|Two types of grassland, side by side
Whenever I take a walk outside, I am always struck by how various grass communities (whether natural or artificial) seem to have very distinct physical appearances. In today's suburban world of course, the most visible grass communities are the lawns that grace many homes. They're composed of grass species that are short and adapted to lots of mowing. But go to some nearby natural area like a pine and wiregrass savanna, and now much taller grasses dominate.
The grass species that are found in each type of community can vary significantly, and this difference was a topic investigated by researchers in Madagascar. In this country, the vast old growth grasslands (which includes savannas) are in peril due to misguided afforestation efforts by some. This is because the common misperception (as in many other countries in tropical areas that were once colonized) is that these grasslands are the products of human activity and derived from areas that were once forests.
|Cynodon dactylon, a major component of the grazing grassland. Yes, that's Bermudagrass, a common turf grass in the USA and elsewhere.
In their study, they examined (1) whether some species were associated together more often than not to form a specific assemblage or community. They also looked at (2) the functional attributes of each species with regards to flammability, palatability and tolerance to fire and grazing. These were:
- Plant height - taller plants tend to be more flammable
- Leaf thickness - thicker leaves tends to be less palatable
- Leaf C/N ratio - leaves with higher C/N ratios tend to be more flammable
- Leaf shape - wide short leaves are preferred by grazers as more palatable, while long narrow leaves ignite easily and burn more intensely
- Plant bulk density - high bulk density grasses provide more forage per bite, whereas low density grasses provide more aeration for fires.
They found several fascinating and important things:
First, they found that, indeed, species tended to cluster together into distinct spatial assemblages. Some species tended to be found together with the same group of species more often than not, and only in certain locations.
Secondly, they also found that the species they studied in Madagascar could be divided into 3 distinct functional groups. The first functional groups showed adaptions for grazing, and are mostly short grasses with high bulk densities, and leaves that are short, wide, and thin. Many of these are mat-forming, using either rhizomes or stolons. The second functional group is adapted for fire, and is comprised of tall bunched grasses with thicker leaves, low bulk density and low leaf width to length ratios. They also found a distinct and third functional group that were intermediate between the two extremes.
|A fire adapted grass (Imperata cylindrica)
Finally, and most importantly, they found a relationship between the spatial assemblages and the functional groups. The functional group of a species influences which assemblage contains it (see table below).
|Solofondranohatra CL et al, 2020