Monodominance in C4 grasslands

A typical view in southern Australia. Here,
Kangaroo Grass dominates the structure
and cover of a native grassland.
(Photo: John Morgan)

One of the things that strikes you about the C4 grasslands in southern Australia is the complete dominance by Kangaroo Grass (Themeda triandra). The concept of monodominance is actully rather rare in grasslands. Only a minority of the world's grasses (around 600 out of 11,00 speces) are documented as being ecologically dominant. These dominant species, however, seem to share a common(ish) evolutionary history.

In an interesting paper on the origins of C4 grasslands by Erika Edwards and collegues, dominant grasses appear to be phylogenetically clustered, suggesting that certain clades of grasses are more prone than others to evolve traits that promote ecological dominance. But what might these traits be?

The answer to this question is not as straightforward as we might presume. While we accept the fact that Kangaroo Grass dominates the grasslands of southern Australia, there is still some uncertainty about why it does so. There's likely to be a few reasons. Some of them are evolutionary, while others are more ecological. Here, I outline a couple that I think are likely to be important.

Kangaroo Grass is a C4 species. "C4 photosynthesis" refers to a suite of biochemical and anatomical traits that increase photosynthetic efficiency in high light and high temperature environments. While C4 enhances the efficiency of photosynthesis, C4 plants only have an advantage over C3 plants in certain conditions - namely, high temperatures and low rainfall. Hence, C4 grasses are conspicuously absent from the world's cooler regions. This may, in part, explain the dominance of C4 grasses such as Kangaroo Grass in southern Australia.

Kangaroo Grass, in Grime's CSR plant strategy scheme, would comfortably be considered a competitive species. Such species are able to outcompete other plants by most efficiently tapping into available resources. Competitors do this through a combination of favorable characteristics, including rapid growth rate, high productivity (growth in height, lateral spread, and root mass), and high capacity for phenotypic plasticity. This last feature allows competitors to be highly flexible in morphology and adjust the allocation of resources throughout the various parts of the plant as needed over the course of the growing season.

Kangaroo Grass might be thought of as a pyrogladiator. It is fire-adapted, resprouting strongly from basal meristems with very little fire-induced mortality. By contrast, our Lab has shown that C3 grasses, such as Wallaby Grass and Spear Grass, can experience substantial levels of tussock mortality after fire (perhaps because of the fire event itself), further weakening their position in C4-dominated grasslands. Kangaroo Grass, by contrast, quickly accummulates biomass between fires, probably because the C4 pathway supports high photosynthetic rates and nitrogen use efficiencies, especially in the high-light environments after fire. The high water-use efficiency afforded by C4 metabolism probably also provides a competitive edge.

Beth Forrestel, from Yale University, admires
a C4-dominated grassland at Vite Vite on the
western plains of Victoria.  (Photo: John Morgan)

The concept of monodominance is not just of academic interest.

Dominant species shape communities and drive ecosystem processes. Our research has shown that healthy swards resist weed invasion. Hence, they should also be of key interest to restoration ecologists wanting to restore resilient ecosystems (see my last Blog as an example of this). Finding ways of returning dominant species (across large scales) might therefore be just as important as returning rare species to ecosystems.

And understanding how dominant species respond to climate change is a challenge we are only just starting to tackle.