Extent of freshwater wetlands

Freshwater Wetland

For this indicator, freshwater wetland refers to areas of land dominated by plant species that grow in freshwater saturated soils or grow tall enough to emerge above areas of shallow freshwater. These include areas of bog, swamp and fen, but do not include open water, flowing water, or salty (saline or brackish) water.

We use classes from the Land Cover Database¹ to identify areas that are predominantly freshwater wetlands: Herbaceous Freshwater Vegetation, Flaxland and Deciduous Hardwoods (where the wetland context attribute is Yes).

Herbaceous Freshwater Vegetation includes areas dominated by herbaceous aquatic vegetation as a component of freshwater wetlands. Common plants include sedges, rushes, or grasses, herbs or moss species.

We include Deciduous Hardwoods that have a wetland context, because an estimated one third of the freshwater wetlands remaining in the Waikato region have been invaded by willow or alder species². Willow and alder stands often mask diverse understories with abundant native wetland species and are important habitat for many wetland fauna.

When reporting on the extent and condition of freshwater wetlands at the regional scale it is important therefore to consider and report on the extent of all wetland types, including areas dominated in the canopy by non-native deciduous hardwoods. Such sites are important repositories of indigenous biodiversity and can be priority areas for wetland restoration.

The extent of terrestrial and saltwater (saline) habitats is covered by other indicators in the State of the Environment Indicator series. Some vegetation types in the Land Cover Database can establish in wetland or terrestrial habitats, such as Flaxland, Fernland, and Manuka and/or Kanuka scrub. For the purpose of this indicator, the Fernland, and Manuka and/or Kanuka scrub land cover classes have been allocated to the terrestrial State of the Environment Indicators.

The extent of freshwater wetland vegetation in 1840 is estimated from the Regional Indigenous Vegetation Inventory (RIVI 1840)².

Threatened environments

In 2007, the Government released a statement of national priorities for protecting rare and threatened biodiversity on private land (Ministry for the Environment & Department of Conservation, 2007). The priorities are also relevant to public land.

Protecting biodiversity in Acutely and Chronically threatened land environments is a national priority (see Table 1). Here, 20 per cent is a critical threshold for measuring ecosystem vulnerability because the rate of biodiversity loss increases dramatically when the amount of available habitat drops below 20 per cent of its original extent. Therefore, habitats in National Priority 1 Environments are the habitats most in need of protection.

Table 1: Threatened Environment Classification description for National Priority 1 Environments

Monitoring area

This indicator covers the entire Waikato region, 2012 boundary using:

GIS_ALL.POL_2012_REGIONAL_AUTH_EW_L1 regional boundary (2012)

Data are reported within four spatial frameworks:

1. Region (2012) - GIS_ALL.POL_2012_REGIONAL_AUTH_EW_L1
2. District/city councils (2012) – GIS_ALL.POL_2012_TERR_AUTHORITY_EW_L1
3. Bioclimatic zones (1994) – GIS_ALL.BIOCLIMATIC_ZONES
4. Threatened environments (2007) – GIS_ALL.SNA_LENZ_TEC

Monitoring frequency

It is estimated that this will be updated every five years.

Monitoring history

This replaces an indicator on Extent of Wetlands published on the Waikato Regional Council’s website because

• the “current” indicator reports on data from 1996, since then three new time periods of data are available
• the Waikato region boundary changed in 2012
• the Landcover Database which was used in the previous indicator had some dataset errors and a corrected version was released in 2015.

Measurement technique

The statistics are compiled using Feature Manipulation Engine (FME) software and Excel. Relevant land cover classes are selected from the Land Cover Database. See Waikato Regional Council document #2991195 for a description of the processes used to calculate the statistics in FME.

Statistics are compiled for four spatial frameworks:

1. Waikato region (2012) 
2. District councils (2012)
3. Bioclimatic zones (1994) 
4. Threatened environments (2007)

Regional boundary

The total land area of the regional boundary has been estimated using data from both the Land Cover database as well as the GIS_ALL.POL_2012_REGIONAL_AUTH_EW_L1 layer.

For this indicator, the 2012 regional boundary is used a baseline for all time periods reported on. In future updates of this indicator, any change in the size or shape of the regional boundary will affect the total area of indigenous coverage, due to areas of indigenous cover being included or excluded from the new boundary. To account for this issue either all previous results need to be recalculated or the update should retain the 2012 boundary as a baseline rather than any new regional boundary layer.

The same holds true for the indicator results pertaining to territorial authorities in which the 2012 territorial boundaries (POL_2012_TERR_AUTHORITY_EW_L1) have been clipped to the 2012 regional land boundary. Retaining these boundary layers will enable consistent change analysis to be conducted. This indicator is a regional analysis and as such does not cover the Waitomo, Taupō and Rotorua territorial authorities in their entirety.

LCBD 4.1 classes

For the current extent of freshwater wetlands in the Waikato region, we use the LCDB 4.1 classes and wetland context attributes listed in Table 2.

Table 2: Target land cover groups and names from LCDB 4.1

This indicator is primarily created using Feature Manipulation Software (FME). A sample of queries were tested in GeoMedia to ensure that results from both are aligned.

For information on data quality (lineage, positional accuracy, attribute accuracy, logical consistency and completeness) see the updated metadata for the LCDB, available on the Terralink International Limited website.

1. Freshwater wetland extent and protection is based on LCDB Version 4.1 data¹. The LCDB gives a ‘snapshot’ of vegetation at the time when the data were collected and should not be considered as a definitive measure of present day vegetation cover.

2. Some vegetation types are incorrectly identified in LCDB 4.1 which may result in errors in extent of freshwater wetland types.

3. The Lake and Pond land cover group is excluded from this analysis, however lake margins with emergent vegetation are included in the assessment where they are mapped by LCDB as one of the target classes.

