Vinterklimat och vårbakslag : lufttemperatur och kådflödessjuka hos gran i södra Sverige

The rapidly developing and widely distributed resin flow syndrom in Norway spruce (Picea abies) in southern Sweden has an unknown cause. One suggestion for its cause is the occurence of six "abnormally" mild winters from 1987/88 to 1992/93. This situation, together with hard spring frost is supposed to pose an unfavourable climatic combination for spruce. Daily temperature data (mean, maximum and minimum) for the period 1961-1994, from 20 synoptic stations from south Sweden have been used to investigate this hypothesis. In order to better analyse the effect of spring frost, a spring frost index (VFI) has been developed. This index is biologically based and defined as: "The largest temperature sum that can be calculated from the last day in a sequence of four days with air mean temperatures exceeding +5 °C, to the next following day with a minimum temperature that falls below -2 °C within the period April 1 to May 15. " The maritime influence on the air temperature in southern Sweden has increased since 1930. In a span of 65 years, the winter temperature in the 1990s has, in spite of this, not been exceptionally high, since mild winter periods appear frequently. It is also apparent that mild winters are commonly followed by a high VFI in the spring. The maritime influence is the main factor for where and when high VFI occur. A high VFI and spring frost during May is much more frequent in inland areas than in coastal settings. Hence, if spring frost is an important factor in developing the resin flow syndrom, inland areas ought to show more frequent damage. However, a survey in southern Sweden in 1994 does not indicate any higher frequency of the syndrom in the inland as compared to coastal areas. The observed resin flow syndrom can not be connected to a mild winter temperature climate nor to a high frequency or severety of spring frost.