What happens when you turn weed management off? A long-term appraisal of the effectiveness of Pteridium aquilinum (L.) kuhn control treatments and the role of sheep grazing

Long-term strategies are needed for the ecological restoration of land invaded by perennial weed species comprising of two parts: (1) control of the invasive species and (2) restoration of native vegetation meeting agricultural/conservation objectives. We investigated this within a statistically-rigorous, 28-year experiment at a site where Pteridium aquilinum had invaded an acid-grass/heathland. Where P. aquilinum-control treatments were applied for 20 years (1993–2012) very good P. aquilinum control was achieved by cutting (once or twice yearly) or repeat applications of the herbicide asulam. Vegetation restoration treatments (±sheep-grazing, seed addition) were also included and an acid-grass/grass heath was stablished. All three P. aquilinum-control treatments were very effective. In 2013 all P. aquilinum-control treatments were stopped and P. aquilinum recovery was monitored along with changes in the understory flora. All P. aquilinum performance variables remained significantly lower than untreated controls but P. aquilinum recovery varied between P. aquilinum-control treatments; slowest in the asulam treatment, intermediate where cut twice yearly and fastest where cut once yearly. P. aquilinum recovery was also faster in ungrazed plots, especially when cut once yearly. The underlying plant community remained relatively stable albeit with a reduction in species diversity. These good results were obtained from a large number of interventions and it is recommended that an adaptive management approach be used with (1) the aim of reducing frond density to ≤2 fronds m−2, (2) continued monitoring, and (3) re-applying control treatments when frond density exceeds this value. We also found that sheep grazing at low densities slowed P. aquilinum recovery, especially in the spray and cut twice yearly treatments almost certainly by trampling. However it is suggested that this needs further investigation using a range of different species of greater body-weights and at increased densities.

This experiment was set up under a DEFRA-funded contract (BD1226), and continuation has been supported by the Leverhulme Trust (EM-2018-073\2 to RHM), a Ramon y Cajal fellowship (RYC-2016-20528 to JGA) and both the Heather and Ecological Continuity Trusts.