4. Flax is a hardy species capable of thriving in wet or dry conditions. The LCDB class Flaxland is described in the Illustrated Guide to Target Classes for LCDB as predominantly a wetland class dominated by the swamp flax Phormium tenax. Therefore Flaxland is treated as a wetland class for the purpose of Waikato Regional Council SOE reports.

5. Fernland, Manuka and Grey Scrub vegetation classes can occur in wetland systems but are more frequently encountered in terrestrial systems. Wetland context attributes in LCDB are considered too unreliable for these classes to allow for a separation of terrestrial and wetland polygons. These LCDB classes were included as scrub classes in the terrestrial state of the environment indicators, and therefore are not included in the wetland SOE indicators.

6. Deciduous hardwoods are described in the Illustrated Guide to LCDB Classes as being “typically willow and poplar species growing adjacent to inland water and rivers, this class also includes stands of planted exotic deciduous hardwoods, such as oak (Quercus spp.), ash (Fraxinus excelsior) and elm (Ulmus spp.)”. An estimated one third of Waikato wetlands have a canopy of invasive deciduous hardwoods² (primarily grey willow, crack willow and, to a lesser extent, alder). Excluding this land cover will greatly underestimate the total area of freshwater wetland in the region. The Wetland Context attribute offers a method to separate out Deciduous Hardwood polygons that represent wetlands. A visual check using Google Earth of 120 Deciduous Hardwood polygons from the LCDB found a high degree of accuracy where wetland context = Yes (over 90 per cent agreement) but a lower degree of accuracy where wetland context = No (62 per cent agreement). This indicates that the extent of deciduous hardwoods with a wetland context is under-estimated in the LCDB. This may be partly offset by loose polygon boundaries that often include areas of terrestrial vegetation, particularly in narrow gullies.

7. The 2012 district council boundaries clipped to the regional land boundary were used for this indicator. Some district councils extend beyond the regional boundary, and statistics compiled at the district scale for the Waikato region may not reflect the pattern for those districts as a whole.

Waikato Regional Council is in the process of creating a region-wide indigenous vegetation spatial layer (Bioveg). When Bioveg is completed it is expected to be a high quality layer which could be used as an alternative to the Land Cover database. However, the scope of the Land Cover database is broader than the Bioveg layer as the LCDB includes land cover classes that possibly may contain indigenous vegetation such as coastal sand and rock. Further, the Bioveg is currently only a snapshot in time and cannot currently be used to present trend data.

Changes to the Land Cover database classification scheme are likely to influence the indicator results. In addition, it is expected that the accuracy of the Land Cover database will increase with new technology and this will influence the extent to which any change in indigenous cover can be accurately interpreted.

Figure 1 shows the location of freshwater wetlands in the Waikato region.

Figure 1: Freshwater wetlands in the Waikato region

See the above map as a larger PDF [PDF, 1.7 MB].

Today 1.1 per cent of the region's land area remains in freshwater wetlands, totalling 27,484 hectares (one quarter of the pre-European extent). This comprises 19,086 hectares of Herbaceous Freshwater Vegetation, 7,900 hectares of Deciduous Hardwoods, and 498 hectares of Flaxland. Deciduous hardwoods have invaded areas of Herbaceous Freshwater Vegetation since European settlers arrived. 

Figure 2: Amount of the region’s freshwater wetlands by vegetation type (2012)

Over 86 per cent of all of the remaining areas of freshwater wetland in the Waikato Region occur within National Priority 1 Environments (23,483 hectares in Acutely Threatened environments and 196 ha hectares in Chronically Threatened environments).

Wetlands are over-represented in these threatened environments because they tend to occur in lowland areas where much of the original native cover has been cleared. Often wetlands are the only natural features that remain in areas of intensively farmed lowlands. Figure 3 shows that while most of the Freshwater Herbaceous Vegetation and Deciduous Hardwoods in the Region occur in threatened environments, Flaxland tends to occur in less depleted or better protected land environments.

Figure 3: Amount of freshwater wetland in National Priority 1 Environments (2012)

Figure 4 shows which bioclimatic zone each freshwater wetland type occurs in.

In the Waikato region, freshwater wetlands are mainly confined to the lowland bioclimatic zone (below 300 m elevation), where 24,444 hectares (89 per cent of the current extent) of freshwater wetlands occur.

Flaxland tends to occur at higher elevations, with 66 per cent of the total current extent found in the submontane zone (300 to 800 m elevation). Less than 1 per cent of any freshwater wetland vegetation type occurs within the montane zone (800 to 1300 m elevation) and no freshwater wetlands are mapped above 1300 m elevation (subalpine and alpine zones).

Figure 4: Proportion of remaining freshwater wetland in each bioclimatic zone (2012)

Most of the region’s freshwater wetlands occur within Waikato, Matamata-Piako, and Hauraki districts (Figure 5). These districts are where the 10,000 ha Kopuatai Peat Dome and the 7,000 ha Whangamarino Wetland Ramsar Wetlands are found. Taupō District also has extensive wetlands associated with the margins of Lake Taupō and Lake Rotoaira.

Figure 5: Area of freshwater wetland in each local authority (2012)

Figure 6 shows that based on the Land Cover Database there has been no significant change in the extent of each freshwater wetland vegetation type in the Waikato region since 1996. Within the region the data set has recorded no change in extent of Flaxland and total net loss of 7 hectares each of Herbaceous Freshwater Vegetation and of Deciduous Hardwoods in a wetland context. There are many wetland revegetation and restoration projects occurring in the region, however these are not yet at a scale, or sufficiently mature to show up on regional scale vegetation maps.

Figure 6: Change in amount of freshwater wetland since 1996