Within-tree variation in transpiration in isolated evergreen oak trees: evidence in support of the pipe model theory

Within-tree variation in sap flow density (SFD) was measured in two isolate d evergreen oak (Quercus ilex L.) trees growing in an oak savannah (dehesa) in southwest Spain. Sap flow was estimated by the constant heating method. Three sensors were installed in the trunk of each tree in three orientatio ns: northeast (NE), northwest (NW) and south (S). Sap flow density was moni tored continuously from May 18 to September 27, 1993. Daily values of SFD r anged between 500 and 4500 mm(3) mm(-2) day(-1). There were significant dif ferences in SFD between orientations; SFD was higher in the NE and NW orien tations than in the S orientation. These differences were noted on both a d aily and seasonal time scale, and were less pronounced on cloudy days and a t the end of the drought period, when SFD was relatively low. Our results s upport the idea that branches of trees can be viewed as a collection of sma ll independent plants.