Large, declining beech (Fagus sylvatica L.) trees (diameter at breast height = 50 cm), growing on heavy clay soils in the highlands near Zurich, Switzerland, were amply irrigated in late summer. During irrigation, the xylem sap flow rate, Q(wt), was measured by the stem-tissue heat balance method with internal heating and sensing. Only a gradual and slight increase in Q(wt) in response to irrigation was observed in the control trees, whereas Q(wt) in the declining trees, whose transpiration rates were only 2-20% those of the control trees, increased 2-5 times within minutes. This suggests, that severe local drought was the major factor limiting tree growth at the site. The extent of the response permits estimation of the supply-limited (soil water) and demand-limited (tree structure) components of stress. Drought caused a decline in Q(wt) in the trees with short crowns and limited root systems that had originally been growing in dense canopies and had become suddenly exposed to full illumination as a result of a severe wind storm and thinning. Trees with deep, narrow, dense crowns, growing in more open places and adapted over a long period to high irradiance remained healthy during drought. Prolonged, periodic water shortage reduced the amount of foliage up to 90% but during drought stimulated the growth of fine roots in the surface and upper soil layers. The stem conductive systems of the declining trees were still partially functional.