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Net woody vegetation increase confined to seasonally inundated lowlands in an Australian tropical savanna, Victoria River District, Northern Territory

Sharp, Ben R. and Bowman, David (2004). Net woody vegetation increase confined to seasonally inundated lowlands in an Australian tropical savanna, Victoria River District, Northern Territory. Austral Ecology,29(6):667-683.

Document type: Journal Article
Citation counts: Scopus Citation Count Cited 11 times in Scopus Article | Citations

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Title Net woody vegetation increase confined to seasonally inundated lowlands in an Australian tropical savanna, Victoria River District, Northern Territory
Author Sharp, Ben R.
Bowman, David
Journal Name Austral Ecology
Publication Date 2004
Volume Number 29
Issue Number 6
ISSN 1442-9985   (check CDU catalogue open catalogue search in new window)
Scopus ID 2-s2.0-12244300516
Start Page 667
End Page 683
Total Pages 17
Place of Publication Carlton, Victoria, Australia
Publisher Blackwell Publishing
Field of Research 0602 - Ecology
HERDC Category C1 - Journal Article (DEST)
Abstract Georeferenced digital aerial photographs were used to assess changes in overstorey vegetation cover since 1948 in the Victoria River District, Northern Territory, Australia, across a range of lowland tropical savanna habitats and with explicit consideration of known and variable site-specific grazing and fire management histories. Vegetation surveys at corresponding locations on the ground identified five distinct woody vegetation communities defined primarily by water drainage and secondarily by soil characteristics. Air-photo analyses revealed that, contrary to popular perceptions and in contrast to results from other habitats, there has been no generalized net increase in overstorey woody vegetation cover across the full range of lowland savanna habitats. Rather, different habitats exhibited distinctly different vegetation change mechanisms: low-lying seasonally inundated 'wet' habitats have experienced woody vegetation increase since 1948, whereas well-drained 'dry' habitats have experienced overstorey vegetation stability or loss. In almost every instance woody vegetation increase could be attributed to the invasion or proliferation of a single species, Melaleuca minutifolia EMuell. The extent of M. minutifolia increase was unrelated to historical grazing/fire regime. Demographic analyses for this species revealed that recruitment was often episodic and that synchronized recruitment events occurred uniformly across the full range of historical management treatments, most likely as a consequence of favourable climatic conditions in years with an extended wet season. Heavy grazing facilitated juvenile survival and/or recruitment, most likely by reducing grassy fuel loads and eliminating landscape fire. We conclude that while there has been no generalized net increase in overstorey woody vegetation cover in lowland environments, savanna dynamics are complex, and multiple change mechanisms have occurred simultaneously in different habitats, some of which have been significantly transformed since 1948. Where net woody vegetation increase has occurred it is primarily a natural consequence of episodic M. minutifolia establishment in climatically favourable years, but the extent and magnitude of this effect is likely mediated by fire.
Keywords fire
grazing
tropical savannas
vegetation dynamics
woody plant encroachment
victoria river district
northern territory
national-park
eucalyptus
woodland
impact
west
DOI http://dx.doi.org/10.1111/j.1442-9993.2004.01407.x   (check subscription with CDU E-Gateway service for CDU Staff and Students  check subscription with CDU E-Gateway in new window)
 
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