Eyeing ring muhly, a clonal grass that forms rings

By Jeff Mitton

Unexpected plant colors and growth forms can surprise us and grab our attention.

At Great Sand Dunes National Park, a wispy grass that grew as hollow circles 1 to 8 feet in diameter fascinated me.

I had come to photograph a vista that included arid grassland in the foreground and then the sand dunes beneath the soaring Sangre de Cristo Mountains. But the light was all wrong, so I decided to wait. It was in this time that I became fascinated with the circles of ring muhly in the foreground.

Several dozen ring muhly were on a patch of sand and gravel on a southwest-facing slope. Ring muhly, Muhlenbergia torreyi, is a clonal species that starts out as a small clump that increases in diameter over the years. That is, a clump of the grass is not a series of individuals, but a single individual.

When the clone's diameter reaches 1 to 2 feet, the density of ramets or shoots of grass at the center begins to decrease. Ultimately, the live growth at the center of the clump disappears, leaving a hollow ring that continues to increase in diameter over the years.

Ring muhly is adapted to arid environments, and an interaction between root density and availability of water creates the hollow circles. When the clump is very small, the central ramets can be fed adequately by the roots beneath them, which are not at high density. However, as the clump gets larger and larger, roots at the center experience higher and higher densities.

In sharp contrast, roots at the periphery of the clump experience high root density on the side toward the center, but low density beyond the edge of the clump. Consequently, ramets at the center struggle and die while ramets at the edge have sufficient water for growth. The clone continues to grow in the direction of low root density, or outwards, so the circle grows larger and larger.

The relation between root density and growth produces ring growth forms in other plant species in dry environments. The most celebrated of these is a creosote bush, Larrea tridentate, growing in the Mojave Desert. Most creosote bushes do not form evident clones, but if they continue to grow as they age, the ring growth form appears.

In an area of very old creosote in the Mojave, numerous rings can be found on a single hillside, and the very largest of the rings measures 70 feet in diameter. Judging from the growth rate of creosote today, biologists have estimated that the large ring is 11,700 years old.

Mojave yucca, Yucca schidigera, also forms rings in the Mojave Desert, and like creosote, the rings are reported to be ancient. However, the estimates of the age of the Mojave yucca circles are controversial, though it is conservative to say that the larger rings exceed 1,000 years in age.

In North America, other plants growing as hollow circles in arid environments include ephedra or Mormon tea, Ephedra viridis, and big galleta grass, Hilaria rigida. But this growth form is by no means limited to North America. An online search quickly turned up photos of three plant species growing as hollow rings in the Negev Desert.

Jeff Mitton, mitton@colorado.edu, is a professor in the Department of Ecology and Evolutionary Biology at the University of Colorado. This column originally appeared in the Boulder Camera.

Sept. 4, 2014