No abstract

Riparian habitats are highly important ecosystems for tropical biodiversity, and highly threatened ecosystems through changing disturbance regimes and weed invasion. An experimental study was conducted to assess the ecosystem impacts of fire regimes introduced for the removal of the exotic woody vine, Cryptostegia grandiflora, in tropical north-eastern Australian woodlands. Experimental sites in subcatchments of the Burdekin River, northern Queensland, Australia, were subjected to combinations of early wet-season and dry-season fires, and single and repeated fires, with an unburnt control. Woody vegetation was sampled using permanent quadrats to record and monitor plants species, number and size-class. Sampling was conducted pre-fire in 1999 and post-fire in 2002. All fire regimes were effective in reducing the number and biomass of C. grandiflora shrubs and vines. Few woodland or riparian species were found to be fire-sensitive and community composition did not change markedly under any fire regime.The more intense dry-season fires impacted the structure of non-target vegetation, with large reductions in the number of sapling trees (<5 cm d.b.h.) and reductions in the largest tree size-class and total tree basal area. Unexpectedly, medium-sized canopy trees (10-30 cm d.b.h.) appear to have been significantly benefited by fires, with decreases in number of trees of this size-class in the absence of fire. Although the presence of C. grandiflora as a vine in riparian forest canopies changed the nature and intensity of crown combustion patterns, this did not lead to the initiation of a self-perpetuating weed-fire cycle, as invaders were unable to take advantage of gaps caused by fire. Low intensity, early wet-season burning, or early dry-season burning, is recommended for control of C. grandiflora in order to minimize the fire intensity and risk of the loss of large habitat trees in riparian habitats.

The shallow tidal and freshwater coastal wetlands adjacent to the Great Barrier Reef lagoon provide a vital nursery and feeding complex that supports the life cycles of marine and freshwater fish, important native vegetation and vital bird habitat. Urban and agricultural development threaten these wetlands, with many of the coastal wetlands becoming lost or changed due to the construction of artificial barriers (e.g. bunds, roads, culverts and floodgates). Infestation by weeds has become a major issue within many of the wetlands modified (bunded) for ponded pasture growth last century. A range of expensive chemical and mechanical control methods have been used in an attempt to restore some of these coastal wetlands, with limited success. This study describes an alternative approach to those methods, investigating the impact of tidal reinstatement after bund removal on weed infestation, associated changes in water quality, and fish biodiversity, in the Boolgooroo lagoon region of the Mungalla wetlands, East of Ingham in North Queensland. High resolution remote sensing, electrofishing and in-water logging was used to track changes over time-1 year before and 4 years after removal of an earth bund. With tides only penetrating the wetland a few times yearly, gross changes towards a more natural system occurred within a relatively short timeframe, leading to a major reduction in infestation of olive hymenachne, water hyacinth and salvina, reappearance of native vegetation, improvements in water quality, and a tripling of fish diversity. Weed abundance and water quality does appear to oscillate however, dependent on summer rainfall, as changes in hydraulic pressure stops or allows tidal ingress (fresh/saline cycling). With an estimated 30% of coastal wetlands bunded in the Great Barrier Reef region, a passive remediation method such as reintroduction of tidal flow by removal of an earth bund or levee could provide a more cost effective and sustainable means of controlling freshwater weeds and improving coastal water quality into the future.

Paddock‐scale Acacia nilotica L. Willd. ex Del. (prickly acacia) spatial distribution, seed production and dispersal patterns were investigated in the Astrebla (Mitchell) grasslands of northern Australia as a step toward predicting future patterns of invasion. A number of hypotheses were tested based on what we know of this species in both Australia and regions where it is native, for example South Africa. It was hypothesized that most A. nilotica seeds would be produced by trees in riparian habitats with access to permanent water. In addition, we predicted that seeds would be dispersed throughout the Astrebla grassland landscape by cattle, following observations that cattle readily ingest and pass seeds and that cattle have access to all areas within paddock boundary fences. Tree density, seed production and seed dispersal by cattle were measured along a series of transects from permanent watering points to paddock boundary fences. Trees associated with permanent watering points produced more seeds per unit area and occurred at higher density than their non‐riparian counterparts. The importance of riparian trees decreased in years with high rainfall and in paddocks with only small areas of riparian habitat. Cattle spread dung and seeds throughout paddocks, with peaks of deposition adjacent to permanent watering points. These results suggest that invasion patterns are likely to be uneven across the landscape and may be reactive to climate. High seedling recruitment and possible thicket formation is expected adjacent to permanent watering points and wherever cattle congregate. Patterns of recruitment in non‐riparian areas are likely to be relatively sparse. The importance of post‐dispersal factors in determining recruitment patterns is discussed.