Why do clumping grasses clump?

[Stephen Tashi]

What mechanism causes clumping grasses to form clumps?

My understanding is that a clump of a clumping grass (such as "Stipa") is not a single organism. What interaction among different individual organisms causes the clumping?

 

[jim mcnamara]

This is a very good question. There are several answers, since you have asked about New Mexico in the past I will limit it to the semi-arid steppe ecosystem.
Bunch grass is a good term for clumping grasses google searches.

Allelopathy - plants in arid regions suppress competition by inhibiting germination and growth of seeds/plants near an existing plant. Rain water washes compounds off the established plant. The concentration of the inhibiting molecules is highest close to the established plant.
Some known allelopathic compounds:
https://www.cambridge.org/core/jour...ic-chemicals/80506ABB13DC975F63797B89F854DEA9
If you walk anywhere out in the scrub you will note that almost every plant species seems to keep its distance from neighbors.

(this is probably the best fit for your question)
Microclimate and plant succession - allelopathy has limits. Switchgrass (Panicum virgatum) is an extreme example. What appears to be single individual in a clump usually is not. It is often a group of genetically very diverse individuals. Stipa and switchgrass are part of later succession. They germinate best in the presence of their own species. The clump changes very local conditions enough to favor growth of its close "cousins". These species do not self-pollinate and very local individuals for pollination are a big plus for reproduction.

Plant succession - https://en.wikipedia.org/wiki/Ecological_succession

Microclimate and germination - A local example is probably best: Go up in elevation (like 6500 feet), in order to get into the PJ's (Pinon-Junipers forest). You will find young pinon pines growing in the middle of junipers. The pines as "juveniles" do best in the conditions found under Junipers. Bear in mind that most of the trees you see are hundreds of years old, and the junipers that got them started are long gone.

Google for "fairy circles" to get some other points of view on spatial relationships of plants

 

[Greg Bernhardt]

Interesting topic! I just bought 4 tufts of some japanese ornamental grass for the garden. I would expect they spread out a bit, but not like lawn grass?

 

[jim mcnamara]

Bunch grass and lawn grass spread by rhizome or stolon growth. If they do spread - some do not.
Creeping grasses are named for their growth habit. They spread by stems that stretch out from the crown of the plant. New shoots develop from nodes on underground stems, rhizomes, or on horizontally growth on above ground stems, stolons. How much they spread is the difference between bunch grass and creeping grass.

Do you happen to know the name of the grass you planted?

 

[Andy Kleinhesselink]

Clumping grasses as you call them are known as bunch grasses or tussocks. There's nothing too mysterious about this, except as you say it can be difficult to decide whether the bunch of grass is a single individual or many.

Generally each bunch of grass starts out from a single individual grass seed. The grass grows through a process known as "tillering". Each tiller is shoot that forms at the base of another shoot (at or just below ground), see: https://en.wikipedia.org/wiki/Tiller_(botany)

As the grass grows, more and more of these tillers are produced. Since they are all grow as offshoots of an older tiller overtime they form a distinct cluster or bunch. Over many years the bunch expands laterally and can become large. Sometimes the older tillers die, but the plant lives on, this can lead to weird patterns. Individuals tillers can be physiologically independent, meaning that you can cut a bunch grass up into individual tillers and plant each one separately and they will live. However, they are usually all descended from a single genetic individual.

Bunch grasses can live like this for many many years sometimes decades or centuries.

In deserts there is a distinct phenomenon of clustered vegetation that includes other processes not described above. That's a whole separate issue and involves subtle changes to the microenvironment. You can read about that here: https://en.wikipedia.org/wiki/Patterned_vegetation

Hope that helps.