Broom (Cytisus scoparius) colonization after grazing abandonment in the French Massif Central: impact on vegetation composition and resource availability

摘要

Broom (Cytisus scoparius) is considered an invasive species in countries outside its native range and a significant colonizing species in Europe. In particular this shrub rapidly invades pastures after grazing cessation in central France mid-elevation mountains. In order to better understand the effects of broom plants on vegetation composition and species richness after grazing abandonment, a study was conducted including stand descriptions and impact on light availability and soil properties. A total of 50 plots were sampled from the following categories: grazed pasture free of broom colonization, abandoned pasture in the process of being colonized by broom plants (1-3 years old), intermediate broom stand (2-4 years old), mature broom stand (4-5 years old) and dead broom stand (8-10 years old). In each plot the vegetation composition was measured; additionally, broom size and density were recorded. Light transmittance was measured beneath the broom canopy. Soil moisture, soil carbon (C), soil nitrogen (N), and nitrate production were measured in a subset of plots. Results showed that broom colonization is associated with a decline in species richness and cover, with a shift toward fast-growing species in the understory compared to uninvaded abandoned pasture. After broom decay stands were dominated by Rubus sp. plants. Concomitant to broom colonization and vegetation changes, light was severely reduced as only 11% of the incident light reached the floor in mature broom stands. In contrast soil moisture, N and C contents increased, respectively, of 37%, 52% and 61% in the upper soil layer from uncolonized pasture to mature broom stands and nitrate production was multiplied by 6.5. However changes in delta N-15 values were not significant. Dead broom stands were characterized by lower values of N, C, and N mineralization as compared with the other types of broom stand. Our data suggest that land abandonment and broom colonization is associated with altered vegetation composition and resource availability and that this shrub tends to occupy a mid-successional position between early successional communities such as heathlands and grasslands and late-successional woodlands. (c) 2006 Elsevier Masson SAS. All rights reserved.