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1 Introduction

The Ministry for the Environment has commissioned a review aimed at:

  • determining whether, with existing information and analysis, it is possible to make credible, ‘order of magnitude’, quantitative estimates of any flooding and erosion reduction benefits (or costs) associated with measures to sequester carbon dioxide or reduce emissions of greenhouse gases in New Zealand

  • providing the initial estimates of the benefits (or costs); or where information is not available, to provide advice on an alternative methodology for measuring such benefits.

This work is a technical contribution to the development of appropriate New Zealand policies for climate change mitigation. Reductions in erosion and flooding are only two examples of the ‘co-benefits’ potentially available through certain climate change mitigation measures. However such reductions may be significant, for several reasons:

  • New Zealand’s rugged terrain, tectonic instability and short steep river catchments mean that we have high rates of natural and induced erosion and flood frequency

  • there are many documented examples of physical and economic losses from such erosion and flooding

  • there is some documentation of reductions in erosion and flooding through afforestation or reversion in certain environments.1

The research brief2 involves three tasks:

  1. (a) to define the circumstances under which tree establishment, as a climate change mitigation measure in New Zealand, will have significant flood and landslip reduction benefits (or costs), and then
    (b) to survey and summarise work that provides a credible quantitative estimate of the physical landslip or flood reduction benefit from tree establishment
  2. to provide national estimates, where available, of physical co-benefits of climate change mitigation measures;3 or advice on alternative methodology where such information is not available
  3. to provide national estimates, where available, of monetary co-benefits of climate change mitigation measures; or advice on alternative methodology where such information is not available.

This report first discusses the available information sources. It then presents the detailed information about estimates of benefits and costs, in three chapters.

Chapter 2 discusses reductions in flood peaks passing down rivers, and also discusses several other hydrological changes that may have economic implications; notably alterations to annual water yields and minimum low flows. Chapter 3 discusses reductions in sediment yield (the product of terrestrial erosion, delivered to waterways). Chapter 4 discusses reductions in terrestrial erosion (processes that affect soil on land). The physical effects of sediment yield are in or close to waterways.

Other published studies and contract reports describe carbon loss from erosion and sediment transport in New Zealand, relative to the carbon sequestration attributed to afforestation and reversion (eg, Tate et al, 2000; Page et al, 2004; Scott et al, 2006). These fluxes may be very significant for overall climate change mitigation policies; for instance carbon transported off-site by soil erosion is between 3 million (M) and 10M tonnes per year (although it has not yet been determined how much of this is volatilised, or how much is trapped through attachment to deposited sediment). The work of these authors leads to the suggestion that “erosive steeplands should be a global priority for tree-planting because of the double benefit of carbon sequestration and erosion reduction” (Scott et al, 2006). However, carbon reduction benefits from reduced flooding, erosion or sediment yield are excluded from Chapters 2 to 4.

In accordance with the brief, our review focuses solely on whether there are significant reductions in flooding, or erosion, or sediment yield. This question must be answered first, before attempting any quantification of associated carbon reduction benefits.

In each section we summarise published and unpublished information in a series of tables, but refrain from comment on the authors’ scientific interpretation of their results; as this has already been done by several excellent published reviews, for which we provide references. Instead, we concentrate on interpreting what the results tell us, about circumstances under which tree establishment would have significant flood and erosion reduction benefits (or costs) in New Zealand. We then move on to discuss studies that estimate the monetary value of these effects (Chapter 5).

Finally we comment on the types of studies that could be undertaken to provide more reliable estimates, and some of the methodological and other issues in extending the work on quantification of benefits and costs of climate change mitigation measures in New Zealand (Chapter 6). The report ends with a set of conclusions (Chapter 7).

Terminology

The brief for this project referred to ‘landslip’ frequency and severity. We have assumed that the term ‘landslip’ was used synonymously with the internationally more common term ‘landslide’, as well as other types of mass movement erosion that occur in New Zealand. In this report we refer to all types of erosion occurring in New Zealand, including:

  • mass movement erosion – movement of material under the influence of gravity

  • fluvial erosion – transport of material by water

  • wind erosion – transport of material by wind.

Some types of erosion, such as gully or streambank erosion, involve both gravity and water in the movement of materials, but for the purposes of this review it is not necessary to distinguish between types of erosion based on their mechanism. All types of erosion may be influenced, to a greater or lesser extent, by changes in land use or vegetation cover such as afforestation or reversion.

Patterns of deforestation, afforestation and reversion in the New Zealand landscape

New Zealand has a very high-energy geomorphic environment, characterised by high rainfall in the headwaters of most major catchments, high relief, high rates of tectonic and volcanic processes, and susceptibility to high-intensity extra-tropical rainstorms. Because of these determining factors, our natural rates of erosion are very high, even without the historical processes of deforestation that have occurred.

