Restoring native grasses to the understorey of The Pinnacle
New knowledge that will aid recovery of degraded box-gum grassy woodlands
A Friends of The Pinnacle project funded by: ACT Environment Grant 2010,
Caring for Our Country Community Action Grant 2010-11, and Canberra Labor Club
The widespread invasion of exotic grasses depletes native biodiversity. Exotic grasses contribute to reducing the number of bird species, reptile species and invertebrates. In the Australian Capital Territory, a number of woodland bird species are threatened with extinction, and habitat degradation by weed invasion contributes to this threat
The most widespread groups of weeds at The Pinnacle are the exotic grasses, along with sorrel and exotic clover species. This cluster of species likely represents the most profound environmental change throughout the park, both because they cover the majority of the area, and because they exclude most other native plants. They also represent a substantial barrier to the restoration of native fauna. Restoration of The Pinnacle therefore requires that this most widespread and difficult problem is addressed.
Based on earlier scientific findings, fotpin proposes to examine methods for reducing nutrients, and the effects of nutrient reduction on weed control and native plant recovery. Our approach is to carefully monitor nutrient levels in trials that compare slash removal, burning, sugar application and crop harvest as means to drive down the nutrient load.
Our approach also includes consideration of the impacts of herbivores. Grazing by rabbits and kangaroos is substantial at The Pinnacle Nature Reserve, evidenced both by the obvious prevalence of these animals, and by our observations that grasses have been grazed back to an extent that has limited flowering. If grazers remove plant material and replace the nutrients through urine and dung, then it may limit our capacity to remove nutrients. Furthermore, grazing by herbivores may also limit the capacity of native plants to re-establish in our experimental plots. Before we can implement widespread restoration activities, it is critical to understand how important herbivore management might be.
A third variable that we regard as potentially important is variation in the level of exotic plant cover. At Goorooyaroo Nature Reserve, McIntyre et al (2010) found plant communities dominated by exotic grasses had elevated soil nutrients, whereas soil nutrients were lower in communities dominated by native grasses. Conceivably, areas of The Pinnacle that have retained some cover of native grasses will have different nutrient loads and so may respond to our treatments in a different way.
Our management questions are:
- Can nutrient levels and cover of exotic species be as effectively reduced by burning, slashing or harvesting a crop plant as it can be by applying sugar?
- Are native species enhanced or exotic species reduced by burning, slashing, harvesting a crop plant or applying sugar?
- Is there a substantial erosion risk associated with any of the treatments, particularly the fire treatment?
- Do high rates of herbivory by kangaroos and rabbits limit our capacity to remove the nutrients contained in vegetation?
- Do sites that retain native grass species have increased soil nutrients and can such sites be restored using nutrient manipulation?