Cataclysmic response to recent cooling of a natural boreal pine (Pinus sylvestris L.) forest in northern Sweden

summary Instant stand‐level dieback occurred in boreal coniferous forests of northern Sweden in 1987. A population of pine ( Pinus sylvestris L.) and subdominant spruce (Picea abies (L.) Karst.) was analysed with respect to defoliation, age structure, radial increment and soil conditions. Defoliation and mortality were due to frost and drought in response to an early winter with abnormal cold and a thin snow cover. A thick mor humus layer preconditioned ground frost preservation. Annual radial increment had declined steadily over c. 40 years, in variance with climatic cooling and local exposure. Possibly, the growth decline depleted energy reserves and lowered the tolerance to climatic stress during extreme conditions. It is obvious that the structure of northern boreal forests responds perceivably to short‐term thermal decline. The outlined mechanism of dieback may prevent the evolution of climax in the absence of other exogenous disturbances. The present event may be a small‐scale analogue of forest responses to rapid cooling during, for example, the Younger Dryas period and the Little Ice Age.