New Zealand was widely deforested in two major phases: one in the first couple of centuries of Polynesian settlement, and one during the expansion of European farming in the late part of the nineteenth century and early twentieth century (in some districts continuing well into the second half of the twentieth century). Extensive erosion and deposition followed both these phases, but in a complex pattern that reflects the geomorphology of the New Zealand landscape. Also, sedimentation and flooding responses to erosion are often long delayed, due to large volumes of sediment being stored in upstream river valleys by up to many decades – until some factor such as a further storm, earthquake or uplift triggers downstream movement or response.

In response to the European phase of deforestation and erosion, public agencies (mainly the New Zealand Forest Service and catchment boards) undertook extensive reforestation between the 1950s and 1980s, throughout the middle and upper reaches of catchments throughout the North and South Islands. Widespread afforestation was also undertaken by private companies but generally on land that was less steep and less erosion-prone, notably in the volcanic plateau on land that had been unsuitable for farming. These areas of widespread afforestation occupy blocks of land tens of thousands of hectares in extent, but are typically located within even larger catchments; and are separated by land under other uses.

Evidence for reduced terrestrial erosion in and sediment yields from these afforested areas has been summarised by Vaughan (1984), Wallis and McMahon (1995), Maclaren (1996), and Hicks (2000b).

During the 1980s and 1990s, extensive afforestation by private landowners took place in many regions. In contrast to the widespread public agency afforestation summarised above, private farm-scale afforestation is diffuse: a typical pattern is farm woodlots, cumulatively several tens of hectares, scattered across a hill country farm several hundred hectares in extent. As well as afforestation, widespread reversion to scrub has taken place in the middle reaches of catchments in many regions in areas where farming could not be sustained, or owners (both private or the Crown) did not wish to continue farming.

In addition, significant areas of deforestation, mainly of exotic plantations, have occurred in the last decade as owners reacted to relatively low prices for exotic forest products and high prices for farm (particularly dairy) products.

The balance of these processes of afforestation, deforestation and reversion is reflected nationally and regionally by ongoing change in the total areas of vegetation shown in surveys such as the Land Cover Data Base. Areas in exotic forest and regenerating scrub are subject to the greatest changes. Historical phases of change are reasonably well documented (McGlone, 1989; MacCaskill, 1974; McKelvey, 1995; Roche, 1994; Pawsey and Brooking, 2002).

More detailed historical discussion is beyond the brief of this project. This sketch merely sets a context for the patterns of afforestation that we seek to summarise in the following three chapters. These consider in more detail the likely consequences of increased afforestation under three scenarios of different intensity:

  • incremental afforestation: diffuse afforestation by individual owners on their own properties that continue to also support farming

  • wide-scale afforestation: whole properties changed in land use and afforested, affecting a large proportion of catchment or sub-catchment

  • whole-catchment afforestation: Crown intervenes to afforest or promote reversion of whole catchment or sub-catchment.

As a background to discussion of the implications of these scenarios, it is useful to summarise the likely patterns of afforestation and reversion under each.

Large forests, established during the 20th century by public agencies such as the former New Zealand Forest Service or by private companies, occupy blocks of land tens of thousands of hectares in extent. But they are located within even larger catchments; and are separated by land under other uses. The Tarawera catchment, selected by NIWA as typical for an investigation of large-scale forestry’s effect on runoff, has pine plantations on just 28% of its 900 square kilometres. It would be possible to find several other medium-sized catchments (100 to 1000 km2) in the central North Island and in Nelson, where a larger percentage has been afforested. Elsewhere in the country, there would be almost no large or medium-sized catchments where blocks of planted forest cover more than 10% by area.

Afforestation by private landowners, in contrast, is diffuse. A typical pattern is farm woodlots, cumulatively several tens of hectares, scattered across a hill-country farm several hundred hectares in extent. When such a land use pattern is aggregated for medium-sized catchments (100 to 1000 km2), small-scale afforestation might exceed 10% by area in some, for example in Northland and the hill country of the eastern North Island (Gisborne, Hawkes Bay, Wairarapa); the western North Island (King Country, Taranaki, Wanganui); the Marlborough Sounds, and parts of Nelson.

Public agencies such as the Department of Conservation, inheriting land from the former Department of Lands and Survey and other stewardship land, have few tracts on which wide-scale reversion has occurred. Perhaps the only instance of scrub reversion, on a scale of thousands of hectares, would be the farm settlements inland of Taranaki and Wanganui, cleared in the decades around 1900 and then largely abandoned at varying times afterwards. Such reversion, though un-measured by area, would occupy a substantial percentage of several medium-sized catchments, notably of the Ohura, Whangamomona, Whenuakura, Waitotara, Patea and Waitara Rivers.

Reversion on private farmland is more widespread, even though most single private landholdings are not large enough for large tracts of land to revert. However, on large farms in rough hill country, it is not uncommon for up to a few hundred hectares of land to be abandoned and reverting, as a large block at the back of a farm, that is otherwise clear and grazed. On easier hills, also on downlands and plains, small blocks of land individually less than ten hectares are allowed to revert for a purpose: riparian protection on a riverbank, or erosion control in a gully, or destocking a steep face that is difficult to graze and muster.

