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b u l l e t i n the University of Sussex newsletter
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WRONG SORT OF GRASS -
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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