The indicator provides information on the extent of significant soil degradation in forests likely to affect productivity, hydrology, ecosystem processes or social and cultural benefits. This indicator is primarily concerned with degradation caused directly or indirectly by human activity.
National data on all aspects of soil degradation are not available. About 25 percent of indigenous forests and 16 percent of plantation forests are located on land with moderate or higher incidences of soil erosion. Disturbance or displacement through erosion can cause nutrient loss in soils. Physical soil damage through compaction can result from the concentrated use of heavy machinery. An environmental code of practice addresses these issues and is widely applied.
Much of the New Zealand landscape is mountainous or hilly, undergoing uplift, and subject to high- intensity rainfalls. As many areas are underlain by soft, erodible materials (such as recent marine deposits), natural rates of soil erosion are high.
Before human settlement, the extensive indigenous forest land cover provided protection for the soil mantle, except during extreme rainfalls. With much of this indigenous forest cleared, parts of the country are prone to mass-movement soil-erosion processes, particularly in the East Coast of the North Island. Across the country, about 10 percent of the land area is classified as severely erodible.
The remaining indigenous forests often continue to fulfil critical soil conservation roles, mostly unaffected by human activities. However, the introduction of invasive animal pest species in the 19th and early 20th centuries, particularly possums and deer, has impacted on the health and regenerative capacity of some forest types. Hence these animal populations are likely to have some influence on the soil conservation role provided by these forests, even though they are subjected to control operations.
The Ministry for the Environment administers national environmental reporting, although many of the data are collected by other agencies. An Environmental Snapshot report on soil health in 2010 summarises the results from sampling 740 sites between 1995 and 2009 under productive land uses, including plantation forestry. Seven soil measures were monitored, providing information about organic reserves, fertility, acidity and the physical status of the soils. About 60 percent of planted forestry sites sampled did not meet all target ranges for soil health.
Key soil measures of planted forests:
However, much of the planted forest estate has been established on eroding and erosion-prone sites, some of which have been subject to soil degradation.
A broad indication of the levels of soil erosion for land under forest cover in New Zealand can be derived from the New Zealand Land Resource Inventory (LRI) and the New Zealand Land Cover Database.
Soil erosion peaks and sedimentation may occur during harvesting operations in planted forests, often due to associated earthworks (the construction of roads and tracks). Earthworks are addressed by the New Zealand Environmental Code of Practice for Plantation Forestry. They are commonly subject to provisions of local government plans prepared under the Resource Management Act 1991 (see Indicator 4.2.a). The period between harvesting and the re-establishment of good vegetation cover and root networks may also see elevated levels of soil erosion.
Very little harvesting occurs in indigenous forests. However, where harvesting is undertaken, it concerns single trees, small groups of trees or small coupes under the sustainable forest management requirements of the Forests Act 1949, and often by helicopter.
Scientists from the Sustainable Land Use Research Initiative developed the New Zealand Empirical Erosion Model (NZeem®), which predicts mean annual sediment yield from a given catchment based on annual rainfall, type of terrain and percentage of woody vegetation cover. The model can calculate the likely extent of erosion under different types of land cover. This will enable prioritising soil conservation work and defining those areas that would benefit from tree cover.
Disturbance or displacement through soil erosion can cause nutrient loss. Although there is no evidence in New Zealand that successive harvests cause severe decrease in soil nutrient supply, some soils may be less able to maintain nutrient supply than others.
Commercial forest managers commonly monitor nutrient levels through foliage and/or soil analyses. Fertilisers are applied where nutrient deficiencies would adversely affect tree growth, and to maintain long-term productivity.
The major cause of soil compaction on forested sites is the concentrated use of heavy machinery (for example, on landings for harvesting operations), particularly when soil moisture levels are high. This issue is addressed under the New Zealand Environmental Code of Practice for Plantation Forestry with guidance notes provided on how to mitigate soil compaction.
No national data on soil compaction under forested land are available.
Government sustainable land management initiatives
Several sustainable land management initiatives supported by government are designed to address soil erosion through forest establishment. These include the Erosion Control Funding Programme (East Coast) (formerly the East Coast Forestry Project), and the Sustainable Land Management (Hill Country) Erosion Programme.
The Permanent Forest Sink Initiative focuses on carbon sequestration and storage, but some of the afforestation is likely to be on eroding land. While not specifically implemented for soil erosion control, this is likely to be a secondary benefit.
The area of forest land with significant soil degradation is likely to have remained similar to the area that existed in 2008.