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Report: Trees for steep slopes

Dean Satchell
Sustainable Forest Solutions

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.

The Case for Land Use Change

Soil loss increases ten fold after conversion of forest to pasture on some hill country (Marden, 2004). This is because the roots of grasses are not able to hold the soil adequately against gravity. Furthermore, because soil dries out more under grass cover than forest cover, fractures form that accelerate erosion by allowing water to percolate through the soil and then seep along the more impervious surfaces, where lubrication occurs, leading to less frictional resistance and greater likelihood of movement (Grange and Gibbs, 1948). The result is erosion that causes significant thinning of topsoil depth, loss of nutrients and moisture holding capacity, with a substantial decline in pasture productivity (Hocking, 2006a).

The well understood issue is that "the absence of a forest canopy and lack of a dense network of intertwining roots in the subsoil is directly related to today’s accelerated rates of earth flow movement on pastoral hill country" (Marden, 2012). However, understanding the issue is one thing, but what is done about it is another.

Erosion and pastoral farming

Removal of indigenous forest cover in hill country and replacing this with a pastoral land use has increased the risk of earth flow initiation (Marden, 2012). There is a strong case for afforestation of erodible hill country currently under pastoral cover. However, can forestry and farming co-exist? Nordmeyer (1978) suggested in 1978 that "we have sufficient information now that, putting aside any entrenched views of land use and classic ways of land management, we can integrate farming and forestry to maintain and improve the land resource for future generations". However, Hocking (2006a) suggested that the reason why between two and five million hectares of erosion-prone hill country, that should be in forest cover, still remains in pasture, is that "our hill country farmers are just not interested in trees and sustainability". This despite minimal reductions required in stock carrying capacity of hill country farms, by only planting the highly erodible country in trees (Hocking, 2006a). 

Hocking (2006a) also questioned whether soil conservation should be the sole responsibility of the landowner or a shared public responsibility. Before the 2017 general election and since the mid-1980's Central Government has not invested in forestry; and at that time responsibility for public benefits such as erosion control and watershed protection were also devolved to regional authorities (Moore, 2017). Hocking (2006a) suggested there is an opportunity for the crown to fund soil conservation plantings as joint ventures and share proceeds with the landowner. However, selective afforestation "requires knowledge, motivation and finance" (Gordon, 2014a). In addition a well-funded education programme, extension activities and technology transfer are all prerequisites to improving our current unsustainable pastoral practices (Hocking, 2006a). In an ideal world the "billion trees by 2027" programme would have a history of research and development supporting it.

While demand and market prices for eroding pastoral land are held artificially high as a result of externalities and do not reflect the lands productive capacity, subsidies are required to encourage planting in forest cover (Moore, H. 2014). Artificially high land prices also force landowners to stretch biological limits beyond reasonable bounds and increase grazing intensity at the expense of environmental outcomes (Moore, 2016). By regulating discharges into water and air, Central Government could influence land use change in favour of afforestation (Moore, 2017). Trading in discharge allowances such as via the Emissions Trading Scheme (ETS) also offers land owners an income stream that substitutes for lost pastoral earnings (Moore, 2017).

Both the positive and negative effects of a land use should influence decisions on economic value and choice of land use. The basis for making decisions on economic value should reflect the true costs and benefits of ecosystem services associated with land use (Harnett and Yao, 2015). However, the contribution that forests make to reducing erosion and sedimentation are currently undervalued in New Zealand (Parker, 2016). Given that farm productivity on steep hill country is severely affected by erosion, and afforestation is effective in reducing soil losses, ecosystem services provided by forestry (e.g. retaining soil capital, restoring water quality, improving in-stream habitat, reducing flood severity and resultant damage to infrastructure) should be a key driver in land-use decisions. However, to justify implementing soil conservation measures would require economic analysis quantifying the true costs and effects of erosion, such as lost productivity and expenditures on containing, repairing and living with erosion (Jones et al. 2008).

For an improved understanding of the beneficial role that forestry provides to New Zealand's economy and environment, Phillips et al. (2015a) suggested that modelling of slope stability, species selection and economics is required. This would link tree species to site characteristics for a better understanding of the trade-offs between ecosystem services and economic outcomes. In addition, by recognising and then actively managing the risk of negative effects resulting from forestry activities, the forest industry could both actively mitigate those risks while at the same time publicise that consequences such as wood on beaches, slips on roads and sediment in waterways are all being managed as best they can (Phillips et al. 2015a).

