An exceptional Calluna vulgaris winter die-back event, Abernethy Forest, Scottish Highlands

Abstract

Background: Calluna vulgaris is a woody shrub forming plant communities of economic and international conservation importance. Following winter 2002–2003, there was exceptional Calluna die-back in the central Scottish Highlands. Aim: To understand the effects and causes of this die-back event at Abernethy Forest Nature Reserve, Cairngorms National Park. Methods: Patterns of weather, foliage moisture and microclimate were interpreted in relation to patterns of Calluna vulgaris die-back. Vegetation development was investigated over 4 years, based on pre-existing monitoring plots. Results: February 2003 included a period of exceptionally low air humidity, during which unusually low Calluna shoot moisture readings were made, particularly in areas that subsequently had severe die-back. In monitored areas, Calluna cover declined by a third, with minimal recovery after three growing seasons. Die-back was more severe where Calluna was longer-stemmed and less abundant, and where topography was flatter or more north-facing. These topographical features, and die-back, were correlated with colder winter microclimates. A doubling in Vaccinium myrtillus cover in forest plots was strongly correlated with the pattern of Calluna die-back. Meteorological data showed an eight-fold increase in the variability of winter humidity minima since 1983–1984. Conclusions: Die-back probably followed severe ‘winter desiccation’, caused by extreme low humidity conditions, combined with low temperatures, lack of snow cover, and Calluna vulnerability due to age. It led to a major vegetation change in the direction of management aims at this site, but contrary to aims at many other Calluna-dominated sites. The importance of climatic variability as a potential driver of major vegetation change is emphasised.