Mapping Grey Willow (Salix cinerea) stand architecture using airborne laser scanning: implications for large-scale tree weed control

Summary We investigated the potential of airborne laser scanning (ALS) for mapping the stand architecture of Grey Willow (Salix cinerea), an invasive wetland weed in New Zealand. In particular, we focused on two metrics, tree height and canopy density, both of which influence the efficacy and nontarget impacts of herbicides aerially broadcast by helicopter to control tree weeds. We compared ground-based measures of Grey Willow height and canopy density with ALS-derived data, and the relationship between canopy density as estimated by each method and aerial herbicide deposition at three wetland sites in New Zealand. Analysis revealed strong linear relationships between ground-based and ALS metrics, indicating that ALS data could be used to generate accurate, high-resolution digital maps of Grey Willow height and canopy density. These maps coupled with computer-guided variable flow rate technologies, which enable optimal placement of herbicide, could maximise Grey Willow mortality while reducing the mortality of nontarget indigenous plants. We recommend the application of ALS-derived maps and computer-guided variable flow rate technology is investigated for more targeted large-scale tree weed control.