Sap flux density in 10 stems of a mature beech forest in the northeastern lowlands of Germany

Sap flux density was measured in 10 stems of a mature beech forest (Fagus sylvatica L.) in the northeastern lowlands of Germany during vegetation periods of years 2002 to 2005. The site represents an intensive monitoring core plot (DE1207) of the ICP Forests level II program (http://icp-forests.net/). The stand was growing on groundwater-distant, weakly degraded soil (Dystric Arenosol; FAO classification 1974). The humus type is moder. It was established in 1888 as a plantation under Scots pine (Pinus sylvestris L.) trees. Investigated trunks are described by stem diameter at breast height and tree height. Sap flow density of several depths of the trunks and data of the air temperature, air humidiy, global radiation, wind speed, and precipitation are given as hourly values. Xylem sap flux measurements performed to trees and tree stands allow differentiation of transpiration by different vegetation layers. The total transpiration of the canopy corresponds to the water flow through the trunk. Scaling from the sap flow measurement to stand transpiration is a two-step-procedure: firstly from sensor to tree and secondly from tree to stand. For the first step a radial profile of sap flow density is used. For scaling from tree to stand the structural inhomogeneity of the stand must be considered. The importance of this point is in even aged stands constricted, however, measurements should incorporate the differences of basal area and crown projected area of the trees.