Extent of freshwater wetlands

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

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 areas of freshwater wetland vegetation: Herbaceous Freshwater Vegetation, Flaxland, and, where wetland context = Yes, Deciduous Hardwoods and Mānuka &/or Kānuka.

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. We also include wet Mānuka &/or Kānuka which is present in a significant amount in the region, particularly in the Whangamarino wetland.

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, including Flaxland and Fernland can establish in wetland or terrestrial habitats. While the LCDB incorporates a wetland context attribute, concerns remain about the accuracy of the data for some land cover types. For that reason, all Fernland and other predominantly terrestrial land cover classes have been allocated to the terrestrial State of the Environment (SOE) Indicators, while all Flaxland polygons have been allocated to the freshwater wetland SOE indicators.

The extent of pre-human freshwater wetland vegetation is derived from published data².

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 and Department of Conservation, 2007)³. The priorities are also relevant to public land.

Protecting biodiversity in Acutely Threatened (AT) and Chronically Threatened (CT) land environments⁴ 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.

In 2012 Manaaki Whenua Landcare Research announced they were no longer using the terms Acutely and Chronically Threatened⁸, and introduced additional categories6, however the criteria for the former AT and CT Environments remain unchanged. To be consistent with the National Biodiversity Priorities, we report on the amount of wetland within each of the first two categories, which mirror the AT and CT categories.

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

Monitoring area

This indicator covers the entire Waikato region, 2022 boundary using:

POL_2022_REGIONAL_AUTH_WRCGIS_ALL.POL_2012_REGIONAL_AUTH_EW_L1 regional boundary (2022)

Data are reported within four spatial frameworks:

1. Region (2022) - POL_2022_REGIONAL_AUTH_WRCGIS_ALL.POL_2012_REGIONAL_AUTH_EW_L1
2. District/city councils (2022) – GIS_ALL.POL_2012_TERR_AUTHORITY_EW_L1POL_2022_TERRITORIAL_AUTH_FULL_WRC
3. Bioclimatic zones (1994) – GIS_ALL.BIOCLIMATIC_ZONES
4. Threatened environments (2015) – LENZ_THREATENED_ENVIRONMENT_CLASSIFICATION_TECGIS_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 Landcover Databases layer which was used in the previous indicator had some dataset errors and a corrected version was released in 2018.
• The 2017 Indicator included terrestrial and freshwater wetland ecosystem types, which have now been separated into their own indicators.

Measurement technique

The statistics are compiled using ArcGIS Pro software and Microsoft Excel. Relevant wetland land cover classes were selected from the Land Cover Database v5.0.

Statistics are compiled for four spatial frameworks:

1. Waikato region (2018)
2. District councils (2018)
3. Bioclimatic zones (1994)
4. Threatened environments (2012)

Guidelines and standards

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 POL_2022_REGIONAL_AUTH_WRC layer.

For this indicator, the 2022 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 2022 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 2022 territorial boundaries POL_2022_TERRITORIAL_AUTH_FULL_WRC have been clipped to the 2022 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 5.0 classes

For the current extent of freshwater wetlands in the Waikato region, we use the LCDB 5 classes and wetland context attributes listed in Table 2.

Table 2: Target land cover groups and names from LCDB 5

Quality control procedures

This indicator was created using ESRI ArcGIS Pro software. The queries for the analysis were completed using model builder scripts. This method enables the queries to be easily recalculated if required. The model outputs calculated values to an excel spreadsheet.

For information on data quality (lineage, positional accuracy, attribute accuracy, logical consistency and completeness) see the updated metadata for the LCDB v5.0, available on the Manaaki-Whenua Landcare Research LRIS portal website.

Limitations

1. Freshwater wetland extent is based on LCDB Version 5 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 or mapped in LCDB 5 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 LCDB⁵ 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 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. Mānuka scrub can also occur in wetland or dryland situations. The Wetland Context attribute in LCDB 5 was used to select wet mānuka scrub for this indicator.
7. Deciduous hardwoods are described in the Illustrated Guide to LCDB⁷ 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. In 2017 Google Earth was used for a visual check of 120 Deciduous Hardwood polygons from the LCDB4. The assessment 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.
8. The 2022 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.
9. The Threatened Environments (TE) data layer has not been updated since 2015. Until a new layer has been updated (incorporating changes to land cover since 2022), this indicator will report on the extent of wetland within our most depleted environments using the 2015 TE layer.

Further indicator developments

Waikato Regional Council created a region-wide indigenous vegetation spatial layer (Bioveg 2012) using aerial imagery captures in 2012, and is developing a biodiversity inventory with more detailed vegetation classes, following Singers and Rogers (2014)7. When the biodiversity inventory is completed, it is expected to be a high-quality layer which could be used as an alternative or to supplement the Land Cover database. However, when completed the biodiversity inventory will only be a snapshot in time predominantly using aerial imagery taken in 2017 and oblique aerials from 2016-2018, and may not be able to present trend data over time.

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 remote sensing information and this will influence the extent to which any change in indigenous cover can be accurately interpreted.

When/ if an updated version of the Threatened Environments Classification is released this indicator will be updated to assess the amount of wetland within the new TE framework. This will only affect the results in Figure 3.

Footnotes

1. The Land Cover Database (LCDB) uses satellite imagery to map land cover in New Zealand every five years. To date five versions have been released since 1996/97.
2. Leathwick, J. Clarkson, B. and Whaley, P. 1995. Vegetation of the Waikato Region: Current and Historic Perspectives. Landcare Research Contract Report LC9596/022. Landcare Research, Hamilton.
3. Ministry for the Environment & Department of Conservation 2007. Protecting our Places: Information about the statement of national priorities for protecting rare and threatened biodiversity on private land. Wellington, Ministry for the Environment & Department of Conservation.
4. Threatened Environments, Manaaki Whenua – Landcare Research
5. Thompson, S. Grüner, I. Gapare, N. 2003. New Zealand Land Cover Database Version 2. Illustrated Guide to Target Classes. Ministry for the Environment. Wellington.
6. Walker W, E. Cieraad and J Barringer. 2012. The Threatened Environment Classification for New Zealand 2012: a guide for users. Landcare Research Contract Report: LC2184.
7. Singers, N.J.D.; Rogers, G.M. 2014. A classification of New Zealand’s terrestrial ecosystems. Science for Conservation 325. Department of Conservation.
8. Walker W, E. Cieraad and J Barringer. 2012. The Threatened Environment Classification for New Zealand 2012: a guide for users. Landcare Research Contract Report: LC2184.