Farm Forestry New Zealand

Report: Trees for steep slopes

Dean Satchell
Sustainable Forest Solutions
dsatch@gmail.com

Reviewed by Mike Marden, July 2018.

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Please note that the web report is regularly updated whereas the pdf download above is dated July 2018.

Introduction

Soil Erosion in New Zealand

Soil is formed by the gradual breakdown of rock material from chemical and physical weathering processes along with decomposition of vegetation and fauna. There is an equilibrium between soil formation and erosion under normal geological erosion processes whereby rocks weather and produce soil, along with "a gradual process of removal of weathered rock from the hills to the lowlands" (Grange and Gibbs, 1948). However "accelerated erosion" results from altering the vegetation cover (Grange and Gibbs, 1948). 

The effects of erosion are either on-site (e.g. loss of productivity) or offsite (e.g. loss of water quality, river aggradation, damage to infrastructure) (McIvor et al. 2011). Loss of soil productive capacity results from the loss of mineral nutrients, lower soil moisture holding capacity and a reduction in soil organic matter (Elliot et al. 1999).

The origins of soil erosion in New Zealand

Prior to the arrival of humans much of New Zealand was cloaked in indigenous forest. Clearance using fire and the introduction of browsing animals weakened the soil matrix and dramatically accelerated erosion, with catastrophic results such as severe flooding and debris carried down to the lowlands (NWSCA 1985).

Forest cover was reduced from approximately 50% of New Zealand's land area in 1840 to 18% by 1920 (Jones et al. 2008). Historical clearing of bush for farming had significant negative effects on water quality and hill country soils in many parts of New Zealand. Howard (1976) described the consequences of clearing bush for pastoral farming in the East Cape between 1890 and 1910: “The roots of the trees that had been burned or cut had rotted, and the ground gave way... We lost a lot of our grazing area... and a few animals”. The erosion itself was described by Howard (1976) as ”Huge earthflows – slow, creeping slips – appeared on the gentler slopes. In other faces, slumping happened. A great crack would appear on the surface, and the ground would sink. Also, floods became a bigger problem as the riverbeds filled up... Some people wrote about the dangers of erosion back in 1920, but nothing was done. The years passed, the farmers went on farming, and things got worse each year”.

Most of New Zealand's erosion-prone land remains in pasture and pastoral farmers have been slow to adopt land use practices that are sustainable (Knowles, 2006). Approximately 200,000 hectares of North Island hill country has a severe, very severe or extreme potential to erode and "New Zealand makes up ~0.1% of the global land mass yet discharges 1-2% of the average annual sediment yield to the world's oceans" (Jones et al. 2008; Hicks and Shankar, 2003). Increased sediment loads in rivers resulting from hillslope erosion results in damage to infrastructure, including tracks, roads, culverts and fences, with sediment deposited on flood plains and near shore sea beds (Hocking, 2006a).

Loss of pasture production results from erosion scars, which can be reduced to 20% of pre-erosion levels, recovering only slowly to eventually be 70-80% of the un-eroded level (Lambert et al. 1984, Rosser and Ross, 2010 in McIvor et al. 2011). Not only is productivity reduced because of loss of organic matter, nutrients and soil depth, soil structure is adversely affected, reducing infiltration rates and water holding capacity (Jones et al. 2008).

Intensification of farming with the emphasis on animal productivity, and in particular the grazing of steeper slopes, not only risks negative market exposure for New Zealand's animal products, but by not recognising the physical limitations of different land classes the negative environmental consequences of erosion remain ignored (Eyles, 2014).

Hill country land never fully recovers from past slip erosion and pastoral production levels continue to decline in erodible lands (NWSCA, 1985). The national economic cost of soil erosion and sedimentation was conservatively estimated to be $126.7 million per year in 2001 (Krause et al. 2001). However, because the economic consequences of cumulative erosion in pastoral hill country are not well researched in New Zealand (McIvor et al. 2011), there remains no immediacy behind any collective mandate for change. Furthermore, land use practices reflect underlying environmental values held by land owners, therefore by not addressing behavioural and social drivers of accelerated soil erosion, market and voluntary actions alone do not result in a more efficient pattern of land use (Basher et al. 2008).

