Modelling cost-effective clearing solutions for invasive alien trees: A case study on wilding conifers

Accurate on-site identification of appropriate alien clearing methods can realize significant cost savings for heterogenous sites. We developed a cost-based model accounting for site parameters such as infestation density, slope, obstructive vegetation density and site-access. These parameters are combined with a unit-costing sub-module to identify the most cost-effective clearing method for a particular site. The model was tested in the heterogenous Cape Fynbos biome of South Africa for three clearing methods for Pinus: traditional felling, drill-and-fill, and the arial-basal bark application (ABBA) method. The model accounts for the above-mentioned site parameters after which it is calibrated with the unit-costing for each method. Various scenarios consisting of different combinations of above-mentioned site parameters are then applied to identify the cost-effective solution for any particular combination of site parameters. Results favoured the drill-and-fill method in most cases, with the ABBA method reserved for sites with isolated Pines situated in dense fynbos with difficult access at slope gradients of 45° and higher. At these site combinations, ground teams experience longer walk times which reduces their productivity to such an extent that ABBA is comparatively more cost-effective. Traditional felling turned out to be prohibitively expensive because of team composition (mandatory higher safety and supervision requirements required for chainsaw operations) and slower on-site walking due to heavier equipment. The information enables site managers to do more accurate planning since the model will ensure that a cost-effective method is chosen for any particular site. It is then up to the manager to implement the chosen clearing method in a cost-efficient way.