A case study suggests positive ecological changes associated with conversion of pasture to mānuka (Leptospermum scoparium) forests in Aotearoa, New Zealand

In Aotearoa | New Zealand, conversion of low-yield high hill-country pasture farms to mānuka (Leptospermum scoparium) forests to support the harvesting of high quality monofloral mānuka honey can provide an economic benefit for rural landowners. However, the effects of such conversions on the local ecosystem and biodiversity are largely unknown. In a cross-sectional (space-for-time substitution) case study, we undertook biodiversity surveys at a single central North Island property that contained four sites representing different time points in the transition from pasture to mānuka forest: pasture (34 ha), three-year-old (36 ha) and five-year-old (23 ha) planted mānuka, and > 30-year-old naturally regenerated mānuka forest (57 ha). Terrestrial invertebrate surveys using window traps and bat and bird audio surveys were performed at each site. Freshwater health assessments were undertaken at three stream sites that were independent of the terrestrial sites. We found that invertebrate and bird abundance and diversity were lower in the three-year-old planted mānuka than in the pasture site. However, both invertebrate and bird abundance and diversity were higher in the five-year-old planted mānuka than the pasture, and were comparable to the > 30-year-old naturally regenerated mānuka forest in terms of community composition and diversity. Calls from the critically endangered long tailed bat (Chalinolobus tuberculatus) were detected more frequently at the five-year-old and > 30-year-old naturally-regenerated sites than at the other sites. Compared to the pasture catchment, macroinvertebrate scores and other stream health measures were higher in streams whose surrounding land use was native forest, naturally regenerated mānuka, or planted mānuka. Although site age was not replicated, our results suggest that there could be considerable positive changes in biodiversity that occur between three to five years after planting mānuka, with the five-year-old mānuka in our study supporting a more diverse and abundant community that was similar to that of the > 30-year-old naturally regenerated mānuka forest. Overall, our study provides supporting evidence that planting of mānuka forests in former pasture likely leads to a positive change in the ecosystem, however replication at different locations is required to confirm these case-study observations. Results from such surveys may aid in developing management systems to further enhance ecological outcomes of planted mānuka forests.