Our research focuses on the population dynamics of plants and how they are influenced by impacts of natural disturbances and global environmental change. We are particularly interested in the interactive effects of fire, grazing and drought in grasslands and woodlands in southern Australia, and how climate change, fragmentation and shrub encroachment affect ecosystems.

Sunday, 10 February 2013

Why does a C4 grass fail to recolonise a C3 grassland when it is the better competitor?

This post is more a question than an insight. But it follows on from previous posts about mono-dominance in the Australian perennial C4 grass Themeda triandra (Kangaroo Grass).

Kangaroo Grass, dominating grasslands near Benambra in eastern Victoria
(Photo: John Morgan, January 2013)
Kangaroo Grass forms mono-specific swards across the basalt plains of western Victoria, and is a dominant feature of grasslands elsewhere in south-east Australia. I've mused about this before (see Monodominance in C4 Grasslands and How to Burn a Grassland) - is it because it is a superior competitor and excludes C3 grasses, or perhaps it is because it regenerates vigorously after fire whereas C3 grasses do not?
But what happens when there is disturbance to the soil, as opposed to disturbance to the vegetation?

Kangaroo Grass dominates on the right. But, on the left, where rocks have been removed, Wallaby Grasses (a C3 grass) dominate. It begs the question. Why?
(Photo: Tim Wills, January 2007)

In this shot, the view is rotated 180 degrees to the last one. Kangaroo Grass is now on the LHS, and the Wallaby Grasses can be seen dominating the grassland on the RHS.
(Photo: Tim Wills, January 2007)

The above photos highlight something that I think is pretty important.

When Kangaroo Grass grasslands are disturbed by ploughing, rock removal and heavy grazing, it declines and becomes absent. 100% cover to 0% in one or two easy steps. We know this from fenceline comparisons, and examining ploughed grasslands with intact ones. What replaces them (in areas where fertilizers are not added) are generally native grasses such as Spear Grass, Wallaby Grass and Plume Grass, plus exotic annuals. These are all C3 grasses.

But when these disturbances are removed, as far as I can tell, Kangaroo Grass seems not to re-assert it's dominance on the 'new' or 'novel' native grasslands. Why is this?

Two things come to mind (well, three if I think about it long enough).
1) Time - one reason that Themeda has not re-established is because it has not had enough time to do so. The rate of secondary succession operates at different temporal scales in ecosystems, depending on disturbance type & intensity, propagule availability and habitat productivity. Here, ploughing is a pretty severe disturbance, and it is likely that few Themeda propagules survive intensive land use. But, you'd think you would see some evidence that Themeda was recovering with increasing time-since-cultivation (assuming a linear relationship, which is dangerous to do) by examining the edges of the ploughed area. But, when you do so, you see very little evidence that Themeda is creeping into the C3 grassland
2) Dispersal - propagule pressure is a key attribute that contributes to the invasion of native ecosystems by exotic plant species. Indeed, the establishment of C3 native grasses after ploughing assumes that these new grasses are readily-dispersed, and there are many seeds available for colonisation. Of course, it may not be this simple. Soil disturbance may facilitate establishment of C3 grasses by breaking soil crusts, reducing above- and below-ground competition with C4 grasses, and generally increasing microsite availability.

But why would dispersal hold up recovery of Themeda after the cessation of ploughing, particularly when it is the dominant grass in the adjoining area?

Dave Tilman, Phil Grime and others have speculated that one of the tradeoffs with being an excellent competitor (e.g. high growth rate, tall stature, etc) is that this comes at a cost. That cost, for dominant grasses (and competitors more generally), is limited dispersal capacity. Many dominant plant species in ecosystems find it hard to re-occupy space because their propagules are not well-designed for long-distance dispersal. They often face an extinction-debt in small habitat fragments because of their inability to disperse through the landscape, making them vulnerable to environmental change. Is that what is limiting Themeda recovery here?

3) Competition - it seems almost unthinkable to suggest that C3 grasses that colonise after disturbance might limit C4 re-establishment because they are better competitors. But, they do occupy the site, and their maximal growth period can overlap with that of seedling emergence of C4 grasses, perhaps making the C4 seedlings vulnerable to water stress early in their life. This has not been experimentally tested (to my knowledge) so we can only speculate about the role that competition between established tussocks and seedlings play here. I suspect it would very much depend on rainfall amount each season.

I'd like to test these ideas with experiments - the most obvious is to spread some seed of Themeda around, well away from the existing plants, and see if the species can develop seedlings with/without C3 grasses in their near vicinity. This would untangle the role of competition and seed limitation for a start. I bet you (a nice bottle of red seems like a good place to start) that we'd get Themeda seedlings and tussocks within two years. And I bet we'd have more seedlings when we introduce more seeds. I think dispersal-limitation is a key here, at least for initial seedling establishment.

What do you think?