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Triffid Tor Grass Threatens Downs

An unwelcome species of grass is causing major problems on the South Downs, threatening the future of an area we all know and love. The fine closely- cropped, springy turf, with its orchids and other wild flowers, its butterflies and bush-crickets, is being invaded and replaced by the long coarse tor grass, Brachypodium pinnatum. Although this is a native grass, it has invaded sites where it has no previous history and, once established, shades out the smaller plants and therefore dominates the plant community, eliminating other species. It is unpalatable to grazing animals, unless they are very hungry, and this allows it to form expanding and proliferating patches of long, yellow-green grass, in an otherwise closely grazed sward.

The special nature of chalk grassland is a product of low soil nutrients and heavy grazing. The spread of tor grass in this sort of grassland is not a problem unique to the South Downs or, indeed, to the UK. In the Netherlands, increases in tor grass seem to be associated with atmospheric pollution which increases soil nitrogen and favours the spread of this coarse and vigorous grass rather than that of the slower growing chalk grassland species. Fertiliser application has a similar effect. In the UK, however, atmospheric nitrogenous pollution is not as high and our chalk grasslands, in the south-east, have a much lower level of soil nutrients than the Dutch problem sites. Nevertheless, increased nitrogen in the soil may still have an important role, and research by Audra Hurst, working with Libby John in BIOLS, suggests that this increase is brought about by the presence of tor grass itself which alters the soil environment to favour its own growth.

Audra found that the level of nitrates in the soil was higher inside the srtands of tor grass than in immediately adjacent areas. Lack of grazing seems to allow nutrients to build up, as little or no plant material is removed from the system. When tor grass dies it produces a vast amount of plant litter, which rots in situ, returning its nutrients to the soil, and litter also plays an important part by shading out more desirable chalk grassland species. The experiments have shown that removal of this litter is essential for controlling this grass, both by lowering soil nitrogen and by making it possible for seedlings of other species to become established. This explains the lack of success of reserve wardens who have become so desperate to get rid of tor grass that they have been driven to use herbicides. This may kill it, but also has the effect of fertilising the soil. Removal by cutting and raking promises much better long term control.

A certain amount of long grass, even Brachypodium, can have its attractions and a mixed sward, with warm well grazed patches and areas of longer grass for shelter and protection, is ideal both for butterflies and for our famous local wart-biter bush-cricket. Left to itself, however, tor grass will let little else survive.

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Friday June 20th 1997

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