Commercially Important Trees as Invasive Aliens – Towards Spatially Explicit Risk Assessment at a National Scale

Alien species that are desirable and commercially important in parts of the landscape, but damaging invaders in other parts, present a special challenge for managers, planners, and policy-makers. Objective methods are needed for identifying areas where control measures should be focussed. We analysed the distribution of forestry plantations and invasive (self-sown) stands of Acacia mearnsii and Pinus spp. in South Africa; these two taxa account for 60% of the area under commercial plantations and 54% of the area invaded by alien trees and shrubs. The distribution of commercial forestry plantations and invasive stands of these taxa were mapped and the data was digitised and stored as Geographic Information System (GIS) (Arc/Info) layers. A series of environmental parameters were derived from GIS layers of climate, topography, geology, land use, and natural vegetation. The current distribution of the two taxa was subdivided into three groups according to the degree of invasion, planting history and the precision of the data collection. We used regression-tree analysis to relate, for each taxon, the distribution of invasive stands with environmental variables, and to derive habitat suitability maps for future invasion. The current distribution of invasive stands in South Africa was largely influenced by climatic factors. At a national scale, the distribution of large commercial plantations was a poor predictor of areas invaded by both taxa. Using environmental factors identified by the regression trees, we found that 6.6% and 9.8% of natural habitats currently not invaded and untransformed by urbanisation or agriculture are suitable for invasion by Pinus spp. and A. mearnsii, respectively. We then derived guidelines for policy on alien plant management based on vegetation type, degree of transformation, extent of invasion, and the risk of future alien spread. These factors were used to identify demarcated areas where these alien species can be grown with little risk of invasions, and areas where special measures are needed to manage spread from plantations.