From John Ludwig, 2000. Vegetation structure,
pattern and function. In Managing for healthy country in the VRD
eds. Tropical Savannas CRC. John Ludwig is from CSIRO Wildlife and
Ecology.
The aim of studies into the structure, pattern and function of
vegetation in the VRD is to gain a better understanding of the
district's broader environmental processes. Combining knowledge of
various plant communities with what is known of other elements of
the environment provides a better understanding of how landscapes
function as a whole. This in turn will enable a better appreciation
of which management regimes are required to conserve natural
resources and maintain habitats for plants and animals.
Three questions were initially posed:
- Have trees thickened in the VRD;
- How 'tough' is the country; and
- How well does it recover from damage?
Evidence shows that trees have thickened in the VRD, but mostly
on heavier, clay soils on river or creek frontages, and
floodplains. Comparisons of ground-based photos from 1973, 1989 and
1994 (Figures 1, 2 and 3), aerial photos from between 1970 and
1991, field surveys and satellite information, clearly shows this
increase.
Click on "Thickening vegetation on black soils" at left, to
view the figures.
(see also the Savanna Links, Issue 14, Tropical Savannas,
Not what they used to be', see linked article below).
On the other hand, trees do not appear to have thickened on most
loamy and sandy soils, including those supporting eucalypt savannas
on hills, plains and broken country. There does, in fact, appear to
have been a decline in woody vegetation on sandstone country where
fires are frequent. Further work is being undertaken to examine
changes in the tree/grass character of savannas in the VRD and some
of this is explained in the 'Ongoing research — Woody
vegetation increase' section of VRD grazing pages.
The second question was examined by looking at the amount and
type of vegetation present at sites within various distances from
watering points. Information was collected from field surveys,
aerial photographs and satellite imagery. This information
indicated that mitchell grasslands on black, cracking clay soils
were more resistant to cattle grazing than eucalypt savannas on
calcareous red loam soils. This finding supported the general
wisdom of pastoral managers in the VRD.
Finally, the study showed that as well as being more vulnerable
to grazing impacts, eucalypt savannas on these red loam soils
tended to recover much faster from long periods of impacts when
starting from a better, rather than a poor condition. This is
evident from comparing vegetation changes from 1971 to 1999 shown
in Figures 4 to 9. Figures 4 to 6 are from a site that started in a
relatively good condition, while Figures 7 to 9 are from a site
starting in a relatively poor condition. Both these sites are in
enclosures built in 1973 in the Conkerberry Paddock at Kidman
Springs.
Click on "Thickening vegetation: red soils", at left, to
view the figures.
Implications for savanna management
These results imply that savannas on red soils will need to be
managed more carefully if changes in vegetation are to be avoided.
If trees have thickened in an area, then fire could be used to
reduce this thickening. However, the level of grazing may need to
be reduced to ensure sufficient fuel for these fires. Grazing may
also have to be reduced to allow areas of degraded red soil to
recover.
Articles
Tropical Savannas: not what they used to be
Many landscapes of the tropical savannas gradually being transformed. From Savanna Links, Issue 14, April - June 2000 [
read more...]
