Belowground competition and growth of juvenile trees in a long-unburnt Australian savanna

Most research on tropical savanna vegetation has focused on the factors promoting the co-existence of trees and grasses, and competition between savanna trees has received relatively little attention. Northern Australian savannas are amongst the most-fire prone landscapes on Earth and are dominated by a uniquely fire-adapted group of trees: eucalypts. Other northern Australian savanna trees tend to be largely suppressed by frequent fires. We used a root-exclusion experiment in a long-term (ca. 40-year) fire-excluded savanna to investigate the growth responses of juveniles of four dominant savanna tree species, both eucalypt (Eucalyptus miniata and Eucalyptus tetrodonta) and non-eucalypt (Terminalia ferdinandiana and Buchanania obovata), with and without root competition from neighbouring trees. Our results provide evidence of juvenile trees are not subjected to strong competition from neighbouring adult trees, as there was no difference in growth between root-excluded treatments and controls. However, Eucalyptus miniata, a dominant tree species in these savannas, had reduced growth, in terms of both height and diameter, regardless of the root-exclusion treatment. Our study suggests that growth of juvenile trees is not strongly limited by root competition from other trees. However, our results provide support that fire exclusion favour non-eucalypts by limiting the growth and recruitment of savanna eucalypts. Large reductions in fire frequency and/or intensity may lead to a shift in dominance away from eucalypts in the tropical savannas of northern Australia.