Date of Award
Doctor of Philosophy
School of Science
Dr Eddie van Etten
Dr David Goodall
Tropical savannas in South East Asia are important yet understudied ecosystems. In fact, the description of savanna is limited in the Indonesian Archipelago, a region which, based on climate alone, would be expected to have mostly forest. In this thesis, I compared and contrasted the vegetation characteristics of four savannas in the wetter part of the Indonesian archipelago (Java – Bali – Lombok) to understand how fire and grazing influence their dynamics, and searched for evidence of savanna origins, maintenance, invasion by exotic/woody species and possible ecosystem transitions. Vegetation surveys, remote sensing techniques and statistical models were used to spatially and temporally analyse the savanna community composition and the environmental variables and disturbances that influence their structure.
there are distinct elevation gradients (along with related climatic factors such as temperature and precipitation), as well as fire regime gradients, linked to tropical savanna community composition across Java, Bali and Lombok Islands. These compositions are characterized by different sets of species, and where invasive alien species are becoming significant components of the ecosystems. Lack of prescribed fire and a range of invasive species threaten to convert savanna at Bali Barat and Alas Purwo into secondary forests or shrubland, whereas the presence of forest pioneer/edge species within the savanna at Rinjani suggests successional change from grassland to forest may occur in the absence of future fires (although the role of soil, topography and microclimate in maintaining grass dominance needs also to be further explored). Compared to the others studied, the savanna in Baluran National Park has characteristics of being relatively old and persistent rather than one being created and maintained via recent human conversion of forests.
Overall in Indonesia, there is much less savanna compared to forests, hence it is expected that a greater percentage of savanna is burned. Using remote sensing analysis, I confirmed that approximately 2% of savanna/open vegetation had burned over a 14 year period, whereas only 0.8% of forest has burned across Indonesia. The extent and frequency of burning is mostly associated with annual Southern Oscillation Index (SOI). Most burning occurred in years when the SOI sustained negative SOI values, which generally means drier conditions across South East Asia.
I also developed species distribution models for the main invasive alien species of the savanna ecosystems studied, Acacia nilotica, to establish its invasion potential, both locally in Baluran National Park and regionally in other parts of Indonesia. Acacia nilotica was different from the other invasive species studied, in that it is promoted by herbivory, and possibly also by fire. It appears that spatially, A. nilotica is rapidly advancing into the savanna of Baluran National Parks where it was observed that over fourteen years the savanna size has decreased (-1,361 ha), whilst the A. nilotica stand has increased in area (+ 1,886 ha). It was demonstrated that fire and grazing play an important role in this invasion. Results also show that global climate change is likely to increase the potential distribution of A. nilotica in Indonesia and the area at risk of invasion. By year 2045, A. nilotica has potential to spread across much of the eastern parts of Indonesia. As fire and grazing are common to savannas of eastern Indonesia, they are likely to facilitate its invasion into these areas.
In summary, I have shown that savanna plant community in Indonesia is formed and maintained by interactions between climatic factors, fire regime and grazing. Invasive species were also present in the studied savannas such as Chromolaena odorata, and Lantana camara. These invasive species together with forest pioneer/edge specialist species (Ficus septica, Laportea stimulans, Melastoma polyanthum, Nauclea orientalis, Rubus rosifolius), may also be increasing in absence of fire and also may be leading to change of state from savanna to dense woody vegetation. Absence of fire seems to be changing structure and floristic of savanna vegetation which has implications for savanna species including rare fauna such as Jalak Bali/Bali Starling (Leucopsar rotschildi) and Javan Banteng/Wild Java cattle (Bos javanicus subsp. javanicus). Results from this thesis showed that Bali Starling range in Bali Island has shrunk to remaining small patches of fire-induced open shrub and savanna woodland found below an elevation of 150–175 m in the north-east part of peninsular Prapat Agung of Bali Barat National Park. The description of the savanna dynamics presented here provides further evidence of the complexity of the savanna ecosystem and its susceptibility to change as a result of changing fire regimes and invasion by invasive species. A greater understanding of the possible ecosystem processes driving the dynamics of the savannas will assist in the formulation of successful savanna management strategies at local and regional scales.
Sutomo (2017). Ecology of Savanna Ecosystems in Indonesia. https://ro.ecu.edu.au/theses/2020