Tom's Blog

Monday, April 3, 2017

Pattern formation in little bluestem prairie

The little bluestem in our tallgrass prairie is so lush that the underlying growth pattern can’t be seen until it is burned. A close look at the burned half of this photo will reveal the individual clumps, which are shown in more detail in the second photo.

Fire moving across the South Slope little bluestem prairie. The bluestem clumps are visible in the burned portion.

Close up of the burned little bluestem clumps. The distance between clumps is about 12 inches (~30 cm).
 Little bluestem (Schizachyrium scoparium) is a classic bunchgrass and is widespread in North America. Each individual “bunch” is a complex of tightly joined tillers that arise from buds just below the soil surface. Little bluestem evolved where the fire regime constitutes frequent, generally annual, burns. In the absence of frequent burns, little bluestem grows poorly.

On dry sites such as ours, little bluestem grows in separate, erect clumps, with bare ground between plants. Each bunch is a clone, derived originally from a seed. The bluestem clone can have several hundred stems crowded into a patch of 5-10 inches in diameter.

Research in Texas has shown that the individual stems (called ramets) of the clone are integrated physiologically, with transfer of resources from older to juvenile ramets.

The mature root system consists of a huge network of roots, which spread out from the clump. Although most clones flower, seed production is not prolific, and is not too important in a mature stand.

The individual ramets in a bluestem clone are fairly short lived (mostly 2-3 years or less), and new ramets are produced to offset mortality. As new ramets are formed, older ones die and decompose. Long-lived clones often have hollow crowns due to death and decomposition of older ramets in the center. The age of a single clone may be as long as 50 years, but eventually the clone dies.

A lot of research has been carried out on the physiological processes by which the bluestem clone grows and maintains itself. Even if seeds of little bluestem are planted in a uniform manner across a field, regular patches develop. It seems evident that clones in a prairie compete with each other for one or more essential resources: water, light, nitrogen, etc. Those clones that get started earlier will have more roots and above-ground plant material to draw on vital resources and will hence win out. The end result is a pattern of clones with bare ground between them.

Although each bunch of little blue keeps other bunches away, the bare spaces between bunches are not barren, as other species can grow there. Here are two photos showing prolific growth of prairie forbs in the bare spaces between the little bluestem.

Violet wood sorrel in flower (with other species) among the little bluestem clumps in mid-May, 2014.
Butterfly milkweed and other forbs growing profusely in the bare patches between little bluestem clumps, July 1, 2014


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