Une nouvelle méthode de mesure du flux de sève brute dans le tronc est utilisée ici pour étudier les transferts hydriques chez le Douglas. Les variations azimutales du flux de sève ont été mesurées pendant 23 jours selon trois orientations et n’ont pas montré de forte hétérogénéité des transferts d’eau. Les variations verticales du flux de sève ont ensuite été analysées à deux hauteurs : base du tronc et base de la couronne vivante. A ce niveau (8 m), si la densité de flux est toujours supérieure à celle de la base du tronc, les flux totaux sont sensiblement égaux pour un arbre bien alimenté en eau. Un dessèchement imposé à l’arbre fait apparaître progressivement un écart de flux entre les deux hauteurs mesurées, correspondant à une déplétion du « réservoir tronc », d’environ 28 litres, soit 4,2 mm. La réhydratation met en évidence deux phases : une réponse rapide de l’arbre qui débute quelques minutes après le début de l’apport d’eau : il s’agit de la recharge partielle des tissus élastiques de l’arbre. La deuxième phase correspond à la réhydratation progressive du bois d’aubier du tronc et dure environ 15 jours.

Water transfers within Douglas-fir trunks are studied through a new method of sap flow measurement. Two trees growing in a forest stand were submitted to artificial drought by suppression of incident rainfall. There were no significant azimuthal differences between sap flow measured during 23 days (fig. 1), along three axis. Azimuthal differences were more important during sunny days, but were always less than 6 p. 100. Sap flow were then measured at two different heights : buttress (1 m) and base of the living crown (8 m). For a well-watered tree, sap flow density was always greatest at the base of the crown. Total flows (estimated by product of sap flow density and sapwood cross sectional area) were however equal notwithstanding vertical level of measurement (see fig. 2). The decrease of sap flow during an induced drought was compared with a model of potential transpiration (see eq. 4). Reduction of transpiration starts when the mean water potential in the first 60 cm of soil was - 0,15 Mpa. Differences in total sap flow according to level of measurements increased progressively during the drought period (fig. 4). The difference between inflow at the lowest level (1 m) and outflow at the highest (8 m) represents the variation in the trunk water reserve (fig. 5), mainly located in the sapwood. Total water loss of 30 days amounted to approximately 28 liters, i.e. 4,2 mm. The estimation of total water loss by the tree is 41 liters, i.e. 6,1 mm. Such amounts of exchangeable water are reported in the litterature. Minimal predawn water potential decreased as low as - 1,25 MPa. Rehydration occured in two phases : first, there occured a refilling of living tissues, starting a few minutes after watering as shown at figure 6. It also can be observed through a rapid increase of tree circumference. Thereafter, rehydration of the sapwood taked place progressively during 15 days following watering. Figure 5 shows a 4-5 days delay between watering and restoration of the sapwood water reserves. These observations confirm the theory of two water storage systems of differing capacities and time constants.