Management trials and demonstrations for invasive Mahonia aquifolium in coastal dunes

Non-native Mahonia aquifolium (Pursh) Nutt. is invasive in Belgian dune ecosystems. This evergreen shrub is a popular garden plant in the area. It grows vigorously on sandy soil and with its clonal growth strongly proliferates and overgrows native vegetation. There was an urgent need amongst conservation managers for data on efficiency of potential management measures. Therefore, an experiment was set up in three heavily infested dune reserves. Individual M. aquifolium plants were located with GPS and received different management treatments: (1) manual removal (digging) with shovels, (2) leaf treatment with a glyphosate Roundup® Max 5% formula on the leafs or (3) stem treatment with either glyphosate or (4) a saturated salt solution as an environmentally friendly alternative. We monitored regrowth one year after treatment (spring 2014). Except for leaf treatment (77%), kill rates were generally low: 38% for stem treatment, 27% for digging and 4% for salt treatment. Excavation of a number of individuals revealed that some roots do reshoot, even if the aboveground parts of the plant look completely dead. Kill rates are therefore probably overestimated. Furthermore, a demonstration on mechanical removal using a heavy excavator was held in a highly infested area (350 m², 100% M. aquifolium cover). Here, invasive shrub removal was combined with large-scale landscape restoration. The crane work was accompanied by intensive raking by hand, which enabled removal of a lot of the smaller rhizome fragments. The effort was documented (cost, effort) and the outcome monitored in terms of regrowth from different depths. The rooting system appeared to be relatively shallow (30-40 cm). Limited regrowth was observed from superficially buried rhizome fragments which could easily be pulled out.