Land use and erosion

The connection between removing forest cover, erosion and flooding has been well accepted for some time, but only since 1941 when the Soil Conservation and Rivers Control Act was passed, has soil conservation been regulated in New Zealand (NWSCA, 1985). 

Slip erosion occurs in hilly and steep land and is considerably accelerated by removing forest vegetation and replacing this with a pastoral land use (Grange and Gibbs, 1948). Conversely, the likelihood of slip erosion occurring is reduced by a cover of dense vegetation, which protects and binds the soil (NWSCA, 1985). The rock type exposed by slips determines the subsequent level of vegetation growth. Where the underlying rock is soft and weathers readily, vegetation establishes more rapidly, whereas where slips expose hard rock, vegetation establishes more slowly (Grange and Gibbs, 1948).

Rainfall and slope are the two key variables that influence the degree to which soil erosion develops (NWSCA, 1985). Severe storms with high levels of rainfall increase the severity of erosion (McIvor et al. 2011), manifested in hill country as mass movement erosion and sheet erosion (Grange and Gibbs, 1948). These are the two dominant forms of erosion associated with hill country in both the North and South Islands (McIvor et al. 2011) where sheet erosion caused by surface runoff and slip erosion caused by water infiltration can be significantly reduced under a forest cover (Elliot et al. 1999).

A well established body of literature supports the benefits of a woody vegetation cover in reducing localised surface erosion and mass-movement processes (e.g. Greenway, 1987; Coppin and Richards, 1990; Phillips et al. 1990; Marden and Rowan, 1993; Montgomery et al. 2000; Phillips and Marden, 2005; Sidle and Ochiai, 2006; Phillips et al. 2012), while the afforestation of whole catchments can reduce the sediment load delivered to waterways by as much as 90% (Hill and Blair, 2005).

Slope is a key determinant of soil stability and susceptibility to erosion (Phillips et al. 1989).

Areas of steepland hill country identified as being at greatest risk to these erosion processes are those where slopes are greater than 28 degrees (DSIR 1980). Although it is recognised that erosion-prone steepland hill country that remains in pastoral use would be better in forest cover to mitigate current and future erosion issues, it is nonetheless well-documented that in production exotic forests it is during the harvest and immediate post-harvest period that cutover is most vulnerable to the initiation of surface and mass movement erosion.

Classification of erosion potential and land use capability

Mapping and classification of erosion types commenced in New Zealand during 1947 (Grange and Gibbs, 1948). Land was mapped on its erodibility or "erosion potential", taking into account vegetation cover in order to select suitable uses for land. This work led to the compilation of the New Zealand Land Resource Inventory (NZLRI) in 1973 (NWSCA, 1985). The NZLRI is a spatial land database in which land is subdivided into units or parcels based on five key physical factors– vegetation, rock type, soil, slope, and erosion presence/severity. Based on these factors, the New Zealand Land Use Capability (LUC) classification was developed to better define the quality of land including its susceptibility to erosion based on the current severity and extent of erosion, potential erosion, and hence the capability of different land use classes to sustain productivity long term. This provided a set of national standards as a basis for land use planning, and mapping. The classification consists of eight major classes of land. Class 1 land has very few limitations and has the capability to sustain a wide range of potential land uses, whereas Class 8 land has little or no inherent productive potential and is normally used for catchment protection and recreational purposes. Classes 1 to 4 are arable while Classes 5 to 8 are non-arable. Thus Class 1 land has greater land use versatility than Class 8 land which because of its physical limitations has fewer land use options that are sustainable, and there are a greater array of hazards associated with this land class (Douglas, 2011). Furthermore, Douglas (2011) suggested that "the importance of matching land use with land use capability cannot be overemphasised". The Land Use Capability handbook was first published by the Soil Conservation and Rivers Control Council in 1971 (NWSCA, 1985) and is currently available from Landcare Research as the third edition (2009).

The NZLRI also includes Land Use Capability (LUC) assessments using pastoral and forestry production parameters and data for key soil attributes, that when combined "are highly flexible in allowing comparative land use studies within a wide range of national or regional areas." (New Zealand Land Resource Inventory – Soil, 2017). However, although useful at national, regional, district or catchment levels, at a scale of 1:50 000 the NZLRI is considered to be too coarse for farm-planning activities (New Zealand Land Resource Inventory – Soil, 2017).

