by John Walsby
Most of New Zealand's
countryside has been changed by deliberate or accidental introductions of plants and animals and we now have an unnatural, managed landscape
made up largely of farms, plantation forests and residential settlements.
A few pockets of original natural habitats survive but most bush
blocks have had their large trees logged out and wetlands beside
lakes, rivers and harbours have mostly been reduced to narrow
fringes where there were once broad sweeps of marsh.
Over the last decade the values of forest cover on steep slopes in helping to control land slippage and retain topsoil and of marshland plants on lowland flats in preventing rivers and lakes from becoming silted up, have been much more widely appreciated. Wetlands have also been shown to be efficient cleansers of pollutants from waste and run-off water and in some areas are starting to be encouraged or planted specifically for water purification purposes.
These changing attitudes have come about as responses to failures of past land management practices and to accommodate further development and changing land use. Over just the last 20 years there have been substantial changes in the farmed land. The size of the national sheep flock has decreased by about 20% and large areas of land that was once grazed is now used for other purposes
Steep country that was never highly productive in grass and prone to slips has seen large scale conversion to pine plantations and this should improve the quality of stormwater run off. In both natural and plantation forest the leafy canopy delays the fall of rain to the ground below where the blanket of rotting branches, leaf litter, mosses, ferns and other forest floor plants soaks up the water from a downpour and only releases it slowly. By preventing it rushing off bare slopes, erosion of topsoil is substantially reduced and therefore is much less silting up of waterways.
Close to towns and cities, especially in northern New Zealand, a large amount of farm land has been "developed" for residential and industrial use to accommodate the growing population and to feed the locals, the area of market gardens has increased.
These changes have in turn led to an increased demand for water for drinking, washing, irrigation and industrial purposes and this is matched by an equivalent increase in waste water. In addition as land is built over or kept open for horticulture, storm water management has become more difficult. Heavy rainfall gushes straight off these areas flooding drains and scouring away exposed soil surfaces.
To clean up these intermittent silt laden torrents, and to cleanse partially treated water running from small sewerage treatment schemes elsewhere, artificial wetlands are being developed with beds of rushes and sedges. The dense thickets of their upright stems slow the water flow allowing fine suspended clay particles to settle out and they become consolidated among the growing root systems of the plants.
The marshes also absorb and process nutrients which elsewhere can be troublesome.
Flushed as excess fertiliser from horticultural blocks and farm paddocks or running out with industrial and domestic waste water, high concentrations of nutrient can stimulate algal blooms in still or sluggish waterways. When these blooms die and are decomposed by bacteria, oxygen depletion results and fish, crustaceans and other natural freshwater animal life can suffocate and die.
Not many plants can flourish with their roots in standing water or waterlogged soil because the oxygen required for vigorous root growth is quickly used up but not easily replenished. However, the stems of rushes and sedges are hollow and this allows oxygen that enters above the water level to diffuse down these pipes to the fibrous root region.
By absorbing and utilising the nutrients, the large wetland plants and the abundant microscopic life that grows over and amongst them, all flourish.Their growth supports teeming populations of small crustaceans and insect larvae and these in turn become food for larger insects like the beautiful damsel flies and dragon flies and for a number of wetland birds.
Though pukekos and herons are still common, draining natural wetlands has destroyed essential feeding and nesting habitat for such birds as the banded rail, spotless and marsh crakes, bittern and the fernbird. Creating new wetlands for improved land and waterway management may also provide more suitable habitat for some of these uncommon and rare birds.