Water extraction by tree fine roots in the forest floor of a temperate Fagus-Quercus forest

Water retention and water turnover were investigated in the forest floor of a temperate mixed Fagus-Quercus forest on poor soil in NW Germany. By field and laboratory measurements the aim was to quantify the water extraction by those tree fine roots that concentrate in the superficial organic layers. The 8-10.5-cm-thick organic profiles stored up to 45 mm of water under Quercus trees but significantly smaller amounts under Fagus (and even less under Pinus trees in a nearby stand). The water retention capacity (i.e. the difference between saturating water content after wetting and water content prior to wetting) and the resulting percolation rate out of the forest floor were measured by infiltration experiments in relation to their dependence on the initial water content of the humus material. The water retention characteristics of the humus material differed from the sandy mineral soil material by i) a much higher maximum water content (porosity), ii) a higher storage capacity for water in the plant-available water potential range, and iii) a marked temporal variability of the water retention capacity. A one-dimensional water flux model for the forest floor of this stand has been developed. According to the model results, the forest floor contributed 27 % (in summer 1991) or 14 % (in summer 1992) to the stand soil water reserves, and 37 % (summer 1991) or 28 % (summer 1992) to the water consumption of this stand. Water was turned over in the forest floor twice as fast as in the underlying mineral soil; however, fine roots in the mineral soil apparently extract more water per standing crop of root biomass and, thus, are thought to operate more economically with respect to the carbon cost of water uptake. ((C) Inra/Elsevier, Paris.).