Australian Journal of Botany Australian Journal of Botany Society
Southern hemisphere botanical ecosystems
RESEARCH ARTICLE

Juvenile tree growth and demography in response to feral water buffalo in savannas of northern Australia: an experimental field study in Kakadu National Park

P. A. Werner A B C F , I. D. Cowie D and J. S. Cusack E
+ Author Affiliations
- Author Affiliations

A Centre for Resource and Environmental Studies, Australian National University, Canberra, ACT 0200, Australia.

B School for Environment Research, Charles Darwin University, Darwin, NT 0909, Australia.

C W. K. Kellogg Biological Station, Michigan State University, Hickory Corners, MI 49060, USA.

D Northern Territory Herbarium, NT Department of Infrastructure, Planning and Environment, PO Box 496, Palmerston, NT 0831, Australia.

E CSIRO Tropical Ecosystems Research Centre, PMB 44, Winnellie, NT 0821, Australia.

F Corresponding author. Email: wernerpa@ufl.edu

Australian Journal of Botany 54(3) 283-296 https://doi.org/10.1071/BT04135
Submitted: 7 September 2004  Accepted: 13 October 2005   Published: 15 May 2006

Abstract

Tree populations in the wooded savannas of northern Australia lack a well developed seed bank, but instead rely on a persistent ground layer of suppressed juvenile plants <1 m of indeterminate ages. The feral Asian water buffalo has been implicated as a factor in inhibiting the movement of juvenile trees to sapling stage. In a 6-year field study in Kakadu National Park, 656 juvenile plants were monitored for growth and survival, and in exploring the effects of buffalo, clipping surrounding vegetation and ambient fire. Buffalo were removed from one half of the sites across an environmental gradient in a landscape-scale experiment. Juvenile trees grew more slowly but fewer became dormant where buffalo were absent. Experimental clipping of surrounding vegetation produced significantly increased height at the buffalo-absent sites, but not at the buffalo-present sites, demonstrating that the positive effect of buffalo on growth was indirect, by reducing the competitive regime for juvenile trees. Within 3 years of removal of buffalo, ground-level green biomass increased 2–10 times and litter 1.3–2 times over a topographic gradient. Mortality of juvenile trees was not significantly different between buffalo-present and buffalo-absent sites when plots remained unburnt. By contrast, at buffalo-absent sites, three times more juveniles died after late dry-season fires than at unburnt sites, but there was no difference among buffalo-present sites. Overall, the total juvenile tree bank decreased by 7% where buffalo grazed and by 18% where buffalo had been removed. The study demonstrated a major mechanism(s) responsible for recorded changes in vegetation patterns of these savannas, whereby buffalo initiate a cascade of effects by changing ground-level biomass, which change competitive relationships and fuel loads, which then have an impact on tree growth and demography. The results are discussed with respect to dynamics of the juvenile tree bank and implications for long-term sustainability of these wooded savannas.


Acknowledgments

The field project was initiated with funding by the USA Man and Biosphere Program through the auspices of the USA Forest Service, Pacific Region (Grant No 59-PSW-82-005-G to P. A. Werner and P. G. Murphy). Dr M. G. Ridpath (CSIRO) conceived of, and set up, the original buffalo experiment; Mssrs P. Pan Quee, P. Brady, and D. Murphy (CSIRO, resident managers at Kapalga) maintained the buffalo-proof fence and protected the study plots from fires more than once with help from CSIRO and Kakadu National Park staff. Thanks go to W. Murray, J. Browne, K. Stagg and D .Murphy (all CSIRO) for field assistance; to J. S. Harrison (dec. MSU), K. Tidemann (CSIRO), S. Pidsley (CSIRO) and C. R. Dunlop (NT Conservation Commission) for technical assistance; M. Haynes (SIROMATH) for statistical analyses; L. D. Prior (Charles Darwin University (CDU)) for production of graphs and figures; and to Drs G. H. Lauff (MSU), R. J. Wasson (ANU) and B. M. Campbell, K. Gibb and G. J. E. Hill (all CDU) for infrastructure support. The paper was greatly improved by discussions with colleagues at Charles Darwin University and by constructive reviews of drafts by Drs R. J. Williams, L. D. Prior, I. J. McNaught, C. E. R. Lehmann and anonymous reviewers. We are very much indebted to all the above for their support, insights and considerable expertise. This is Contribution No. 1157 of the MSU Kellogg Biological Station and CSIRO TERC Publication No. 1312.


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