By plugging the information-decision gap, Eyles (2014) suggested a new approach could emerge that integrates forestry and agriculture together as sustainable land management, for a mosaic of pasture and trees according to land capability (Eyles, 2014). Realising such a vision would require training of land managers and regionally-based forest advisors (Eyles, 2014), which aligns well with Moore's (2017) vision of a climate change future that "looks like an extended mosaic landscape where pastoral farming, cropping and forestry co-exist" (Moore, 2017).

The need to address air and water emissions at a national level offers the opportunity for implementing large scale afforestation as the most effective and lowest cost solution to the issue of erosion, requiring planting of 30,000 hectares of grassland per year over the next 30-50 years (Moore, 2017). Steeplands are very likely to receive the highest attention because of low pastoral productivity and high risk of erosion. Regulatory encouragement for afforestation (exotic or indigenous) could be in the form of an improved emissions trading scheme that actually encourages behaviour change; or even-handed nitrate discharge allowances (Moore, 2017). Such actions would encourage farmers to retire pasture and plant trees for income, or alternatively promote reversion to an indigenous forest in steep and remote areas not suitable for harvest (Moore, 2017).


Forestry for erosion control

In 2010 central government policy was that "the use of forests for controlling or reducing the impacts of rain events on erosion-susceptible land has and will continue to be important in many regions of New Zealand. Forests continue to be actively promoted as an erosion mitigation tool integral to many sustainable land management programmes" (Ministry of Agriculture and Forestry 2010b as cited in Phillips et al. 2012).

The use of woody vegetation is an effective method for both prevention of erosion on erodible country and rehabilitation of eroded country, becoming increasingly effective over time (Van Kraayenoord and Hathaway, 1986). Although forest cover does not necessarily provide total protection of the soil where erosion risk is severe, mitigating erosion with forest cover has been well researched and reviewed in New Zealand with the conclusion that forest cover significantly reduces sediment yields and that soil can be protected from erosion, provided sufficient trees are planted (Blaschke et al. 2008). Forest can also protect against gully development and accelerate gully stabilisation (Scion, 2012).

Grange and Gibbs (1948) described vegetation cover as the most important factor related to soil erosion, followed by rainfall, slope, parent material and then soil type. Since then numerous studies have come to the same conclusion, and that closed forest cover "reduces soil erosion on unstable slopes by around 90% compared to pasture cover" (Hocking, 2006a; Basher et al. 2016a).

There are a number of mechanisms that explain how trees and forests reduce soil erosion:

  1. Dense vegetation or a forest canopy absorbs the force of rainfall impact, protecting the soil surface from surface erosion (Van Kraayenoord and Hathaway, 1986; Hocking 2006a);
  2. The canopy intercepts rainfall which is then removed by evaporation, reducing water yield (Fahey and Payne, 2017). Water flow from a mature pine plantation can be 30% less than for pastoral farming (Quinn, 2005) with peak flows reduced by up to 50% (Fahey et al. 2004);
  3. Trees reduce excess moisture in the soil by transpiration (Hocking 2006a) and this reduces pore water pressures (Ekanayake et al. as cited in Phillips et al. 2017), reducing the risk of erosion occurring;
  4. Most importantly, tree roots provide mechanical reinforcement of the soil (Ekanayake et al. 1997; Hocking, 2006a). Reinforcement of soil by roots is achieved by their tensile strength (O’Loughlin and Watson, 1979), along with frictional resistance and soil bonding properties, which together are influenced by tree density stocking, tree size, tree species and soil physical characteristics (McIvor et al. 2011).
  5. Grafting, or the interlocking of roots between adjacent trees can form a raft that forms a reinforced, semi-rigid layer that enhances mechanical reinforcement of the soil (Marden, 2012).

Plantation forest cover also reduces nutrient levels in both ground water and surface runoff when compared with pastoral land use (Quinn, 2005; Hock et al. 2009). The improved water quality delivered off-site from forest cover (Parkyn et al. 2006) is attributable to less soil erosion, less streambank erosion and less surface runoff (Quinn, 2005). Pine plantations also export much less nutrient than pasture because of the uptake and retention of nutrients by trees, along with their low fertiliser requirements (Quinn, 2005).

However, forests are also at risk of erosion, in particular during the period of time after clearfell harvesting and until the replacement forest becomes well established, a period known as the "window of vulnerability" (Phillips et al. 2012). It is during this period (and only if extreme rainfall events were to occur during this time) that plantation forests established on erosion-susceptible steeplands for the original purpose of stabilising these slopes, are particularly vulnerable to the initiation of landslides, which in turn mobilise slash and debris from those slopes and deposits it in stream channels. On-site consequences of landslide initiation include an increase in erosion while increased sediment yield, when combined with excessive slash in the form of a debris flow, can result in severe off-site consequences.

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.


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