Cold‐induced dieback of montane spruce forests in the Swedish Scandes – a modern analogue of paleoenvironmental processes

In 1987 particularly, but also in 1988, primaeval montane coniferous forests of northern Scandinavia experienced needle loss on an unprecedented scale, which affected the character of the entire landscape in certain regions. This study elucidates the dieback event For Norway spruce [Picea abies (L.) Karst.] in an area of the Swedish Scandes. The study includes visual assessment of percentage needle loss, age structure, radial growth and various environmental parameters, along altitudinal transects and within a larger sample plot. Frequency and magnitude of foliage damage were found to be related to elevation. The dieback process of closed forest stands was interpreted as a response to short‐term climatic variability. Damage mainly developed during one of the coldest early winters (December January 1986/7) of this century. In combination with sub‐normal depth of the snow caver, the severe cold promoted exceptionally deep and persistent soil frost in spruce forest stands. It is hypothesized that damage was mainly due to winter/spring drought (frost‐drought). Phenotypically closed forest stands may change in just a few years in response to cooling, a feature which has not been realized previously. The possibility of major retrogressive responses lo short‐term climatic events should therefore be considered in studies of historical biogeography and when modelling vegetation succession. The forest studied was probably established after a severe wind storm in 1837. The restocking process was facilitated by the post‐Little Ice Age warming, but the stands are at present suffering severe dieback due to the recent cooling of the climate. The results stress the potential importance of short‐term climatic disturbance in boreal forest succession. Reasonably, rapid cold‐induced dieback of forest stands may have been a recurrent feature of Holocene forest history in the Boreal zone.