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EnviroNorth > VRD-Sturt > Landscape and Climate > Vegetation structure and function

Vegetation structure and function

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.

Study's aim

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?

Vegetation change

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.

Resilience

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.

Grazing impacts

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...]