Patterns of soil water depletion under coexisting oak and beech trees in a mixed stand
Soil moisture regimes were studied under two European beech trees (Fagus sylvatica L.) and two Durmast oak trees (Quercus petraea (Matt.) Liebl.) in a mixed oak-beech stand on the Lüneburger Heide, NW Germany. In the long run, oak is out-competed by beech in this stand. Water is among the factors that might influence competitive interaction between beech and oak. In the growing seasons 1990, 1991 and 1992, soil water matric potential was monitored with a large number of tensiometers which were installed at six soil depths and within four distances from the stem under the four trees. Troughfall and stemflow were recorded separately under the canopies of the two species. Overall patterns of matric potential fluctuations were similar under both oak and beech. Significant differences between the species only occurred in periods of extended drought in the summers of 1990 and 1992, when up to 100 hPa lower matric potentials were recorded in the soil profile under oaks. In the less drought-affected summer of 1991, however, no difference was visible. Since the oaks had a considerably larger leaf area per tree than the beeches, it is assumed that a higher water extraction rate under oak trees was responsible for the differences in depletion found in 1990 and 1992. Thus, in this stand, competitive superiority of Fagus exists despite its apparently lower capacity to extract water in periods of drought.