By combining climatic data with erosion severity potential as identified in the New Zealand Land Resource Inventory (NZLRI) and the Land Use Capability (LUC) databases, a tool called the Erosion Susceptibility Classification (ESC) that maps 'risk classes' was produced by the Ministry for Primary Industries (MPI) for the National Environmental Standard for Plantation Forestry (NES-PF).

Types of Erosion

Two types of erosion are important in New Zealand hill country, fluvial erosion and mass movement erosion.

Fluvial erosion is surface erosion caused by water scouring. Sheet erosion is the removal of the thin surface layer of soil by surface water. Sheet erosion can become Rill erosion when the action of water scours small channels in the soil. Rill channels are no larger than 60 cm deep and 30 cm wide (as defined by NZLRI, 1973) and develop where soil is disturbed and the vegetation cover removed. Rill channels can develop into gullies. Gully erosion is also caused by the channelisation of water causing scouring during periods of heavy rain. Tunnel gully erosion is caused by water flowing through a tunnel beneath the soil surface. Eventually the roof of the tunnel collapses to expose the underlying gully.

Large-scale gullies develop when the protective vegetative cover is lost and once initiated their development is difficult to stem. Gully erosion is a very destructive form of erosion and is the largest sediment-producing process in many New Zealand river systems, particularly in the East Coast region of the North Island (Marden, 2009). The most effective method for controlling actively eroding gullies and reducing sediment production from them is reforestation (Marden, 2009). Although reforestation is also the best means of preventing the initiation of new gullies, once gullies form they are most easily stabilised while new and small, because if allowed to develop into larger gullies reforestation can take considerably longer to stabilise them (Marden, 2009).

Mass movement erosion is gravitational driven movement of soil mass downhill. Landslide and soil slip erosion is where subsidence is rapid. Nearly a third of the land area in the North Island is affected by slip erosion, making it the "most extensive and economically important erosion process in the North Island" (NWSCA, 1985). Consequences include silted drains and watercourses and reduced production potential. The occurrence and severity of landslide erosion is described by NWSCA (1985) as "depending mainly on the steepness of slope, the underlying rock type and the vegetation cover. Soil slips are most likely to occur on steep slopes and are rarely found where slopes are less than 16° ". Where landslides are deeper, failure can occur within the subsoil deposits or at the contact between the subsoils and the underlying bedrock. Typically the depth of failure is less than 1 m below the original soil surface (Marden et al. 1991; Page et al. 1994). 

Where the landslide involves movement of a mass of soil and underlying material while leaving the surface vegetation relatively intact, this is known as an earthflow. Debris avalanches occur on steep slopes and incorporate large volumes of material. Failure of the slope occurs rapidly and as the failed material moves downslope it creates a scar referred to as a debris trail. Where a large mass of hillside fails and also rotates backwards as it slides on a concave slope, this is known as a rotational slump.

Risk of erosion

The risk of the occurrence of erosion is determined by factors that predispose land to erosion, along with the likelihood of events that trigger erosion (Saunders and Glassey, 2007). Estimating erosion susceptibility involves the interaction between these two factors (Bloomberg et al. 2011). Increased slope and higher rainfall are the most important factors that influence "risk". The result is that unstable geological terrain located within high rainfall zones generate high sediment yields (Blaschke et al. 2008), a negative environmental consequence that can be related back to land use.

Topographic and physical characteristics such as slope, drainage, type of bedrock and soil type influence the likelihood for land to erode (Bloomberg et al. 2011). However, perhaps more importantly, management practices also either mitigate erosion risk, or increase it (Bloomberg et al. 2011). Removal of vegetation cover increases the risk of erosion and afforestation reduces the risk (Bergin et al. 1993). Earthworks can mitigate erosion by stabilising slopes, or prepare land for erosion by inadvertently changing drainage patterns.

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Disclaimer: The opinions and information provided in this report have been provided in good faith and on the basis that every endeavour has been made to be accurate and not misleading and to exercise reasonable care, skill and judgement in providing such opinions and information. The Author and NZFFA will not be responsible if information is inaccurate or not up to date, nor will we be responsible if you use or rely on the information in any way.