Variations in the radial gradient of sap velocity in trunks of forest and fruit trees

We studied the radial pattern of sap velocity (v) in trunks of six forest species: Cupressus sempervirens L; Eucalyptus camaldulensis F. Muell; Pinus halepensis Mill. (Alleppo pine); Quercus aegilops L. ssp. ithaburensis [Decne] Boiss; Quercus calliprinos Webb; Quercus rotundifolia Lam. (Syn. Quercus ilex spp. Rotundifolia) and three fruit species: Citrus sinensis L. Osback; Malus domestica cv. Golden Delicious; Persea americana Mill. Data was selected from measurements made over a period of 15 years. For each species, at least data of 1–2 weeks measured on days with clear sky and after a rain event or irrigation were analyzed. Measurements were made with the Tmax heat pulse method, and six points were measured along the radius of the trunk. Two types of radial pattern were found; in one type the highest v value was recorded near the cambium, decreasing toward the heartwood; and the other type had low v near the cambium increasing toward a depth of 12 to 20 mm below the cambium and decreasing with depth to the heartwood. There was large interspecies variability of v in both the pattern of radial gradient with depth and the distance between the cambium and the border of conducting sapwood. Variations in radial pattern of v between trees within species were generally small with a coefficient of variation of 4–20%. The mean fractions of volumetric flow in the 0–16 and 0–24 mm layers below the cambium were 59% and 79% of the total flow, respectively. The distance between the cambium and the border between conducting and non-conducting sapwood varied from 31 to 66 mm in the various species. The radial distribution of sap velocity with azimuth around the trunk had a coefficient of variation similar to that found between trees of the same species. The amplitude of the azimuthal variation changed during the day; it was high in the mornings and evenings and low at noon. The variation of sap velocity in three azimuths was more affected by the structure of each individual trunk than by the position with respect to the sun.