Pre- and Post-Reforestation Gully Development in Mangatu Forest, East Coast, North Island, New Zealand

RIVER RESEARCH AND APPLICATIONS River Res. Applic. 21: 757–771 (2005) Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/rra.882 PRE- AND POST-REFORESTATION GULLY DEVELOPMENT IN MANGATU FOREST, EAST COAST, NORTH ISLAND, NEW ZEALAND MICHAEL MARDEN,a* GREG ARNOLD,b BASIL GOMEZc and DONNA ROWANa a Landcare Research Ltd, PO Box 445, Gisborne, New Zealand b Massey University, Palmerston North, New Zealand c Geomorphology Laboratory, Indiana State University, Terre Haute, IN 47809, USA 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-km2 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 kmÀ2 yrÀ1, and during the period of maximum sediment production (1939–1960) 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). Copyright # 2005 John Wiley & Sons, Ltd. key words: gully erosion; reforestation; gully stabilization; sediment production; Mangatu Forest INTRODUCTION Erosion ...(predominantly gullies, earthflows and shallow landslides) ...is as spectacular in its quick and often irreversible degradation of otherwise good land as it is tragic in its economic implications for individual prop- erty owners and for the district as a whole. Where such erosion has developed on a large scale the cost of erosion control far exceeds the value of pastoral production from the land concerned. There is therefore no known way of economically controlling erosion of this kind other than by complete reforestation. (NWASCO, 1970) Accelerated erosion by gullying can make a major contribution to the sediment yield of steepland catchments (Le Bourdiec, 1972; De Rose et al., 1998). Gully initiation is commonly explained in terms of process thresholds for incision (Patton and Schumm, 1975; Prosser and Abernethy, 1996), and is often linked with the degradation or eradication of vegetation cover (Lyell, 1849; Graf, 1979; Prosser and Slade, 1994; Prosser and Soufi, 1998). Stu- dies of the morphology and evolutionary development of gullies (relatively deep and rapidly eroding channels) *Correspondence to: Michael Marden, Landcare Research Ltd, PO Box 445, Gisborne, New Zealand. E-mail: [email protected] Received 8 February 2004 Revised 6 August 2004 Copyright # 2005 John Wiley & Sons, Ltd. Accepted 12 August 2004 758 M. MARDEN ET AL. suggest they have a limited lifespan and rapidly evolve to a condition of relative stability (Ireland et al., 1939). Some gullies appear to be a natural component of landscape evolution, but many contemporary gullies formed after native forests were cleared and agriculture intensified in the 19th and 20th centuries (Ireland et al., 1939; Wells and Andriamihaja, 1993; Prosser et al., 1994; Harvey, 1996; Belyaev et al., 2004). In either case, gully exten- sion represents a major adjustment to the landscape that is imprinted on the drainage network and has a profound impact on basin sediment yield. Concern over accelerated erosion induced by human activity has generated a substantial literature on gully development (Harvey et al., 1985), not least in New Zealand where, in the North Island, gully erosion affects some 10% of the land area (Eyles, 1985). However, it remains that in locales where gully erosion has developed on a large scale the cost of erosion control often far exceeds the value of production from the land concerned (NWASCO, 1970), and the most economic way of controlling erosion of this kind is by reforestation (see Pie´gay et al., 2004). Government-funded reforestation programmes over the past 40 years have had considerable on-site success in reducing gully erosion in the headwaters of the Waipaoa river basin, North Island, New Zealand (Figure 1), which was initiated following clearance of the indigenous forest by European settlers. However, the literature describing the ameliorating influence of the exotic forests is, in large part, anecdotal (Allsop, 1973), seldom quantitative (Hicks, 1991; Phillips et al., 2000; Gomez et al., 2003), and has never been fully evaluated. In this paper we quantify changes in gully area over a 21-yr period of pastoral production followed by a 24-yr reforestation period, and evaluate the contribution sediment derived from the gully-prone Cretaceous-age lithologies makes to the Waipaoa River system, for the periods 1939–1960, 1960–1970, 1970–1988, using the relationship derived by De Rose et al. (1998). We also discuss the long-term implications of sediment production from treated and untreated gullies elsewhere in Waipaoa basin, in the context of the capacity of the Waipaoa River Flood Control Scheme. Our results are relevant to other areas of New Zealand where gully erosion is still widespread, and reforestation is, as it has been in the past, the preferred management option for erosion control on 195.5 km2 of land identified as severely eroding soft rock terrain in the East Coast region (Parliamentary Commissioner for the Environment, 1993). STUDY AREA The 140-km2 Mangatu Forest is located in the headwaters of the 2200-km2 Waipaoa River basin (Figure 1). Climate in the study area is temperate maritime, with warm, moist summers and cool, wet winters. At the lowest elevation (200 m) annual precipitation is 1350 mm, increasing to 2500 mm at elevations of 800 m (Pearce et al., 1987). Tropical cyclones periodically accelerate both gully and earth flow activity. The largest recorded cyclonic storm (Bola) occurred in March 1988, and generated 500 to 700 mm of rain in a 5-day period. Since the turn of the 20th century there have been 29 extreme rainfall events, when the discharge of the Waipaoa River (at Kanakanaia) exceeded 1500 m3 sÀ1, and there is evidence of widespread shallow landsliding and accelerated gully erosion (Cowie, 1957; Phillips et al., 1990). There is a 29% chance that an extreme rainfall event will occur every year, and a >99% chance that such an event will occur every decade (Kelliher et al., 1995). Clearance of indigenous podocarp-hardwood forest began c. 1894, and had essentially been completed by 1914 (Black, 1977). Photographs (c. 1903–1910) suggest the terrain revealed by the clearances was susceptible to mass movements (that is, large, deep-seated earthflows and shallow landslides). Mass wasting accelerated and gullies were initiated following the change in soil moisture status, pattern of hillslope runoff (from subsurface, diffuse drainage, to surface runoff and its concentration along preferred drainage channels), and the loss of root strength after deforestation (O’Loughlin, 1974a, 1974b). The chronology of the early stages of gully development is anecdotal, but it has been suggested the acceleration of mass wasting and gully initiation in the upper Waipaoa basin took place in the first decade of the 20th century (Hill, 1895; Henderson and Ongley, 1920; Hamilton and Kelman, 1952; Gage and Black, 1979). The largest (Tarndale) gully complex (Figures 1 and 2) is thought to have been initiated in the winter of 1915 on the site of an extant slump, and many other gullies were initiated during the subsequent winters of 1916, 1917 and 1918. By 1910–1912, gully-derived sediment had begun to impact on the river system, and in headwater streams the cobble-sized bed material had been replaced by fine gravel and sand Copyright # 2005 John Wiley & Sons, Ltd. River Res. Applic. 21: 757–771 (2005) PRE- AND POST-REFORESTATION GULLY DEVELOPMENT 759 Figure 1. Location of Mangatu Forest area affected by gullies during the period 1939–1988 as interpreted from aerial photography (‘T’ indicates location of the Tarndale gully complex), and geological formations (after Mazengarb and Speden, 2000) Copyright # 2005 John Wiley & Sons, Ltd. River Res. Applic. 21: 757–771 (2005) 760 M. MARDEN ET AL. Figure 2. Pre- (1961) and post-reforestation (1988) photography of a large gully-mass movement complex known locally as Tarndale Slip. When planted (1964) with Douglas fir (Pseudotsuga menziesii) this gully was 21.4 ha in size.

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