输有很大影响，但在所研究的针叶树种中，仅在少数物种中观察到了水力安全性和相关解剖参数的适应性变化。
