4.3.b Area and percent of water bodies, or stream length, in forest areas with significant change in physical, chemical or biological properties from reference conditions

Rationale

This indicator provides information relating to water quality in forests. Significant changes in the physical, chemical or biological properties of water in forest lakes, rivers and streams may reveal the extent to which management activities or natural events are affecting water quality. Maintaining water quality is important for human use and consumption and to support healthy forest and aquatic ecosystems. Where water quality is being adversely affected by human activity, forest management practices may be adapted to protect water values.

Current state

At the national level, New Zealand has no river quality monitoring by land use cover, and no substantive information available on changes in physical, chemical or biological properties of streams in forest areas. Monitoring is undertaken on a regional basis. The quality of water from forest catchments is generally considered to be high. For planted forests, the most significant measured changes to water quality have been increases in suspended sediment.

No detailed quantitative national information is available to identify changes in physical, chemical or biological properties of water bodies that flow through New Zealand’s forest lands.

The Ministry for the Environment (2013) reports that river condition indicators monitored nationally (but not by land use) were either stable or improving at most sites. The exception was nitrate concentrations that were increasing in about a quarter of the sites.

The National River Water Quality Network consists of 77 sites on 35 rivers distributed across New Zealand. Monitoring a range of physical and chemical variables has been undertaken since 1989. National trends
for the period 1989 to 2007 (but not by land use) showed no significant change in water temperature and percentage of dissolved oxygen, but significant increases in visual clarity, dissolved reactive phosphorus, total phosphorus, oxidised nitrogen and total nitrogen.

Compared with Europe, North America and Asia, river water quality in New Zealand is considered “fairly good”, although conditions vary greatly from place to place depending on land use:

  • water quality is very good in rivers draining conservation lands, most of which have indigenous vegetation cover
  • rivers draining planted forests sometimes approach the quality of rivers in indigenous vegetation cover, although periodic harvest operations can mobilise fine sediment.

An assessment of water quality at the national level in low-elevation streams by land cover types indicated that:

  • nitrogen oxide, ammonium and Escherichia coli concentrations were significantly lower, and clarity was significantly higher, in indigenous and planted forest classes compared with urban and pastoral classes
  • differences between indigenous and planted forest classes were not statistically significant for any parameters
  • the median E. coli concentration in indigenous forest streams, and conductivity in planted forest streams, exceeded the guideline values.

For water bodies in planted forests, the most serious changes that have been measured in water quality are increases in suspended sediment concentration. These increases are associated with the construction of forest roads, tracks and landings for harvesting; accelerated landsliding; and other forms of erosion after harvesting from steep, unstable slopes.

An analysis of the River Environment Classification and the Land Resource Inventory enables estimates to be made of the distances of rivers flowing across forested land areas against the degree of surrounding land soil erosion. This provides one indication of the potential for sedimentation.

 

At the sub-national level, regional councils collect and hold a large amount of water resource information.
This substantial resource is used for monitoring, planning and reporting activities (including forestry) that relate to water resources. Land and Water Aotearoa has used models to estimate nutrient concentrations for different land uses based on data collected from hundreds of regional monitoring sites over the five years to 2012.

Trends

Since 2008, the national level information available on the properties of streams flowing through forested land has changed little.

Trend Status

Data Quality L

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