New Zealand Department of Conservation, Dunedin Office, 265 Princess Street, Dunedin 9016, New Zealand
The New Zealand high country has very extensive areas of montane and sub-alpine tussock grasslands that are susceptible to invasion by introduced conifers. However, the detection and control of these small, isolated and low-density infestations prior to seed production is problematic and expensive. Here I describe the development and evolution of a control and detection system for isolated wilding conifers, especially those developments that have led to significant improvements in cost-effectiveness.
The objective of a control operation is to systematically locate, map and destroy all adult and sapling conifers before they can set seed. Because of the large expanses of land involved (usually >30 000 ha) and the relative isolation of the wilding conifers, aerial operations are the only practical methods for finding trees. Initially, using GIS and GPS technology, helicopters carrying 5 – 6 staff used a method called skid-hopping to systematically search the high country for wilding conifers. Each tree found was cut down by a staff member using chainsaws or hand tools who had alighted from the aircraft. Often cut trees would regrow so it became standard practice to use an herbicide gel to poison the cut stumps. This methodology is costly, dangerous and time consuming.
The search and destroy control methodology has been improved through the development of a new and more efficient method called “Aerial Bark Application”. This method involves using only one operator to apply an oil-based herbicide from a helicopter to the bark on the lower trunk and branches of the target trees. This single staff member can now cover 5–7 times the area treated with this methodology compared to skid-hopping. Aerial bark treatment has significantly increased productivity, reduced flying costs and eliminated the dangers associated with skid-hopping.
To eliminate unnecessary helicopter flying to search for trees, the detection methodology is being improved using fixed-wing, multispectral remote sensing. Testing very high resolution (VHR) imagery against field data has given us confidence that we now have a robust tool for the accurate detection and classification of small pre-coning sapling wilding conifers in high contrast environments. Approximately 90% of pre-coning conifer saplings that are >30 cm in diameter are able to be detected in tussock grasslands. Analysis of the cost differential between current best practice control (e.g. random search and destroy missions in helicopters) and more directed approaches using remote sensing suggest a significant advantage in using imagery for detection of small trees.
The improvements to the detection and control system was instrumental in the New Zealand Government investing in a three-year National Wilding Conifer Programme (Phase 1 of programme 2016–19), to coordinate control efforts by multiple agencies within the highest priority areas.