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Brittleness Scale:
A Critical Insight into Landscape Function
Why does overgrazing in
Otago, Canterbury, and Marlborough lead to bare
soil and erosion but the same practice year after year in Southland and
West
Coast doesn’t produce the same effect? The
answer lies in a concept known as brittleness.
The
Brittleness Scale relates to the annual
distribution of soil surface moisture. At
one end of the scale are brittle environments, places where t here is no soil
surface moisture on any day of the year. A
true desert would lie at this end of the scale, but
places in the
South Island are only a whisker away like Seddon, Marlborough, and the
Hakataramea, North Otago. Rainfall in
these seasonal environments is typically low or erratic in its
distribution.
At the
other end are non-brittle
environments, those places that experience 100% humidity at the soil
surface
everyday of the year. Traditionally a
rainforest would fit this category and places like the West Coast and
coastal
Southland would be very close to this end of the scale.
Rainfall here is relatively reliable.
The relevance
of the brittleness scale
becomes clear when combined
with the carbon cycle. Most living
organisms are carbon based and organic matter is an essential food
source for
soil life. In non-brittle areas plants
tend to rot from the base upward allowing carbon to cycle through the
soil and
add to the organic matter. However, it
is the lack of moisture to drive the decay process in brittle areas
that causes
problems. Instead plants oxidise and
breakdown through chemical and physical weathering allowing precious
carbon to
escape into the atmosphere.
Large
areas of the South Island lie around
5 or above on the brittleness scale. This
is why bare earth is a common site on many properties
along the
east coast. When bare soil smoothes over
and caps it reduces the effectiveness of the total rainfall because the
water
can’t penetrate the soil surface. If
total rainfall is the basis of stocking rate calculations, not the
effective
rainfall, the higher stocking rate will aggravate any overgrazing and
increase
the area of bare soil.
Good
organic matter levels are extremely
important in any soils but arguably more so in brittle environments
where the
rainfall is low or erratic. It is
essential
in such environments that landscapes absorb water rapidly and release
it slowly
to encourage biological decay and prolong the cycling of carbon.
Problems
occur in brittle environments when
left undisturbed or rested. Growing
plants mature and die with no way of breaking down and returning to the
soil
within the same season. Standing dead
plant material blocks light to the growing points for the next season’s
growth
thereby starving the plant of light and eventually killing it. Over a period of time the old plant blows
away leaving bare soil. This leaves the
landscape with a “patchy” look of grass stands and bare earth.
Much of
New Zealand tussock country looks
this way and is testimony to the rest these landscapes have experienced.
The
practice of set stocking exacerbates
the problem by running low numbers of animals over large areas. The lack of stock pressure through low stock
densities results in some areas being over-rested, allowing the
accumulation of
plant material to choke plants to death. Simultaneously
other areas are overgrazed where all
surface litter is
removed leading to bare soil.
 Any horse paddock shows
these classic signs
no matter how many animals are present. They
dung at one end and refuse to graze there while overgrazing the other. In larger landscapes, low animal densities
lead to trailing and the trails become prone to wind and water erosion.
Conventional
rotational grazing also adds
to the problem by not allowing plants sufficient recovery from severe
grazing. Favoured plants eventually
disappear over
time leading to oversowing with seed and fertiliser.
Bare
earth is seldom if ever generated in
non-brittle areas because of the constant levels of moisture driving
the decay
process. Therefore, plants are always
growing
and covering the soil even when overgrazing occurs.
To
counteract many of the problems brittle
areas face, we can look at how animals and plants coexist in natural
grassland ecosystems. In seasonal rainfall
environments like
Africa, huge herds of animals roamed the grasslands yet there is little
evidence of overgrazing until fairly recently (last 200 years).
Animals herd for
protection from predators. As a result the
herbage not eaten is trampled
into the soil surface to decay when the rains arrive.
Overtrampling doesn’t occur because the
animals are always moving on fresh grass away from their own dung. They only return to an area once their own
fouling
has worn off and this determines their rotation length. The
digestive systems of these animals
substitute the microbial activity of the soil in such situations.
Many
rotational grazing systems try to mimic
this pattern but fears of plant quality drive less than adequate
recovery
times. If farmers selected for plants
that cured better and maintained their quality longer this fear would
prove
false and provide greater flexibility to buffer times of moisture
stress. A greater diversity of pasture
species would
also improve grazing flexibility.
 Only through large numbers of
animals bunched
in high densities moving over the landscape can brittle environments
increase
their productivity through improving water absorption and retention,
better
carbon cycling, greater photosynthetic area and activity, as well as
increasing
the biodiversity of the plants and soil life.
Removing
livestock from brittle tending
environments is a recipe for disaster, as the landscape will continue
to erode
and fill the rivers and dams with precious topsoil.
Only by using the tools of grazing and animal
impact can we cheaply regenerate the productive capacity of brittle
areas like
much of the east coast and the high country. Changing
grazing management will not only improve farm
production but
also benefit the community by reducing the severity of droughts and
floods.
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