**1. Introduction**

Following arrival by European settlers in the 1800s, extensive deforestation for pastoral farming resulted in a geomorphic landscape response consisting of high erosion and sedimentation rates [1–5]. Increased erosion rates have led to a variety of adverse consequences, including i) reduced land productivity, ecosystem services, and food security through loss of productive soil; ii) increased damage to infrastructure; iii) adverse impacts on water quality and aquatic ecosystems from increased sediment delivery to streams; and iv) negative impacts for cultural values related to

soil and aquatic environments [6–13]. Erosion processes in New Zealand remain very active [14], due to a predisposed natural environment with steep slopes, weak sedimentary rocks, and a climate featuring high annual rainfall and relatively frequent high magnitude rainfall events [15–17]. Climate change is predicted to result in large increases in sediment loads, primarily due to increasing storm magnitude-frequency of mass movement erosion in soft-rock hill country [18].

Mass movement processes are geographically the most widespread type of erosion in New Zealand [15]. The most common types of mass movement in New Zealand are shallow, rapid slides and flows involving soil and regolith [1, 19]. Such landslides are generally triggered either by high-intensity-rainfall events or by small rainfall events on top of saturated soil moisture conditions [18]. Shallow landslides make up the largest source of sediment from pastoral hill country in New Zealand [20]. In these steep and highly dissected pastoral landscapes, bioengineering—either through widely spaced trees, blanket afforestation, or through natural reversion to indigenous forest—has been the most common method to increase slope stability and reduce soil erosion [21–23]. Tree roots are more effective than pasture roots in binding soil and preventing shallow landslides in pastoral landscapes [21].

Sheep and beef production depends on hill country for the supply of breeding stock as well as prime animals for meat processing. Wool has been a significant commodity product of hill country farms in the past and may well regain importance in future. Converting native forest to farmland was seen as a necessary activity to provide livelihoods for new immigrants and generate national wealth from the supply of essential export products, wool, and meat. The conversion happened quickly without an awareness of the inherent instability of the slopes and their vulnerability to landslides when the soil becomes saturated.

Hill country landscapes in scope for this chapter include those low altitude lands (<1000 m a.s.l.) that feature rolling and steep slopes (>15°), are not regularly cultivated on a large scale, are dominated by diverse pasture systems (but may include various woody vegetation components), and are managed for mixed livestock operations (mainly sheep, cattle, and deer) [24]. The area covered by this loose definition is about 5.2 million ha [25] or approximately 20% of New Zealand. Most of these landscapes have been developed into productive pastures from indigenous broadleafpodocarp forest over the last century, but in many cases, the prevailing vegetation has seen cycles of reversion to scrub, or establishment of plantation forestry as the economic and social drivers have shifted over decadal scales [23]. While active measures to reduce soil erosion in pastoral hill country have been undertaken at central and local government level, the land remains in private ownership, and as such, erosion management is dependent on individual landowners carrying out erosion control measures to stabilize their pastoral slopes.

The key contaminants for hill land waterways are sediment, P, N, and fecal microorganisms. Sediment loss from large-scale erosion events, in terms of both the immediate and ongoing quantities of soil loss, is the biggest environmental management issue for hill country [23]. Phosphorus is included ahead of N as most surface waters in New Zealand are more P-limited than N-limited [26], and total P losses in hill environments are strongly linked to sediment [27]. In general, relative to waterways in forested catchments waterways draining pastoral-dominant catchments have greater water yields, peak flows, nutrient concentrations, suspended sediment concentrations, and fecal coliform concentrations [28].

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