Pre‐ and post‐reforestation gully development in Mangatu Forest, East Coast, North Island, New Zealand

Abstract

Following clearance of the indigenous forest and conversion of the land to pasture early in the 20th century, gully erosion became a pervasive feature in the headwaters of the Waipaoa River basin, and was notably problematic in the 140-km 2 area now covered by the Mangatu Forest. In this area, before reforestation in 1961, gully erosion affected c. 4% of the terrain. After a 24-year exotic reforestation programme the area affected by gullies was reduced to 1.5%, but of the eight gullies larger than 10 ha in 1960 none had stabilized by 1988, although four had at least halved in size. Estimates that a gully will stabilize or increase in size under a range of conditions suggest that in the case of gullies <1 ha in area, formed in terrain underlain by Cretaceous rock, there is a >80% probability of stabilization after one forest rotation (c. 24 years). For gullies between 1 and 5 ha in area the probability of stabilization is c. 60%. Gullies of 5 ha have an even chance of stabilizing over the time frame of a single rotation. The key determinant is gully size and shape at the time of planting and, within this size range, these relationships were stronger for linear than for amphitheatre-shaped gullies. Between 1939 and 1988 sediment production from gullies in the portion (76%) of the Mangatu Forest underlain by Cretaceous-aged rock was c. 22 000 t km À2 yr À1 , and during the period of maximum sediment production they may have accounted for c. 17% of the Waipaoa River's average annual suspended sediment load. Reforestation reduced the contribution to c. 8% in the period between 1970 and 1988. However, the off-site (downstream) impact of sediment generated by the remaining 420 active gullies in the Waipaoa River catchment is significant, not least on the capacity of the scheme that protects high-value agricultural land on the Poverty Bay Flats from flooding. A targeted reforestation programme may be an alternative to raising the height of the existing artificial levees. It is estimated that additional exotic plantings totalling c. 15 400 ha (c. 7% of the Waipaoa River basin area) would produce a >64% reduction in sediment production from gullies on pastoral hillslopes within one forest rotation (c. 24 yr).

Modelling of root reinforcement and erosion control by ‘Veronese’ poplar on pastoral hill country in New Zealand

“…The trial was established on a low-lying, even-surfaced alluvial flat terrace adjacent to the Taraheru River, at the site of previous 'plant growth performance' trials to allow comparisons between species (e.g. Marden et al, 2005). The soil is a free draining 'Te Hapara' Typic Sandy Brown Soil (Hewitt, 1998).…”

Observations of root growth of young poplar and willow planting types

“…While woody vegetation (trees) has been shown to reduce erosion in many situations (e.g. Phillips et al 1991;Marden et al 1992;Zhang et al 1993;Watson et al 1999;Phillips and Marden 2005;Marden 2012), it may not be effective in treating the largest active gully systems (Marden et al 2005aHerzig et al 2011). Vegetation is particularly suited to reducing shallow landslides and this has been demonstrated in a number of situations (e.g.…”

Towards an Understanding of Catchment-scale Sediment Dynamics: Cascades & Connectivity in Steepland Systems

“…Large amphitheatre-like features develop and the combined mechanisms give rise to gully-mass movement complexes, where sediment is generated both by fluvial incision and mass movement [Betts et al, 2003]. In the steepland East Coast Region of New Zealand's North Island, which is prone to gullying due to the underlying highly weathered and crushed Cretaceous shales and Tertiary sand/mudstones, more than half of the sediment load of the Waipaoa and Waiapu Rivers is derived from these gully-mass movement complexes [Marden et al, 2005;Page et al, 2008]. De Rose et al [1998] suggest that ~3% of the annual sediment load of the Waipaoa River (15 Mt) is derived from a single gully mass movement complex known locally as Tarndale Slip (Fig.…”