The proportion of medium-sized catchments occupied by such reverted land has not been specifically measured but could be (see Chapter 7). In the absence of quantitative estimates, our knowledge of the country’s farming districts suggests to us that catchments where private reversion is in the 10 to 20% range, could be expected in the following regions: western Northland, Waikato-King Country west of the Waipa River, farmed parts of inland Taranaki and Wanganui. Many catchments from Marlborough through Canterbury and central Otago to interior Southland might be added, on account of sparse woody scrub in retired tussock grasslands. Only in the Bay of Plenty east of Opotiki, the East Coast north of Tolaga Bay, perhaps the Catlins district of South Otago and the Waiau district of western Southland, might we expect reverted land on private farms to exceed 20% of catchments’ area.

This list might be expanded somewhat under a scenario of wide-scale afforestation, in which whole properties are purchased for afforestation, for the purposes of permanent timber production and/or carbon storage – thereby affecting a larger proportion of a catchment or sub-catchment. There is reported investor interest in such possibilities (A Shrivastava, MAF, personal communication, August 2007). Current interest is focused on areas with proven forest growing conditions, such as the East Coast, inland Manawatu, and Northland. We suggest that such whole-farm purchases would not greatly expand the above list of regions where extensive afforestation could be anticipated.

Deliberate government intervention to afforest or promote reversion of a whole catchment or sub-catchment has been much more recent. Up till now, in the rare cases where the Crown or catchment boards / regional councils took ownership or management of extensive areas of hill country, this was driven by soil or nature conservation purposes. Very recently the Department of Conservation (DOC) has offered land to tender to commercial investors for climate change initiatives (Minister of Conservation, 2007). So far the projects described have involved six areas totalling about 40,000 hectares. The projects are being designed to either replant or promote natural regeneration of forest on land that was not in forest before 1989; or to undertake major pest control in order to remove animal pests (eg, goats) that emit methane, or in order to promote increased plant growth in the absence of such pests.

Target areas on the public conservation estate that are potentially suitable for such management have not been identified but are potentially very large. For example, 150,000 ha of forested hill country within Whanganui National Park and adjoining public conservation land are not currently receiving goat control yet have the potential for increased growth of woody biomass through goat control (Arand, 2007). Additionally, large areas of public land (in the order of hundreds of thousands of hectares) currently have a non-Kyoto forest land cover that is theoretically available for indigenous afforestation, although afforestation would undoubtedly not be sensible nor acceptable for a large proportion of this land.

Potential benefits of afforestation, reversion, or pest control of such land have so far been described by DOC solely in terms of greenhouse gas reductions. Associated potential benefits in terms of reduced flooding or erosion do not appear to have been considered, let alone other potential ecosystem or biodiversity conservation benefits.

Information sources

There is an extensive literature on the subject of benefits from soil conservation and afforestation, further discussed in Chapter 4. The literature that is relevant for this review of flood and erosion reduction benefits is highly dispersed; much of it is unpublished and was undertaken by agencies that have since been disbanded, hence the literature is difficult to obtain. We use both published and unpublished literature, but take a critical attitude to the reliability and applicability of either source. We have only examined New Zealand literature.

Information cited for our review includes:

  • regional-average percentage reductions in eroded4 area under different land uses, from State of the Environment reports by region councils since 1990

  • storm-specific percentage reductions in eroded area under forest plantations, native scrub, and native forest (from storm damage surveys by local and central government agencies, 1970s–1990s)

  • catchment-specific information on reductions in flood peaks, and sediment yields, from research investigations by a number of agencies

  • a small number of reports attempting to put monetary values to reductions in flooding or erosion

  • other reports available to us, from a range of sources.

Where appropriate we comment further on the literature sources (reliability etc) as we cite them.

Such a multiplicity of sources might lead to the expectation that there is a great deal of information on the subject. Unfortunately most of it is of low value for undertaking the sort of nationwide estimation that MfE seeks, particularly for flood-related issues.

To be reliable, flood frequency / magnitude data need to be collected long-term. Likewise, information about the extent and recurrence of terrestrial erosion, and the amount of sediment it contributes to waterways. It is true that these parameters have been measured long-term in some large New Zealand catchments, but land use in these catchments is mixed, and this makes interpretation of land-use effects difficult. There are some comparative investigations (paired catchment or before / after studies), in catchments where land use is uniform, but these are almost all short-term.

Furthermore, the variability of the New Zealand landscape means that a large number of studies need to be undertaken to achieve meaningful national estimates.

1 As referenced in later ‘approaches’ chapters.

2 Wording used in Request for Proposal; see later discussion of erosion terminology. We have referred to ‘co-benefits’ and ‘co-costs’ simply as ‘benefits and costs’, with the implicit assumption that these benefits and costs are linked to climate change mitigation measures.

3 We have also considered adaptation measures.

4 We include all forms of terrestrial erosion in our assessment where data is available.

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