1. Interactive effects of alien species invasion and landscape development on an endangered coastal forest community. This project, soon to be published (I hope!), consisted of a broad scale survey of an endangered coastal swamp forest invaded by the alien turf grass Stenotaphrum secundatum along the south-east coast of Australia. I surveyed sites across a gradient of urbanisation in the surrounding landscape ‘matrix’, in order to examine the interactive effects of invasion and anthropogenic landscape modification on the forest community. I expected to find that such interactive effects, if at all present, would be negatively synergistic, such that the magnitude of native species loss in response to invasion would increase with urbanisation in the matrix. Unexpectedly, the opposite effect was evident: the number of native species occupying invaded sites increased with urbanisation, to such an extent, in fact, that invaded sites in highly urbanised areas had a similar number of species as non-invaded forest! Indeed, the magnitude of native species loss in response to invasion was about two-times higher in patches of forest associated with relatively non-disturbed, ‘pristine’ landscapes with a very high cover and connectivity of indigenous forest!
The obvious question is: does urbanisation improve the capacity for native plants to coexist with S. secundatum and, if so, what is the mechanism behind this? Urbanisation is well-known to increase the amount of nutrients (mainly of nitrates) in coastal vegetation. One possibility is that increased nutrient availability reduces the ‘strength’ of the invader-native competitive interactions, either by diminishing the competitive ‘vigour’ of the invader or by enhancing the competitive ‘vigour’ of the natives. This is all speculative, of course, but I’m testing the ways in which increases in nutrient availability modulate the competitive interactions between the S. secundatum and native plants with a mesocosm experiment (see below), the results of which should be out shortly.
2. Seed bank responses to alien grass invasion. Seed banks play a fundamental role in the recruitment and assembly dynamics of plant communities. They are also a plant community‘s primary means of regeneration following disturbances, such as fire and alien plant invasion, which damage or remove the standing vegetation. As such, they can be thought of as a community’s ‘insurance’ policy against environmental change. In this study I investigated the effects of alien grass invasion and the accumulation of alien litter on the diversity of native seed banks in an endangered coastal forest. I found that invaded sites had significantly fewer native species represented in the seed bank, and significantly different germinant compositions. I found that alien litter, despite doubling the mass of litter on the forest floor, had no effect on the accumulation of seed, or the incorporation of seed in the seed bank.
3. Variation in invasive species impacts across native plant functional groups. In this, my most recent of studies, I’ve found that impacts of the invader S. secundatum on native plant functional diversity in the seed bank depend on species’ seed size (which represents maternal investment in offspring) and mode of dispersal. The number of propagules of wind and water dispersed species in the seed bank did not vary between invaded and native sites. However, the number of vertebrate dispersed species, including those spread by frugivores as well as carried externally on animal’s fur and feathers, was significantly lower in invaded soil, which I speculate may be caused by differences in the densities or behaviours of seed dispersal vectors, such as red-neck and swamp wallabies, between invaded and native forest.
4. Interactive effects of alien grass invasion and nutrient addition on community assembly and reproduction of native plants.
5. Impacts of grass invasion on litter dynamics in coastal forest
6. Are the competitive effects of alien grasses with contrasting photosynthetic strategies (C3 vs. C4) modulated by ‘extreme’ heat? As part of this project, I’m also interested in examining how the patterns of extreme heat (i.e. heatwave vs. non-consecutive days of extreme heat) influence native-alien competitive interactions.