A plan for pioneers
In the wake of more East Coast land lost to a severe weather event and an estimated 1 million tonnes of forestry slash washed into waterways, Whangara farm-forester Nick Seymour sees the reintroduction of pioneer species such as manuka, kanuka, tutu, flax and ferns as a way to preserve precious topsoil on steep country.
Thirty years ago, on Wednesday 7th March 1988, Cyclone Bola was a benchmark event for the East Coast region. At Tauwhareparae Station, 900 millimetres of rain fell in 36 hours. During the recent heavy rain event the top end of Whangara got 270mm over 12 hours.
The forces of nature can be huge, and the weight of sea, wind, fire and water is devastating. We cannot change that, but as landholders we must manage the outcomes of each the best we can. We have only a short period of time in this beautiful country.
The recent heavy rain was short and sharp. In any high-intensity event, how water-flow impacts downstream depends on the shape of the catchment. The Waiomoko catchment at Whangara is long and winding with small, contributor catchments. Water, and some groundwater, builds up at each catchment. If a catchment is pear-shaped, water rises quickly and it all comes down together.
Pine harvest contributes to erosion. We cannot blame anyone for planting out whole areas but the problem is some of that class 6 land should not be planted in pine, and heavy timber trees should not be planted on slopes at more than 30 degrees. Steep land cleared for agriculture has much less root reinforcement and is vulnerable in a high-rainfall event.
Slash is created during pine harvest as tops of trees and branches break off. We have excellent engineers who have designed slash catchers that are put into catchments during harvest. The catchers are designed to catch big logs and can hold only so much.
Sections of trees should be left intact at the bottom of catchments to catch debris. At present, there is a lot of slash still up there and ready to come down. It is not over yet.
During Bola, steep hill country in pasture lost most of its topsoil. The soil raced down rivers and on to the flood plains where houses, fruit orchards and crops were ruined by silted floodwater. After the cyclone, a lot of damaged hill country was planted in pinus radiata. This species holds one quarter of the immediate rainfall within the tree, then releases it in a timely fashion.
Grow pioneer species on steep landDamage done to the land by Bola had to be covered because once topsoils are lost, what is left is papa, a soft sediment rock, and that creates slips and landslides. That was the motivation for the establishment of the East Coast Forestry Project (now the Erosion Control Funding Programme) in which landowners tendered for money to plant out areas.
Trees were planted on 6 to 7 class land. In hindsight, some of the steep class 7 land should not have been planted. Class 7 land is regarded as unsuitable for arable cropping and pastoral grazing, and has low suitability for production forestry. After trees are planted in steep class 7 land, the land is susceptible to erosion for the first seven years. Over the 20 following years, tree cover and soils protect the land during heavy rain.
Once trees are harvested, the soil is opened up again. The roots will hold the country for four years. By then two-year-old plants are in the ground and for the next four years the soils are unproductive.
Those four years are critical.
There are some things we cannot change and water is one, as is the uncertainty of climatic high rainfall and wind events that are commanded by atmospheric high and low pressure systems. Climate change has happened in New Zealand since this land came out of the sea, and the climate, with heavy rainfall events, has shaped the topography. Thirty years ago, and prior to that, we would get a high-intensity tropical cyclone on average every nine years.
Earthquakes can fracture soil structure. After a rolling motion on the land in 1958 we had a number of drop-out slips in unlikely places on our farm Wensleydale.
The sculpted hills in our East Coast region are mainly of a mudstone base, which is highly erodible and soluble. River flats were laid by the erosion on the hills and with large trees felled by the weight of rain and silted water. This happened before indigenous bush was established and happens today with trees of all sizes in earth flows and slips.
Bola-damaged areas left to revert to manuka and kanuka have little erosion.
Pioneer species need to be allowed to grow on this region’s steep land.
Some damaged country is too remote or it is physically impossible to plant trees on it, but nature clothed similar country with time and pioneer plant species.
The first to grow would be manuka, kanuka, tutu, tauhinu, tawini, coprosma, hebe, pittosporum karo, ferns and flax. These species slowly built up humus to create top soils. I have read it takes 1000 years to create 25mm of top soil.
Larger forest species of trees arrive with birds and establish cover for the pioneer species. It will take hundreds of years under natural processes to cover these raw, eroding areas that a cyclone like Bola destroyed in a few days. Nature has done it before, but introduced pests are now present.
The first step is to eradicate pests such as goats, possums, deer and stoats, as they will destroy seedlings and the bird population.
The next step is for local people to collect seeds of all the pioneer plant species then inoculate and lime-coat them.
Application has to be from the air. Helicopters can fly to these remote locations. The lime-coated seed will spread evenly and provide a starter pack for germination.
Such a programme should continue for at least 10 years. Funding would be from the left-over funds of the East Coast Forestry Project.
This is probably too hard for politicians to get their heads around but there needs to be mutual agreement to start the process.
The weight of heavy rain events immenseWorking on Wensleydale, north of Gisborne, for 55 years is not a long time in New Zealand history but we have had our weather challenges.
Our longest drought was in 1982-83 with 151 days without rain. Our biggest flood since 1938 was a localised event in 1977 with 425 millimetres in 17 hours.
The 1977 high rainfall event is a good case to follow, as the Waiomoko Catchment is 7500 hectares. That is small compared with the many catchments in the East Coast region.
With an area of 7500ha, 25mm of rain at 1700 tonnes per ha, is useful rain that is absorbed into the earth.
In a heavy rainfall event there is little time for absorption. Cracks and weak spots take the rainwater and, with its weight, it lubricates earth movements in soil slipping and earthflows.
Rainfall in the 7500ha catchment totalled 425mm (17 inches) in 17 hours. The result was 12,167,500 cubic tonnes of water that flooded to the sea.
Wensleydale, which is in the centre of this catchment, had the same rainfall. Half of that weight — 6,083,750 cubic tonnes — of water rushed down a winding, roaring Waiomoko River and over the river flats to deposit a quarter metre of silt that also ended up in our woolshed and sheep yards.
By 1959, the East Cape Catchment Board was active with farm maps, five-year soil erosion planting programmes and grant money for interested land owners who wanted to protect their hills and waterways with debris dams, willow and poplar poles.
This opportunity was too effective to let go. We completed five five-year plans and strengthened our vulnerable country with tree roots.
Our Catchment Board plantings held up well during the high rainfall of 1977 but unplanted, unprotected, steep country was a mess, with near 70 percent loss of pasture.
That land is now planted in pinus radiata.