Vegetation development on upland and marginal land treated with herbicide, for bracken (Pteridium aquilinum) control, in Great Britain

A chronosequence, or space-for-time substitution, survey was carried out to investigate the impact on associated vegetation of spraying with asulam for the control of Pteridium aquilinum. The survey covered 117 sites in upland and marginal areas of Great Britain. A series of environmental variables were measured including those defining or relating to regional spread, mesoclimate, microclimate and site condition. The vegetation showed high beta-diversity with a considerable number of sites representative of upland grassland, heath and woodland. Species alpha-diversity increased after spraying. However regional differences were found. Indirect gradient analysis (unconstrained ordination using detrended correspondence analysis) suggested strong spatial, slope and sea distance gradients in determining species composition. Site conditions and time since spraying were also correlated There was a high degree of collinearity between environmental variables. A hybrid ordination (using canonical correspondence analysis) was employed to investigate further the role of time (the only constrained variable) since spraying. The ordination showed a trend of species in time-after-spraying starting with those lost early (Oxalis acetosella) and ending with stress-tolerant species (Juncus squarrosus). This ordination also revealed a second gradient almost parallel to axis 2 spanning the dichotomy between species rich grassland and degraded or invaded heath. Though control procedures appear to be reducing bracken successfully, there are strong regional variations in vegetation development Thus, policy changes are suggested whereby either improved cost-effectiveness, or, more effective treatment may be available. (C) 2000 Academic Press.