What is a Wetland?

Wetlands are determined by:

  • Hydrology (‘wetness’)
  • Soils
  • Plants and animals

Hydrology

Wetlands do not have to be saturated with water all year round… but they do have to be saturated intermittently. The hydrology needs to be monitored over at least a year which makes it impractical for most assessments.

Soils

Wetland (hydric) soils are defined by the water saturation. Soils are useful in defining a wetland because spot samples are definitive. However, the Resource Management Act definition is silent about soils.

Plants and Animals

Various plants and animals that are adapted to wet conditions are good indicators of wetlands. There is a national protocol for defining wetlands based on vegetation (Clarkson, 2014). All plants have been given ratings on a scale of 1-5 for how much they need wet conditions. Some plants have a specialized need for a wet root zone.

Wetland Habitat Classes

To help assess whether an area is a wetland these classes are used:

Class Description Probability

OBL

Obligate Wetland. Rarely in uplands (drylands). Estimated probability >99% in wetlands
FACW Facultative Wetland. Usually in wetlands, occasionally in uplands 67-99%
FAC Facultative. Commonly occurs in wetlands and uplands 34-66%
FACU Facultative Uplands. Occasionally in wetlands but usually in uplands 1-33% in wetlands
UPL Obligate Upland. Rarely in wetlands, almost always in uplands <1%

 The first three indicator groups (OBL, FACW, FAC) are indicative of a wetland. In order for an area to be called a wetland, its dominance test must be over 50% and its prevalence index = or < 3.

Dominance Test

The dominance test is the percentage of dominant species across all strata that are OBL, FACW or FAC. The three strata are trees/canopy, understory/shrubs and herbs/groundcover. Dominant species are all plant species that cumulatively cover immmediately in excess of 50% of the total vegetation cover in each stratum (when ranked from highest to lowest cover), and any additional species that cover at least 20% of the total cover in the stratum.

Prevalence Index

The prevalence index is a vegetation based system of weighted averages incorporating the percentage cover of species and their affinity for wet conditions. The species cover percentages are multiplied by 1 for OBL species, 2 for FACW, 3 for FAC, 4 for FACU, and 5 for UPL, and the total is divided by the sum of percentage covers.

Determining a Wetland

The presence of a couple of isolated OBL plants alone does not mean it is a wetland, just as the presence of a couple of isolated UPL plants alone does not mean it is dry land.

If a wetland is dominated by FAC plants and the dominance test and prevalence index do not agree, it is necessary to also look at the hydrology, soils, and whether the conditions are natural or disturbed.

This method is realistic and pragamatic and will  result in smaller areas being mapped as wetland than using the “20% wetland plant cover” definition used in the past. The method is more scientifically robust and consistent with practices nationally. However, it is complex and needs relatively detailed botanical knowledge.

‘Boggy’ areas of pasture grasses and exotic rushes are excluded from the wetland definition (and rules that go with that) under the Tasman Resource Management Plan, and so are most wetlands not formed by natural processes.

If there is a wet area on your property you are uncertain about, and would like to develop or restore, feel free to contact council for a free site visit by a wetland ecologist. The ecologist can determine whether it is a wetland, define the boundaries and provide advice on how you can restore the wetland, or keep adjacent pasture dry while keeping the wetland wet.