**3. Current approaches to erosion management in pastoral hill country**

#### **3.1 Government legislation requirements for erosion management**

The Water and Soil Conservation act of 1941 mandated statutory bodies to manage erosion control and flood management. However, under this Act, landowner response to erosion control was largely voluntary.

The Resource Management Act 1991 (RMA) is now the main piece of legislation that sets out how New Zealand should manage its environment. The RMA is based on the idea of the sustainable management of resources and encourages communities and individuals to actively engage in environmental protection.

More recently, the 2020 National Policy Statement for Freshwater Management (NPS-FM) has required regional authorities to manage freshwater in a way that considers the effects of land use, including the effects on receiving estuarine environments (New Zealand Government, 2020). Moreover, Freshwater Farm Plans (FFP) have been established as a legal instrument under the RMA to identify environmental actions on farms in consideration of objectives for the catchment. The Act specifies that an FFP must *"identify any adverse effects of activities carried out on the farm on freshwater and freshwater ecosystems"* and *"specify requirements that (i) are appropriate for the purpose of avoiding, remedying, or mitigating the adverse effects of those activities on freshwater and freshwater ecosystems; and (ii) are clear and measurable"* (Section 217F, RMA). Therefore, understanding the impact of erosion and sediment control is important to achieve the desired environmental outcomes and—more specifically sediment standards."

The Resource Management Act 1991 and National Policy Statement for Freshwater Management 2020 both require and assist landowners to carry out erosion control and freshwater quality measures on their properties using farm plans and on farm advice and through the provision of woody vegetation planting materials (poplar and willow poles, native plant seedlings) and financial assistance for plant protection.

#### **3.2 Passive erosion management**

Financial incentives (e.g., fencing) may be provided to retire steep land with low productivity and high erosion vulnerability [30] from active grazing and allow natural regeneration of woody native vegetation. Native forested catchments have been shown to generate lower soil erosion loads to rivers compared to pasture catchments [31–33]. Quinn and Stroud [33] reported suspended sediment loads of 988 kg ha−1 yr.−1 in a pasture catchment and 320 kg ha−1 yr.−1 in a native forest catchment. Furthermore, when compared suspended sediment in a pastoral catchment (180 ha) and native forested catchment (10 ha) on similar topographies and soil types ~8 km apart, Bargh [31, 32] measured loads of 1400 kg ha−1 yr.−1 in the pastoral catchment and 120 kg ha−1 yr.−1 in the forested catchment.

Native vegetation regeneration is likely to be a slow, incremental process and of itself leaves the slope no less vulnerable to erosion until any establishing woody vegetation with appropriate root systems can stabilize the slope. In a study on Oashore Station (farm property), Banks Peninsula, 3.3% of the retired area showed observable increase in natural revegetation between 2003 and 2016 despite the property being managed to support natural regeneration [34]. Significant factors influencing natural regeneration were the distance to existing woody vegetation (p < 0.01), woody vegetation within 25 m (p = 0.008), and

### *Drivers and New Opportunities for Woody Vegetation Use in Erosion Management in Pastoral… DOI: http://dx.doi.org/10.5772/intechopen.112241*

years without cattle (p < 0.01) with other nonsignificant factors being topographical wetness, years without sheep, solar radiation, slope, elevation, and aspect [34]. These findings are consistent with studies on natural woody regeneration in tropical regions of Brazil where tree cover was found to increase by 0.3–0.4% per year (e.g., [35]).

Passive regeneration, while being slow and incremental, goes some way to satisfying societal aspiration to reduce soil erosion, increase ecosystem biodiversity, improve water quality, and restore parts of the rural landscape to primeval native forest with its incumbent birdlife. For instance, it is estimated that landslides reduced by 65% in hill country with 10-year-old regenerating scrub (mānuka (*Leptospermum scoparium* and kānuka (*Kunzea* spp.)) compared to open pasture, and there was an estimated 90% reduction in landslides for 20–year-old scrub [36]. However, to achieve this protection, scrub density was 20,000 tree ha−1 at age 10 years and estimated at 10,000 tree ha−1 at age 20 years [36], reducing grazing pasture considerably. In the meantime, the pastoral slope may be invaded by woody exotic species such as barberry (*Berberis* spp.) or gorse (*Ulex europaeus*), which colonize much more readily but which may improve conditions for establishment of native species. Because of small size and slow growth of the woody vegetation, passive natural regeneration is unlikely to generate income from carbon credits under the ETS.

Research is needed across all climatic zones on rates of passive regeneration, and ways to blend passive and active approaches in erosion management.

#### **3.3 Active erosion management**

The objective of active erosion management is to target potential sediment source areas with tree planting to increase slope stability and thereby prevent landslide initiation and deposition of debris into adjacent streams. Active erosion management operates at a policy level (e.g., local authorities offering financial assistance to counter erosion, particularly slope erosion, and promoting/producing environmental farm plans as a tool to guide landowners in identifying areas where management activities can effectively reduce erosion) and at an operational level (e.g., providing advice, providing woody plant materials, placing and planting woody plant material, assessing success of planting projects, forming catchment groups of landowners).

