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|The why and what of wetland plants|
What are the plants in your wetland doing?
There are so many variants of form and use of constructed wetlands that no two perform alike. They range from open water settling ponds with virtually no vegetation, to densely planted ponds designed to give a final ‘polishing' to water prior to discharge.
Some are designed to catch only the severest floods, while others have those heaviest flows diverted away so as not to disturb the normal processes. Each has its own requirements and different design capabilities. The measurement of effectiveness must be based on the wetland's purpose, but most have the primary objective of improving water quality by some means. The purely mechanical methods are perhaps the most easily measured while those that rely on plants to remedy/mitigate water pollution are more difficult to assess.
The Auckland Regional Council (6-2 Auckland Regional Council Technical Publication #10)
Wong et al (1998) list the advantages of vegetation in a constructed wetland stormwater treatment system as follows:
During baseflow the vegetation provides for the following benefits over unvegetated ponds:
During storm events vegetation also provides the following physical benefits:
It is clear that for most wetlands there is an advantage in having plants performing one or more functions to purify the water before discharge. The two fundamental issues when selecting the plants for your wetland are the conditions they will encounter, and the tasks you want them to perform. The plants' requirement to grow vigorously and your requirements of the wetland, will dictate what plants you choose and where in the wetland they will perform best.
Immediately in front of the water inlet you may require a species capable of controlling the more severe floods. While Phormium tenax (flax) is the most robust native species, it has the disadvantage in a major flood of resisting the waters to the point of being torn out roots and all, taking the soil with it. In such circumstances a more giving plant that will slow most flows, but bend under floods, would be a better choice. The Baumea species might be suitable as they are robust but with their creeping rhizome are less likely to be dislodged. Baumea are particularly capable of growing in acidic soils/waters with high concentrations of nutrients and contaminants.
Where wave action is a concern around the edge of wetlands, choose a species that can tolerate the movement and break the eroding action. In nature Bolboschoenus fluviatilus is found growing along the edge of estuaries and it tolerates not only the action of waves but the rise and fall of tides, and the salinity that varies with its position within the estuary. It does have the disadvantage of being deciduous and least effective in winter when perhaps most needed. In that case choose another species growing along estuaries Apodasmia similis (formerly Leptocarpus), commonly known as oioi. This plant has the added advantage of being an efficient filter for sediment and suspended particles in the water.
If the wetland's catchment is prone to erosion the design of the pond may require a special ‘holding tank' which can be cleaned periodically by mechanical means. Traditionally in such circumstances exotic plants such as Glyceria maxima and, more recently Schoenoplectus californicus, were used. When the sediment was cleaned out, the plants were also removed and dumped to eventually find their way into the nearest waterway and establish as weeds. By using species that are local to the area, any ‘escapees' will not become a nuisance weed and they may, indeed, enhance the natural environment.
The sedge genus Carex provides a range of species for differing conditions - from the amphibious sward which may vary periodically from very dry to very wet where Carex virgata will do well, to totally inundated where Carex secta is recommended. For eroding banks, either in the pond or along streams, the best species is one of the Carex lessoniana/geminata complex. With their tough rhizomes and pliable foliage they anchor the soil and lie flat during flooding to provide a bed for silt deposition, sprouting quickly back up to again anchor the new soil.
Other bottom dwelling plants in water to one metre deep are Myriophyllum propinquum and Potamogeton cheesemanii. They provide a floating carpet on the surface of the pond which gives maximum contact between the water and the leaf surfaces on which micro-organisms can grow. Similarly the free-floating fern Azolla filiculoides absorbs nutrients and provides rapid and extensive surface cover. In the northern half of the North Island it is being dominated by the introduced Azolla pinnata.
One category of wetland plant that is little understood and seldom grown is the native aquatic algae. These inhabit clear water sometimes as deep as twenty metres or occasionally grow alongside shady stream banks. Two main genera Chara and Nitella occur throughout the country and are probably a good indicator of unpolluted waters. Establishment of algae in constructed wetlands is theoretically possible in larger ponds, but they would best be introduced at a later stage when stability in growth of other plants has been achieved.
The most numerous species belong to the genus Juncus, with Juncus pallidus being the largest at about two metres tall. It tends to inhabit shallower water, but provides a sturdy vigorous structure to protect other species. The many other species of Juncus range in size shape and function. The commonest are unfortunately mostly exotic weeds. Although they can perform the same functions, they oust our native species from their narrow habitat niche.
We ask a lot from our wetlands, requiring them to fulfil the functions of natural wetlands that nature would have made ten or twenty times the size. Only by constant management and surveillance can we expect them to meet our requirements.