The irrigation experiment was situated in the Rhone Valley near Leuk (Valais, Switzerland, 46°18′ N, 7°37′ E, 615 m a.s.l.) in a Scots pine (P. sylvestris) forest with occasional interspersed pubescent oak (Q. pubescens). Permission for the field experiment was issued by the forest service of the canton Wallis (CH) (Kantonaler Forstdienst, Kreis Oberwallis, Kantonsstrasse 275, 3902 Brig-Glis). Additionally, the permission for use of the forest for research purpose was approved by the owner of the forest, the Burgerschaft Leuk (http://www.burgerschaft-leuk.ch). The geological properties are dominated by gravel input from the Rhone river and from the Illgraben alluvial cone. A more pristine pedogenic event was the landslide from Siders. The mean annual precipitation measured in Sion (20 km) was 518 mm and the mean annual temperature 10.7°C from 2003 to 2012 [48] . The irrigation experiment had 8 plots (25 × 40 m) of which four were randomly chosen for irrigation, whereas the remaining four served as control. The plots were separated by a buffer zone of 5 m (Fig. 1). From 2003 to 2012, the irrigation system was activated in rainless nights during the vegetation period (May-October), doubling the annual rainfall amount. Water from the Rhone-channel situated along the experiment site (Fig. 1) was used for irrigation. Nutrient input through irrigation was minor: phosphate was below the detection threshold (PO4 <0.15 kg ha−1 yr−1) and the input of nitrogen (2.4–3.3 kg ha−1 yr−1) was less than the amount that could be expected to be deposited by a doubling of rainfall (N ≤ 3.5 kg ha−1 yr−1) [49] (link), [50] . Three identical trees per plot with the lowest crown transparency value, which refers to trees with the highest foliation, were chosen for our study [23] (link). In the first two plots, the volumetric soil water content was monitored hourly at a soil depth of 10 cm at four different locations using time domain reflectometry (Tektronix 1502B cable tester, Beaverton, OR).
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