The potential distribution and impact of bracken in upland Scotland: An assessment using a GIS-based niche model

Bracken spread could have very selective impacts on the vegetation of uplan d Scotland (UK). Information on this would be of value in targeting appropr iate management measures. However, it is difficult to establish accurately the current distribution of bracken, so modelling bracken spread using proc ess-based models is not possible at present. An alternative modelling appro ach is described which uses knowledge of the climatic, soil and topographic limits to the existing bracken distribution in upland Scotland. This knowl edge is formalized as a rule-based model. This is applied through a geograp hical information system (GIS) to identify the maximum potential bracken ni che over the whole of Scotland with a nominal spatial resolution of 1 ha. T he model is validated against field data from 14 sites around Scotland. Bet ween 64-97% of the existing bracken lies within the predicted bracken niche . Locally low accuracies are ascribed to scale-effects derived from the inp ut datasets, the nature of the ground data, as well as the universality of the model rules. The bracken niche is differentiated into two types: contin uous and discontinuous. The total area of the niche is 6036 km(2) or 7.75% of the land area of Scotland with a broad 60:40 split between the continuou s and discontinuous niche types, respectively. However, there is a strong r egional bias in their distribution with the discontinuous niche type princi pally occurring in the west of Scotland. Overlaying the potential niche on the Land Cover of Scotland 1988 dataset shows that it impacts on 70% of the national area of 'good rough grassland' and 31% of the 'poor rough grassla nd' category. In addition, 16% of the national area of 'heather moorland' i s impacted. There are strong regional variations in these impacts which may have implications for future bracken management. The limitations of the ap proach are discussed and the potential For linking it to dynamic models is identified. (C) 2000 Annals of Botany Company.