Pockman W T, Sperry John S
Department of Biology, University of Utah, Salt Lake City, UT 84112, USA fax: 801-581-4668; e-mail:
Oecologia. 1996 Dec;109(1):19-27. doi: 10.1007/s004420050053.
We investigated the occurrence of freezing-induced cavitation in the evergreen desert shrub Larrea tridentata and compared it to co-occurring, winter-deciduous Prosopis velutina. Field measurements indicated that xylem sap in L. tridentata froze at temperatures below c. -5°C, and that this caused no measurable cavitation for minimum temperatures above -7°C. During the same period P. velutina cavitated almost completely. In the laboratory, we cooled stems of L. tridentata to temperatures ranging from -5 to -20°C, held them at temperature for 1 or 12 h, thawed the stems at a constant rate and measured cavitation by the decrease in hydraulic conductivity of stem segments. As observed in the field, freezing exotherms occurred at temperatures between -6.5 and -9°C and as long as temperatures were held above -11°C there was no change in hydraulic conductivity after thawing. However, when stems were cooled to between -11°C and -20°C, stem hydraulic conductivity decreased linearly with minimum temperature. Minimum temperatures between -16 and -20°C were sufficient to completely eliminate hydraulic conductance. Record (>20 year) minimum isotherms in this same range of temperatures corresponded closely with the northern limit of L. tridentata in the Mojave and Sonoran deserts.
我们研究了常绿沙漠灌木三齿拉瑞阿（Larrea tridentata）中冻融诱导空穴化现象的发生情况，并将其与共生的冬季落叶植物绒毛牧豆树（Prosopis velutina）进行了比较。野外测量表明，三齿拉瑞阿的木质部汁液在约-5°C以下的温度时会结冰，而对于最低温度高于-7°C的情况，这并未导致可测量的空穴化现象。在同一时期，绒毛牧豆树几乎完全发生了空穴化。在实验室中，我们将三齿拉瑞阿的茎冷却至-5至-20°C的温度范围，保持该温度1或12小时，以恒定速率解冻茎，并通过茎段水力传导率的降低来测量空穴化现象。正如在野外观察到的那样，在-6.5至-9°C之间的温度时出现了结冰放热现象，并且只要温度保持在-11°C以上，解冻后水力传导率就没有变化。然而，当茎冷却至-11°C至-20°C之间时，茎的水力传导率随最低温度呈线性下降。-16至-20°C之间的最低温度足以完全消除水力传导。在相同温度范围内的记录（>20年）最低等温线与莫哈韦沙漠和索诺兰沙漠中三齿拉瑞阿的北界密切对应。
