Kidman Springs Vegetation Recovery

Kidman Springs Exclosure Photos over 25 Years

Gary Bastin, CSIRO Wildlife & Ecology, PO Box 2111, Alice Springs NT 0871
John Ludwig, Robert Eager & Adam Liedloff, Tropical Savannas CRC
and CSIRO Wildlife & Ecology, PMB 44, Winnellie NT 0822
Reg Andison, Dept. Primary Industry, Bowen QLD 4805
Mike Cobiac, Dept. Primary Industries & Fisheries, PO Box 1346, Katherine NT 0851

(This article is reproduced here with the kind permission of the Australian Rangeland Society and the authors. It was originally published in the Range Management Newsletter, 2000, Vol 00/2, pages 1-5. To join the ARS, please contact - The Subscriptions Manager, Australian Rangeland Society, PO Box 235, CONDOBOLIN, NSW 2877. rrichards@dlwc.nsw.gov.au)

Introduction

Last year Ali Valamanesh told us how exclosures can be a valuable tool for studying long term vegetation change (Valamanesh 1999). For those of us who have worked with exclosures, it is often tricky to separate the effects of exclosure (i.e. protection from grazing) from year-to-year variation in rainfall. Fortunately, sometimes, photos alone can tell a powerful story. This is the case with the Kidman Springs exclosures.

Kidman Springs is a research station run by the NT Department of Primary Industry and Fisheries in the Victoria River District, 220 km south west of Katherine. The exclosures were erected in 1973 and measured annually between 1974 and 1979 (see Foran et al. 1985). Casual observations continued through the 1980s and the plots were next measured in 1989 (Bastin and Andison 1990) and in 1994 (RA & MC). Two of the exclosures were again measured in May 1999 (GB, JL, RE, AL).

The exclosures, 500 m square, were erected on three sites:

  • Calcareous red soil in "good" condition carrying mainly short-lived limestone grass (Enneapogon spp.).
  • Calcareous red soil in "poor" condition; eroded and growing a sparse pasture of native couch (Brachyachne convergens) and other annuals.
  • Cracking clay soil dominated by golden beard grass (Chrysopogon fallax) and flinders grass (Iseilema spp.).

Recovery on Red Soils

Pasture yield increased rapidly in the first few years following exclosure (Photo 1) and the Poor condition exclosure recovered to equal the Good condition area (Photo 2).

Photo 1. Pasture within the Good Condition exclosure in April 1973 (top) and June 1978 (bottom). Yield had increased over the five years and perennial grasses replaced much of the short-lived limestone grass. Rubberbush had invaded the exclosure by 1978 (see following comments).



Photo 2. The Poor Condition exclosure in April 1973 (top) and June 1978 (bottom). By the latter date, grasses had reclaimed most of the bare areas and pasture yield equaled that in the Good Condition exclosure (see Photo 1). Rubberbush also invaded this exclosure.

From 1973 to 1978, there was little difference in pasture yield and composition outside exclosures, suggesting a slow recovery on these grazed areas (Photo 3).

Photo 3. Pasture yield remained low from April 1973 (top) to June 1978 (bottom) on grazed areas outside exclosures. Native couch was the dominant species in 1978. Contrast this photo with the preceding 1978 photos (Photos 1 and 2) taken inside the exclosures.

There was little difference in measured yield inside and outside both red soil exclosures in 1989 and this remained the case over the next ten years (Photo 4). Pastures outside the exclosures have improved because of better cattle management as part of Kidman Springs Experiment Station operations and because of control of feral donkeys and horses.

Photo 4. In 1999, pasture yield was similar on grazed (top) and exclosed (bottom) areas. Black speargrass had colonised much of the grazed area producing sharp transitions in yield similar to that visible in the top photo.

Within exclosures, tall perennial tussock grasses such as whitegrass (Sehima nervosa), bluegrass (Dichanthium sericeum) and black speargrass (Heteropogon contortus) have covered areas of bare soil (Photo 5).

Photo 5. Large areas of bare soil occurred in the early years of exclosure (top). Tall perennial grasses covered these same areas 25 years later (bottom).

Wet-season rainfall was well above average in the mid 1970s (Fig. 1) and this no doubt drove the rapid improvement in pastures within exclosures. Rainfall during the 1980s was more variable with some wet-season totals being well below average. Despite this variability, perennial grasses have continued to increase on both exclosed and stocked areas, which are now more lightly grazed. An important feature of northern Australia is that at least some rain falls during each wet season. This feature, coupled with strategic spelling or lighter stocking, should mean that savanna pastures have an enhanced ability to recover from grazing compared with the more arid rangelands.

Numbers can sometimes supplement the story told by pictures. Figure 2 shows how pasture yield has changed in the poor condition exclosure, and on the surrounding grazed area over the last 25 years. The dominant features are:

  1. Yield rapidly increased in the first few years following exclosure. (It was depressed in 1976 because cattle broke into the exclosure.)
  2. By 1979, yield within the exclosure greatly exceeded that on the grazed area.
  3. Since then, yield has increased on the grazed area and is now similar to the destocked area.

Trees on the March

The density and composition of woody species have changed dramatically within the exclosures over the last 25 years. Initially, rubberbush (Calotropis procera) increased rapidly inside both exclosures to approach a density of 1,000 stems per ha (Photo 6). It then declined to obscurity with only five plants being observed within one of the exclosures in 1999.

