In Scots pine forests in the inner valleys of the European Alps high mortality of unknown causes has recently been observed. In 2002-2003 we assessed forests in the Swiss Rhone valley on a systematic grid ranging in altitude between 440 m and 1550 m a.s.l. We tested if mortality rates since the National Forest Inventory (NFI) in 1983/85 differed between species, geographic regions, altitudinal ranges or potential drought stress (extrapolated difference between annual precipitation (P) and modeled potential evapotranspiration (PET) for the period 1961-90). We also correlated annual pine mortality for a local long-term forest research site (LWF) with climatic data.
At the LWF site 59% of the pines died between 1996 and 2004. Annual mortality rates varied substantially between years and were highest following the drought years 1998 and 2003. Mortality related best to P-PET of the prior summer (May - August). Pine mortality on NFI plots was higher than elsewhere in Switzerland, while mortality of other species was not. Below 1100 m a.s.l. mortality rates of pines, but not of other species, were higher than above. We found a significant negative relationship between mortality rates and P-PET in pines, but not in the other species. Therefore, we postulate that drought stress triggers the observed pine decline.