Freezing-induced xylem cavitation and the northern limit of Larrea tridentata

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 degrees C, and that this caused no measurable cavitation for minimum temperatures above -7 degrees 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 degrees 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 degrees C and as long as temperatures were held above -11 degrees C there was no change in hydraulic conductivity after thawing. However, when stems were cooled to between -11 degrees C and -20 degrees C, stem hydraulic conductivity de creased linearly with minimum temperature. Minimum temperatures between -16 and -20 degrees 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.