Photo 6. Rubberbush invaded the exclosures in the 1970s to reach a maximum density of about 1,000 stems per ha in 1978 (top). It then declined during the poorer wet seasons of the 1980s, and by 1999 had almost completely disappeared (bottom, same area in 1999).

Native species have replaced rubberbush since 1979 and the woody density remains much higher within the exclosures (Photo 7). Common hakea (Hakea arborescens) and conkerberry (Carissa lanceolata) comprise the two main species.

Photo 7. Compared with their original open state (Photos 1 and 2), more native trees and shrubs were present in 1999, with grazed areas outside the exclosure having a lower density of woody species (top) than inside exclosures (bottom).

The thickening of woody species has probably occurred because the exclosures have been ungrazed and unburnt, except for a fire in April 1997 that burnt parts of the grazed paddock and inside one of the exclosure. This early dry-season "cool" fire was patchy and probably had little overall effect on woody density. Fire was an integral component in the evolution of savanna vegetation (Stocker & Mott 1981) and no doubt its absence has contributed to the greatly increased density of native woody species following long term protection from grazing.

Stability of Pasture on Cracking Clay

There have been slight year-to-year variations in pasture yield on the cracking clay site but no significant differences due to exclosure (Photo 8). Golden beard and flinders grasses have dominated throughout. Rosewood (Terminalia volucris) has increased in some parts of the exclosure, but not massively. The fire in April 1997 may have reduced tree thickening on this site, and promoted the current abundance of flinders grass.

Photo 8. Grazed cracking clay site in April 1973 (top) and May 1999 (bottom). Golden beard and flinders grasses were dominant on both occasions. These photos demonstrate the stability of this landscape type under continuous grazing.

Understanding and Managing Long Term Vegetation Change

Can knowing something about the past through a sequence of photos such as the above help us to manage for the future? We think so.

The photos of the cracking clay site clearly show that the golden beard and flinders grass pasture is stable through time and remarkably resistant to a moderate level of grazing. Further, they suggest that low-level photopoint monitoring and appropriately recorded notes are probably sufficient to guide grazing management decisions for this pasture. The main requirement is that photopoints be strategically placed to detect the most probable type of vegetation change — increase of woody species, particularly rosewood and perhaps bauhinia (Lysiphyllum cunninghamii).

A different train of events has occurred on the calcareous red soil. Degraded areas have a considerable propensity to recover in terms of pasture yield where control over grazing is possible. We consider that this is due to the occurrence of at least some rainfall each wet season. However, the outcome in terms of pasture composition is not always predictable. Complete protection from grazing has resulted in a rich variety of perennial grasses, and a composition very different to that originally perceived as being in "good" condition. We note though that composition of the herbage layer within the exclosures still may not truly represent the "good condition state" because of absence of fire. Continuous, but more moderate, grazing has gradually allowed pasture yield to increase - but to the point where this pasture is dominated by the one perennial, black speargrass.

The photos show us how pasture yield has increased and tell us a little about compositional change but they lack the panoramic view and clarity to provide the complete picture. In this case, long term measurement has provided valuable background about how the pasture has changed under grazing. This tells us that photopoint monitoring for management needs to be complemented with at least some data about pasture species composition.

The most striking feature of the photo sequence on the calcareous red soil is the dramatic change in the woody layer. The photos clearly show how rubberbush initially invaded the exclosures and was subsequently replaced by a high density of native woody species. Unfortunately, neither the photos or the collected data explain why this has occurred. We know that fire is a natural part of northern savannas and undoubtedly has an important role in shaping the long term tree-grass balance (Stocker & Mott 1981). We can only speculate on how its absence for 25+ years has influenced the current density of trees and shrubs within the exclosures. The photos tell us that native trees and shrubs have increased in the absence of grazing but it would be naïve to assume that this was the only causal factor.

To conclude, long term photos can tell us much about vegetation change in landscapes. In some cases, change is minimal and continued photographic evidence alone may be sufficient to guide management of the vegetation. Elsewhere, change can be substantial such as a large increase in woody density. Although photos do not explain why these changes occur, they can provide sufficient evidence that direct management intervention is required. The Kidman Springs sequence clearly illustrate both of these examples.

References

Bastin, G. and Andison, R. (1990). Kidman Springs country — ten years on. Range Manage. Newsl., No. 90/1, 15-19.

Foran, B.D., Bastin, G. and Hill, B. (1985). The pasture dynamics and management of two rangeland communities in the Victoria River District of the Northern Territory. Aust. Rangel. J., 7: 107-113.

Stocker, G.C. and Mott, J.J. (1981). Fire in the tropical forests and woodlands of northern Australia. Pp. 425-439. In: A.M. Gill, R.H. Groves and I.R. Noble (eds.), Fire and the Australian Biota. Australian Academy of Science, Canberra. 582 p.

Valamanesh, A. (1999). Exclosures as a tool in studies of rangeland vegetation dynamics. Range Manage. Newsl., No. 99/1, 7-10.



     
Contact the Audit Office
Copyright and Disclaimer Notice
Contact our Webmaster
© 2001 Commonwealth of Australia
Rangelands Information System v2.0