Effectiveness of operational control of invasive conifers: Mortality, residual populations, and reinvasion

Invasive conifers are widespread and impactful weeds, resulting in significant ecological change and management costs. Treatments to control invasive species are expensive and require long-term commitment, yet their effectiveness under operational conditions is rarely quantified. The effectiveness of two operational control approaches – aerial application of foliar herbicides and ground-based manual methods – applied to manage invasive pines in New Zealand (NZ) in typically high-density and low-density infestations, respectively, was assessed. The objective was to quantify the effectiveness of each method in reducing Pinus contorta populations and to characterize the post-treatment population structure over time to assess reinvasion risk. Pine mortality and regeneration were surveyed in operationally treated infestations 1–4 years after control across a range of invaded environments in NZ. Mortality following aerial boom spray exceeded 90% on average across all height classes 3 years after treatment. Ground-based methods achieved high mortality among larger individuals but were significantly less effective for pines shorter than 60 cm. Both operational approaches allowed populations to persist via surviving individuals in the initial years post-treatment; however, the resulting population structures differed between treatments. Reinvasion risk also differed between treatments. Following aerial boom spray, reinvasion was driven primarily by surviving larger trees, whereas following ground-based control, it was driven by missed seedlings that persisted after treatment. Despite high mortality, aerial boom spray left an estimated 1,094 surviving pines ha -1 4 years after treatment, approximately half of which exceeded 2 m in height. In contrast, the large number of seedlings missed by ground treatments could lead to rapid invasion recovery. Follow-up treatment within 2–4 years is likely to be necessary for both control methods, although management should target contrasting residual population structures and reinvasion pathways.