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How do earlywood, transition-wood and latewood deal with increasing xylem tension? Addressing this question is important to understand how hydraulic safety functions within the growth ring and therefore the role of latewood, earlywood and transition-wood in terms of drought resistance. We measured the conductive area on radial-longitudinal wood cuts of the last growth ring of eight selected 10 year-old Douglas firs. Trees were selected according to their vulnerability to cavitation (P50) measured with the air-injection technique. Vulnerable and resistant trees where characterized by mean P50 of -2.4 MPa and -3.4 MPa, respectively. In order to simulate increasing water demands (higher tension in xylem), the fresh wood samples were submitted to increasing positivepressure levels ranging from 0 to 4 MPa. Subsequently the conductive area was stained by introducing through the sample an aqueous colored solution (1 % Phloxine). Samples were then oven dried and polished; wood surface was scanned and resulting digitalized images were analyzed. We found significant differences in the conductive area between the three parts of the growth ring at all xylem pressures. The conductive latewood area was very small, even before pressure application, and decreased very rapidly with increasing pressure. Both earlywood and transitionwood conductive area started to decrease just above 2 MPa. However, transition-wood remained conductive under higher pressures than earlywood. Assuming that the decrease of the conductive area corresponds to the propagation of xylem cavitation, these results suggest that cavitation propagation would not follow a trend related to cell dimensions in Douglas-fir, but would rather follow a latewood-earlywood-transition-wood sequence.The conclusion discusses the functional and adaptive implications of these results.

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