• Post category:2022 / News
  • Reading time:3 mins read
22 August 2022 | By Willem de Lange

The idea of using helicopters to control alien invasive trees is not new.  But when exactly are these expensive machines worthwhile mobilizing and which control method should be used?

This study assessed important trade-offs when comparing helicopters with two alternative ground-based clearing methods (felling and a kill-standing approach called “drill-and-fill”) to find out under which circumstances the higher cost of helicopters are justified.

Several scenarios were compared in terms of site conditions such as infestation density, slope, on-site obstructive vegetation density, and site access.  Other key variables included in the assessment included: herbicide application rate, droplet size, fuel consumption, load carrying capability, altitude performance, endurance time, ferry time, mortality level, timing of application, collateral damage, and operating cost.

Results clearly shown that ‘team composition’ is an important cost determinant and that decisions based on ‘operator productivity’ alone can lead to cost ineffective choices.   This finding was quantified by showing that the higher productivity of drill and fill teams outweigh their higher total daily team rate and slower individual operator work tempo compared to felling.  The difference was big enough to make drill and fill more cost-efficient than felling under all site conditions, except those where helicopters were better.  Helicopters proved to be the most cost-effective option for cases where isolated Pines situated in dense fynbos with difficult site access at slope gradients greater than 25 degrees are targeted.  All these factors lead to significantly longer walk times of ground teams which decreases their productivity to such an extent that a helicopter becomes comparatively more cost-effective.  This means that the management emphasis for helicopters lies on containment and not on clearing heavily infested areas.

When deciding the merits of using a helicopter site managers could practically ignore slope as decision variable and concentrate on whether a particular site is a “rare” infestation density class and whether or not on-site obstructive vegetation density is considered “dense”.

The results make the choice of matching clearing methods with site conditions far easier.

Read the paper in Journal of Environmental Management

De Lange, W.J.; Boast, K. and Kleynhans, T.E. (2022) Modelling cost effective clearing solutions for invasive alien trees: A case study on wilding conifers.  Journal of Environmental Management. https://doi.org/10.1016/j.jenvman.2022.114985

For more information, contact Willem de Lange at wjdel@sun.ac.za

MD500 helicopter applying herbicide at high altitude and slope (Photo credit: Peter Willemse)