Understorey vegetation interactions and the effect of wet season burning in a tropical savanna, Northern Territory

  • Annette Margaret Lane

    Student thesis: Other thesis - CDU


    The understorey of tropical savannas in the Kakadu region of northern Australia is dominated by species of annual Sorghum {sorghum) (Poaceae). Reduction in the abundance of sorghum by wet season burning in such savannas is likely to impapt on the abundance and composition of other understorey elements. Further, the reduced rate of litter biomass accumulation (as a consequence of reduced sorghum biomass), may change the germination, emergence and growth conditions experienced by understorey plants. There have been no studies on the factors influencing the composition and abundance of savanna understorey vegetation or the impact of wet season burning on understorey interactions.

    In this study, I examined the abundance of understorey elements in relation to sorghum density, time since last fire, and fire regime (wet season or dry season) during the wet seasons of 1994-95 and 1995-96.

    The biomass of several understorey groups - forbs, Galactia tenuiflora ( Galactia, a dominant perennial forb), annual grasses, perennial grasses, and sedges - was not related to natural variation in sorghum density. In contrast, when sorghum was thinned from high densities {in both wet season burnt and dry season burnt sites) non-sorghum annual grasses, rare in dry season burnt sites, showed a significant response to variation in sorghum density in wet season burnt sites. The fecundity and mass of sorghum itself was also density-dependent in both fire regimes.

    At both natural and thinned sorghum densities and across fire regimes, forb biomass, and the biomass of non-sorghum annual grasses, were negatively correlated with litter biomass, and sorghum density was positively correlated with litter biomass.

    Species richness of forbs and annual grasses was significantly higher in wet season burnt sites, and, whilst the biomass of forbs was similar between fire regimes, the species composition was substantially different, with about half the species from each regime being exclusive to that fire type.

    Across a range of natural sorghum densities, application of fertiliser did not increase biomass of forbs, perennial grasses or sedges, indicating that these groups are not affected by competition with sorghum for nutrients. Sorghum, in contrast, responded positively to the increase in nutrients with a significant increase in fecundity. Other annual grasses were rare and their response could not be tested.

    Experimentally increasing shade over the understorey did not significantly affect the biomass of forbs, sorghum or other annual grasses. It seems unlikely, therefore, that competition with sorghum for light at these sites was a factor limiting the abundance of forbs.

    The immediate effect of wet season burning and variation in ground surface litter abundance on species richness and biomass of forbs was assessed in the 1995-96 wet season. There was no significant effect of fire or litter on species richness of forbs or on the biomass of non-sorghum annual grasses. The biomass of forbs other than Galactia, however, was significantly higher in wet season burnt areas. Wet season burning, therefore, enhanced establishment and growth of forbs other than Galactia.

    This study has demonstrated that the abundance of understorey elements was not directly determined by competitive effects of sorghum. Rather, it appeared to be primarily determined by the indirect effect of litter biomass. Increasing biomass of litter was negatively correlated with the abundance of forbs. Wet season burning in the short term leads to a change in species composition and structure of the ground stratum. Over a few seasons of relatively low sorghum density and consequently reduced levels of litter accumulation, forbs and other annual grasses increased in species richness and biomass. Replacement of sorghum with less flammable annual grasses leads to a reduction in fuel loads and consequent reduction in fire intensity. Over time, this is likely to subtly alter habitats in tropical savanna woodlands.

    Date of Award1996
    Original languageEnglish

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