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

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.

Are the eucalypt and non-eucalypt components of Australian tropical savannas independent?

“…in particular, dominate the tree biomass of Australia's tropical savannas, yet they account for only a small fraction of the tree diversity (Taylor and Dunlop 1985;Bowman and Prior 2004;Prior et al 2006Prior et al , 2009Russell-Smith et al 2010). Among the key processes affecting tree abundance of tropical savannas are fire and herbivory (Werner 2005;Prior et al 2006;Werner et al 2006;Russell-Smith et al 2010), although complex synergistic interactions with other environmental determinants are likely, such as the seasonality and amount of rainfall Communicated by Peter Clarke. (Sankaran et al 2004;D'Odorico et al 2006), soil characteristics, such as soil depth , and the eco-taxonomic grouping and stand density of trees ).…”

Fire controls population structure in four dominant tree species in a tropical savanna

“…In central north Australia the average Wre frequency is once every two years (Russell-Smith et al 1997). A distinct lack of sapling-or pole-sized trees (>150 cm in height and <5 cm diameter at breast height) has been repeatedly observed in many Australian mesic savannas (Bowman 1986;Werner 1986;Bowman et al 1988;Fensham and Bowman 1992;Bowman and Panton 1993;Bowman and Panton 1994;SetterWeld 1997;Vigilante and Bowman 2004;Werner et al 2006). This is despite high densities of co-occurring juvenile trees (<150 cm in height as deWned by Prior et al 2006) of the same species (Fensham and Bowman 1992;Wilson and Bowman 1994;Lehmann, unpublished data;Williams, unpublished data).…”

“…Factors other than Wre also aVect sapling growth and recruitment. In general, removal of understorey herbaceous vegetation by grazing or clipping increases tree growth (Werner et al 2006). Both rainfall variability and topographic position can inXuence the availability of water and hence sapling growth.…”

Savanna Responses to Feral Buffalo in Kakadu National Park, Australia

“…Mortality of juvenile trees in savanna woodland in buffalo-present vs. buffalo-absent plots, averaged across all time intervals over the six years of the experiment, in plots that received no fire vs. in plots that received high-intensity, lateseason fires. The figure is from Werner et al (2006), used with permission.…”