Effect of fire regime on grass-layer plant dynamics in a tropical savanna
Creators: name: Scott, Kenneth
Publisher: name: Charles Darwin University
Year: 2008

description: Despite the regularity of fire in tropical savannas, the impact of different fire regimes on species abundance, and the role of fire in ecological processes, remains poorly documented. This thesis examines the vegetation dynamics of grass-layer plants in savanna, with particular focus on the effects of fire. It aimed to examine the relative importance of fire regime as a determinant of species composition, and the influence of fire on ecological processes, which are mechanisms of population change. These aims were addressed by describing the distribution of plant species at the community-scale, and undertaking experiments in a replicated manipulative fire experiment, at the Territory Wildlife Park near Darwin, Northern Territory, Australia. Six fire regimes were allocated as treatments in a randomised complete block design, where regimes comprised burning at different yearly intervals in either the early or late dry season (June and October respectively), including an unburnt control treatment. Shade house and laboratory experiments complimented field observations and experimentation. After three years, the species composition of grass-layer plants remained unchanged by the different fire regime treatments. Rather than fire regime, an increase in grass-layer plant density during the experiment coincided with an increase in rainfall, and grass species distribution was highly correlated with top soil moisture in the wet season, tree / shrub cover and the cover of tree leaf litter on the ground. These environmental factors drive species distribution patterns in part through their influence on seedling establishment. In a field sowing experiment, canopy and litter cover both significantly reduced the establishment of the annual grass Sorghum intrans F.Muell. ex Benth. In a shade house experiment, litter cover significantly reduced seedling emergence in seven annual and perennial grass species, regardless of whether seeds were placed above or below a 5 cm litter layer. Consequently, areas of the site with lower canopy and litter cover, as well as higher top soil moisture during the wet season, supported more diverse and species rich grass-layer communities. This resilience of the grass-layer to fire regimes over the short term (three years) was attributed to the dominance of annuals, which avoid fire-related seed mortality in a dry season soil seed bank, and the fire tolerance (resistance) of the perennial species. An examination of soil seed bank dynamics revealed no significant change in the density or richness of germinable seeds in the seed bank immediately after fire, and so the seed bank remains a source of seedling regeneration in the subsequent wet season. Adult perennial grass tussocks were also highly resistant to fire. After three years, there were no significant differences in the survivorship of three common species (Eriachne triseta Nees ex Steud., Eriachne avenacea R.Br. and Chrysopogon latifolius S.T.Blake) between different fire regime treatments. In contrast to this resilience over the short term, differences in fire frequency over the longer-term (14 years) appeared to considerably influence grass species composition. An examination of composition between areas of contrasting fire frequencies during the previous 14 years revealed the apparent loss of regionallydominant, taller grass species (Sorghum intrans and Heteropogon triticeus (R.Br.) Stapf) in areas of lower fire frequency. The approach taken here of examining life cycle processes as causes of population change has application in the management of plant populations elsewhere, because it has a mechanistic basis rather than being purely phenomenological.