Study site
Our experiment was conducted in a naturally regenerated, mature Scots pine (Pinus sylvestris ) forest in Pfynwald, Valais, Switzerland (46°19′27″N, 7°34′40″E, 610 m a.s.l.). The forest site is located in a dry, inner-Alpine region of Switzerland with repeated occurrence of Scots pine mortality events within the last decades (M Dobbertin et al., 2005; Rigling et al., 2013). The mean annual precipitation is around 657 mm and the mean annual temperature is 9.7°C (M. Dobbertin et al., 2010). A long-term irrigation experiment was started in 2003, where four plots of 1000 m2 are irrigated with water from a nearby channel during the growing season (+700 mm year-1, resulting in 1300–1400 mm total precipitation per year = irrigated). Four additional plots of the same size are used as a naturally dry control (= non-irrigated). The dominant species in this forest (> 10 km2) is P. sylvestris , withQuercus pubescens occurring occasionally. The pine trees are over 100 years old, with a mean height of ∼11 m and a diameter at breast height (DBH) of ∼12 cm (Schaub et al., 2016). The soil type is a Rendzic Leptosol derived from limestone (Brunner et al., 2009). Many of the Scots pine trees are severely infected by pine mistletoe (V. album ), with variations in the density and age of the mistletoes (mostly more than 10 years old) (M. Dobbertin et al., 2010). Since the irrigation experiment started, the environmental conditions (i.e. air temperature, air humidity, precipitation, soil temperature and soil water potential) have been continuously monitored.