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.