### *3.3.1 How modeling landslide susceptibility informs active erosion management*

The policy and operational level represent different scales at which erosion mitigation is planned and implemented. At both levels, the aim is to design and implement cost-effective, targeted erosion control measures to conserve soil and meet water quality targets. Spatial modeling can help make the connection between catchment erosion sources, sediment loads in rivers, and sediment-related water quality. In particular, statistical landslide susceptibility models based on empirical observations of previous landslides (landslide inventories) can help determine where landsliding can be expected in future heavy rainfall events [37, 38]. Statistical models assume that locations with similar physical characteristics to where past failures have occurred are also likely to fail in future. This assumption is tested through validation of models by training in one area and testing in another or by splitting the landslide inventory into train and test sets. By coupling landslide susceptibility models (probability of future landslide occurrence) and sediment

connectivity models (probability of sediment-delivery to adjacent streams), potential source areas of landslide-derived sediment can be identified and targeted [39]. The importance of targeted approaches to erosion control was demonstrated by Spiekermann et al. [39] who found a 10-fold increase in cost-effectiveness of targeted mitigation of landslide-derived sediment compared to a non-targeted (random) approach.

Scale considerations are important at different levels of erosion management (policy and operational). Models that use national data inputs are of greater utility at catchment to regional scales in determining where the problem areas are within a particular catchment (**Figure 1**) [37]. Increased spatial refinement of models (e.g., including data on individual trees) can support planning at farm to slope scale by identifying specific locations on a farm that are prone to landslide erosion and/or potential sources of sediment [38].

Following careful erosion mitigation planning, the key activity is planting of woody vegetation within the pastured area, usually on slopes where erosion events are expected to occur in future. Other operational activities exist to support the key activity. Choice of planting material is determined by its compatibility with future use of the land, whether continued pastoral farming or retirement.

#### **Figure 1.**

*Scale considerations: Different data products available at different scales can serve different purposes to support decision-making for erosion and sediment mitigation. Shown above are shaded relief maps of digital elevation models (DEMs) at different scales: From left to right: 15-m (based on topographic contour lines), LiDAR-based 5-m and 1-m ground-sampling-distance (GSD). Below are landslide susceptibility models using topographic variables derived from the differently scaled DEMs (15 m, 5 m, 1 m) shown above. The 15 m and 5 m DEM-based models using the land cover database of New Zealand (LCDB) are based on Smith et al. [37], the 1 m model using individual trees based on Spiekermann et al. [38]. Other issues determined by the scale of data used for modeling include its impact on the accuracy/performance of models, as well as requirements of computing power.*

*Drivers and New Opportunities for Woody Vegetation Use in Erosion Management in Pastoral… DOI: http://dx.doi.org/10.5772/intechopen.112241*

### *3.3.1.1 Spatial decision support for targeted erosion mitigation*

#### *3.3.2 Spaced planting of poplar and willow*

Poplars and willows have been the species of choice within pastoral systems since the early days of active erosion management. They are planted as 3 m vegetative stem poles in winter when soil moisture is high, and at distances of 8–15 m apart, with the closer spacings applied particularly in gullies with high erosion risk. Each pole is planted in such a way as to capture surface runoff, avoid stock paths, and promote root development (https://www.horizons.govt.nz/HRC/media/Media/Land/Growing-poplarsinfo-sheet-2014.pdf). The pole is protected from any browsing activity by sheep but vulnerable to browsing by larger animals. For this reason, it is recommended that cattle be excluded from planted areas for two years. Survival rates for poles are >90% in good years, and annual height growth from poles can be 2–3 m (**Figures 2** and **3**). The practice has limitations in that higher positions on the slope are more difficult for poplars to establish because of shallow soil depth, low soil moisture, greater exposure to wind, or little topsoil resulting from a previous erosion event.

Mature poplars and willows planted at low densities (<70 tree ha−1) reduced landslide occurrence in pastoral hill country by 95% [21] compared with pasture-only protection.

#### *3.3.3 Change of land use to commercial forestry*

Whole farm conversion from traditional pastoral farming in hill country to either production forestry or carbon forestry has accelerated in New Zealand since 2011 (https://www.rnz.co.nz/news/country/473898/overseas-firms-buy-more-sheepbeef-farms-for-forestry-conversion), prompting a change in regulations for foreign investors. The primary forestry species that is planted is radiata pine (*Pinus radiata*).

**Figure 3.** *Mixed clone poplars at age five years and planted as 3 m poles.*

Carbon forests are expected to be profitable within the carbon market, without the market uncertainties associated with pastoral farming and without the risks of slope erosion associated with harvesting. Since this trend is recent, there is much debate about the future consequences of establishing permanent forests of fast-growing pine trees on valuable pastoral land. Furthermore, there are concerns with the downstream effects of 'slash' (the woody material left behind after the trees are harvested) after storm events, which is deposited in rivers and beaches (https://environment. govt.nz/what-government-is-doing/areas-of-work/land/ministerial-inquiry-intoland-use). Close planted forests have been shown to be more effective than spaceplanted poplars or willows in reducing slope erosion, moderating runoff, and improving water quality [15]. An alternative approach promoted by New Zealand Farm Forestry Association is to plant slopes vulnerable to erosion with close-planted timber tree species while retaining gentler slopes and less erosion-prone land for pastoral farming.
