Effectiveness of vegetative mitigation strategies in the restoration of fluvial and fluvio-mass movement gully complexes over 60 years, East Coast region, North Island, New Zealand

Abstract

Background: Early (1957–1997) remediation strategies, predominantly Pinus radiata D.Don afforestation and indigenous shrubland reversion, proved effective in treating gully erosion in the East Coast region, North Island, New Zealand. However, the expansion of untreated gullies and initiation of new ones necessitated additional investment in land-use change (1997– 2017) to reduce their on- and off-site impacts. Methods: Gullies were digitised from aerial photography flown in ~1957 (before reforestation) and again in 1997. The region was re-flown in 2017, providing an opportunity to assess remediation successes and failures. For hill country areas, gully location and size were recorded by vegetation type, for major catchments, for two contrasting geological terrains, and for land designated in Gisborne District Council’s Combined Regional and District Plan as Land Overlay 3A (LO3A). Results: Between 1997 and 2017, 601 treated gullies fully re-stabilised and 315 new gullies formed. There remain 1864 gullies, comprising 5347 ha. This represents a 13% reduction in numbers and a 31% decrease in area during this 20-year period. Over the longer 60-year period, and across all land uses, 2942 treated gullies successfully stabilised. However, with the initiation of 1446 new gullies there are currently 5347 ha of gully requiring further treatment, with the largest of the affected area equally distributed across pastoral and reforested hill country and primarily located within the Tertiary terrain, Waiapu catchment, and areas designated as LO3A. Conclusions: Although attempts at gully remediation since the early 1960s have resulted in a 45% reduction in gully number, the current area of hill country affected by gullying is only 5% less than 60 years ago. During this period, gully initiation and development have outstripped mandated erosion control targets set by the East Coast Forestry Project (ECFP), for land designated as LO3A, and for the ‘Restoration of the Waiapu Catchment’ by 2020-22. Addressing ongoing on- and off-site impacts of gully erosion will require further significant long-term investment in the prioritisation and completion of these unfulfilled targets. For gullies identified in the National Environmental Standard for Plantation Forestry (NES-PF) as high erosion risk (orange zone) or where the erosion risk is very high (red zone), we recommend: (i) a revision of remediation strategies for the larger and more actively eroding of gullies destined for future afforestation, and (ii) for gullies within exotic production forests, the replanting of species (exotic or indigenous) better suited to providing long-term stabilisation, post-harvest.