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

Abstract

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.

Can aerial herbicide application control Grey Willow (Salix cinerea L.) and stimulate native plant recovery in New Zealand wetlands?

“…Assessing the cost‐effectiveness of additional targeted or aerial boom‐spray control of surviving and reinvading Grey Willow would also be a useful line of enquiry (Griffiths et al . ).…”

“…We took this approach because tree size influences the effectiveness of herbicides broadcast for tree weed control (Horsley & Bjorkbom ), whereas light availability or its correlate willow canopy cover influences subcanopy spray deposition (Griffiths et al . ) and was expected to determine the distribution of nontarget herbicide impacts. Further, we expected baseline light availability to explain substantial variance in precontrol nonwillow plant community composition.…”

“…Currently, the only practical method for controlling tree weeds at large spatial scales (>10 ha) involves the aerial boom‐spray application of herbicides by helicopter (Reeves & Champion ; Bodmin & Champion ; Hutchinson & Langeland ; Griffiths et al . ). However, efficacy of aerially broadcast herbicides for large‐scale Grey Willow control is variable, and mortality of nontarget indigenous plants can be high (Reeves & Champion ; Bodmin & Champion ; Champion et al .…”

“…We expected that glyphosate, a broad‐spectrum herbicide, would cause significant nontarget mortality in fen areas where Grey Willow was small and scattered, and native plants were directly exposed to herbicide (Griffiths et al . ). But, where Grey Willow formed a continuous canopy, we expected that native plants would be sheltered from herbicide exposure and would increase due to increased light availability after Grey Willow control.…”