Plant natives and exotics

Hawke’s Bay farmer Peter Arthur puts up an argument for mixed planting. It might mean more work, but the results would be worth it, aesthetically and financially.

In Environment11 Minutes

Most people enjoy seeing native pohutukawa or kowhai when they are in flower and can even name them. Neither tree has any real economic value, though pohutukawa was used for ribs in boat building and woolsheds were made of kowhai in the Taihape area, but both are appreciated for their beauty.

Exotic pines on the other hand have great economic value, but planted en masse have become a much-detested tree. What would happen if we mixed the two – natives and exotics – together?

The native brigade tends to stick strictly to natives while the growers of exotics – pine, eucalypts, cypresses – ignore the natives.

Recently I received the meeting agenda for a plant group I belong to. Among other things they were going to look at a group of five Illawarra flame trees (Brachychiton acerifolius) in a Northland native reserve. The trees would be in full, spectacular flower at the time of the meeting. However, just prior to the meeting this visit was cancelled as the trees had been cut down because they weren’t natives.

Some years ago a farmer north of Gisborne wanted to do a big planting of natives as a commercial timber crop. As he would probably be dead by the time the trees were ready to harvest, he wanted to ensure the trees could be logged.

Because of all the red tape regarding the logging of natives, there was no way he could get a guarantee that the trees could eventually be felled. He ended up planting pines, currently not flavour of the month in the Gisborne area.

Let us mix things up and plant natives for timber mixed in with some flamboyant exotics for some spectacular colour. It’d certainly make the place more exciting to look at than our exceedingly dull pine forests. We could also brighten up some of the all-native riparian plantings with a few rhododendrons, liquidambars and magnolias.

With our wonderful climate we can be harvesting big trees only 20 years old. In many European and Canadian timber producing areas it would take 80 to 120 years to reach the same size.

Forty years ago, the Australian blackwood (Acacia melanoxylon) looked like a tree with great timber potential and the then Forest Research Institute planted a stand among natives in the Kaimai Mamaku Forest Park between Rotorua and Tirau. Even those well-versed in natives did not recognise this exotic intruder.

Farm foresters at the time busily planted this acacia, but it had terrible form. They tried pruning it like Pinus radiata but with little success.

I was on a farm forestry trip in the South Island and the farmer had planted blackwoods along the top edge of a basin that the house looked on to. Sometime later the missus decided to plant the basin with daffodils, went out with a spade, cut slits and dropped a bulb in each.

When we visited, the basin was a forest of nice, straight blackwoods. For every spade slit she had made she had cut the tree roots and up sprang dead-straight suckers.

This tree produces nicely coloured finishing timber, is a legume and has flowers for the bees. However, for the growers it was a flop as there was no market for it. One of the problems with these alternative timber trees is there is not enough volume to make it worthwhile setting up a special sawmill to handle them.

A monoculture, such as we have with pines, is not a good thing for all the other forms of life, such as birds, insects, lizards above ground, and the fungi, mycorrizha, earthworms and creatures that live below the surface.

In the 1950s the fertiliser works added DDT to superphosphate to control grass grubs. It killed the grubs but also the starlings that helped control the tortoise-shell beetle Paropsis, which ate eucalypt leaves.

With no starlings, Paropsis went mad, totally defoliating and killing some of the 120 species of eucalypts to be found in New Zealand. However, there were certain species of this Australian tree that Paropsis didn’t like and they are still with us today. The starlings are also back and Paropsis is not the problem it was.

European foresters have gone for a monoculture of Sitka spruce and in the last few years millions of trees have been killed by a bark beetle. There are more than 100 different species of pine trees, our P. radiata being just one of them. In the United States, thousands of hectares of various pines have been killed by bugs or disease. I have read there are about 18 different pests or diseases which, should they ever reach our shores in a tramper’s backpack, could attack our radiata forests. It would have the same economically devastating effect as foot and mouth disease.

Historically, European forests were a mixture of different types of trees that were selectively logged, often with horses. The very best trees were not harvested but kept as a seed source for natural regeneration.

The trouble with selective logging or continuous cover forestry is getting the individual tree out. A good system of tracks is needed in the first place. Mobile mills can be taken into the forest with the tree being taken out as sawn timber, or expensive helicopters used to lift the logs out. If the log is to be hauled out, care has to be taken not to damage other trees.

The joy of a monoculture is the ease of management. You plant the trees, prune them all at the same time, thin at the same time and clear-fell the lot when they are ready. The trouble on steep country is slips and erosion once the trees have been felled.

Selective or continuous cover crop forestry involves planting different species and taking out the individuals as they mature. Some species will be ready for harvest before others but there will always be a good tree cover, lessening the chances of Cyclone Gabrielle-type damage.

We have this mantra of right tree in the right place but still tend to be looking at it in a monocultural way – big blocks of a single species. I have a horrible-looking trial shelterbelt about 800 metres long. It runs up a slope with four very wet patches and at the top one very windy spot.

In the drier spots I have planted about eight different species of evergreen Mexican oaks in groups of 10, in the wet spots swamp cypress, dawn redwoods, kahikatea, tupelo or Nyssa sinensis and scarlet oaks.

In the windy spots I’ve used Wellingtonias (giant redwood) and scattered ginkgos throughout. It is a spotty mess with everything planted too close together, which is going to require a good thinning to keep the best trees. In that short distance of 800m there is a great variation of site conditions. Originally it was a pine shelter belt, but those in the wet spots were very poor trees.

For areas like Gisborne, belts of pines could be planted with big gaps in between the rows for natives or other exotics. Or the plantings could be done in groups or coupes using trees suitable to the site.

As the trees would be maturing at different times they would have to be selectively logged, which is much, much more expensive than clear-felling. However, there would always be trees on the site and eventually some beautiful timber from kauri, rimu, totara, kahikatea, beech, tawa, oaks, elms, ash, cedars, redwoods, paulownias, eucalypts and black walnuts. Though these trees are slow-growing compared to radiata, the timber would be worth much more.

To make things perfect I would add some really colourful trees, either for their autumn leaves or flowers. Absolutely worthless financially, just like the pohutukawa, but there is a tad more to life than just money.