Flooding as an ecological disturbance in native grasslands



The native tussock grasslands in northern Victoria, unlike those on the Victorian volcanic plain, occur on alluvial floodplains and hence, flooding is a natural but rare disturbance. Indeed, the last time the plains were extensively flooded was 1974, so trying to study flood impacts on plant community dynamics isn't the easiest thing to do. In January 2011, northern Victoria received more than half its average rainfall in a single day with the result that large areas of the grassland plains flooded. What an opportunity to document an ecosystem's response!



The heavens are about to open on the plains near Echuca (Photo: M.Kohout)



Flooding is likely to be an important long-distance seed dispersal agent, but this has been under-appreciated in the literature on grassland dynamics (although I note that the Australian Centre for Biodiversity at Monash Uni is interested in similar events in terms of aquatic plant community organisation; see Aquatic Dispersal Project for more information).



Extensive flooding near Kerang (Photo: C. Moxham)

  

Tracking seed movements during flood events must be hard to do, but I reckon it's worth investigating for one simple reason. On the flat plains of northern Victoria, plant species with seeds that might normally fall in close proximity to maternal plants are potentially capable of moving many tens of kilometres during flood events, particularly if they have small or bouyant seeds. This has important implications for species distributions (perhaps allowing species to migrate in response to climate change), the transfer of genetic material amongst populations, and the re-assembly of plant communities by species that may rarely come into contact with one another.



Native grasslands were inundated for weeks (Photo: C. Moxham)

 

When the Loddon River broke its banks near Serpentine, native grasslands dominated by Spear Grass were inundated by tens of centimetres of water and the flooding lasted for several weeks. This included a research site I've been using as part of a global collaboration on factors that impact on grassland diversity (but that's for another day....). No doubt this has large effects on seed dispersal, but it's also likely to impact on plant survival (because of waterlogging), and nutrient enrichment (with the potential to lead to a dramatic increase in invasive species). And what impact it'll have on germination this winter is anyones guess!

It'll be interesting to see just how much change occurs in the grasslands we've been following during the last decade of drought. Stay tuned.....revisiting our permanent plots this spring should be really interesting. Perhaps we have seen the 'event' that'll determine the structure and function of these grasslands for the coming years.