Post-grazing Scots pine colonization of mid-elevation heathlands: Population structure, impact on vegetation composition and diversity

Abstract

Scots pine (Pinus sylvestris L.) pioneer woodlands have established spontaneously on former grasslands and heathlands on Mont-Lozère (Cévennes National Park, France) since their abandonment in the 1950's. Subsequently, late-successional beech (Fagus sylvatica L.) has progressively invaded the ageing pine stands. We studied 44 plots according to a gradient of forest colonization and maturation, from heathlands to mature beech stands. The objective was to assess the impact of forest colonization on the vegetation composition and richness of legally-protected grasslands and heathlands. Dendrometric characteristics and demography were assessed on each 20 x 20 m plot. An extensive ground vegetation analysis was carried out, focusing on plant functional traits that may be sensitive to canopy closure. Results indicated the persistence of most heathland species under pure and young Scots pine stands, but quite high species turnover. Xerothermic and strictly shade-intolerant species rapidly disappeared, whereas thermophilic forest species were apparent. In submature and mature pine stands, the establishment of a beech understorey entailed considerable species turnover, and prevent pine regeneration. In pure beech stands, ruderal and stress-tolerators were replaced by shade-tolerant and competitive species. Ellenberg's scores for light, nutrients and water confirmed the changes in vegetation composition and plant life traits, whereas the Shannon diversity index remained quite constant among pure and mixed pine stands.