Viewpoint

"Natural" Grasslands

By Pat Palmer

After the last great retreat of the ice about 10,000 years ago, and by the time of the arrival of the Maori about 1,000 years ago, the South Island became covered with forest from the bushline to the sea. Tussock grasslands were confined to areas recently cleared of forest by flood or natural fire, or the thinnest soil or the driest parts of the inland basins.

With the Maori came uncontrolled fires. Grass pollens replaced tree pollens, and charcoal in the deposits of this period confirms Maori traditions of widespread forest destruction. The whole of the region was visited regularly by food gatherers and so was liable to repeated burning, which suppressed forest regeneration. The wonder is not that so much forest was destroyed, but that any survived at all.

A thousand years of intermittent burning culminated in reducing millions of hectares of primeval forest to tussock grasslands. The grasslands were made and are maintained by Man, fire and sheep. They are only marginally more natural than the Christchurch Botanic Gardens. But now various groups such as sheep farmers, the Canterbury Regional Council and the Maruia Society wish to perpetuate them.

I wonder at their wish to preserve this brief historical hiccup for posterity, and at their disregard for the ecological processes involved. I wonder why the early botanists believed that the tussocks were climax vegetation growing in a "grassland climate" too dry for forest, despite the evidence of the charred logs around them.

When vegetation is burned, the nutrients go up in smoke or are reduced to ash and flushed away in the next rain. Plants with lower fertility needs and lower production potentials, such as the now despised Hieracium, replace the higher producers. If the burning cycles persist often enough, the ultimate result is regression to desert. Parts of the Mackenzie Country are now near this state.

The land can be farmed permanently if it is fertilised to grow enough grass to support enough animals to keep the trees out without periodic burning. This would doom the tussock grasslands to be converted to swards of introduced plants. It is not likely to happen, because returns from farming are not likely to pay for the investment.

There is another alternative: the progression back to forest could be permitted or even encouraged but, strangely, this process the conservationists wish to prevent. We know that conifers produce 10 to 50 times as much biomass as depleted tussock grasslands, and the Forest Research Institute has shown that conifers have an unexpected ability to rejuvenate worn-out soils. In their plots at Craigieburn, pines planted on running shingle scree have built a deep soil in 20 years. The soil is much more fertile than the surrounding grassland soils, and has a much higher phosphate content. Many of the tree species establish and spread naturally. These three pluses could provide a key to the recuperation of both the monetary and natural economy of these beleaguered lands.

What are the problems? Pastoralists see the trees as a threat to their cherished way of life, but could alter their views. Foresters themselves label some of the wilding species as weeds -- in other lands these "weeds" are forestry mainstays. Conservationists see the conifers as alien intruders in the natural tussock grasslands, but are not very concerned about much more widespread invaders such as hares-foot trefoil or the yellowhammer. It seems that the pines are being picked on, and that multiple standards are being used in judging the tolerability of introduced species.

Contrary to the views of early settlers who thought native things inferior, native forests establish under and emerge above scrub associations such as gorse and heath. They also establish in pine plantations and, given time, will suppress them. Conservationists could eke out their scarce resources by planting native trees strategically to re-seed the tussock.

Canute would not recommend trying to turn back the tide of the pines, which we would better regard as progress on the way to a natural reforested future, rather than as alien invaders of the "natural" tussocks.

Pat Palmer is interested in encouraging debate on alternative floras.