Sunday, December 6, 2015

Plant Ecology--Tumbleweeds, the Lifestyle

young tumbleweed
young tumbleweed
In the dry fall of the Front Range of the Rocky Mountains, the tumbleweeds let go and roll around.  Drifts on fences from the region make dramatic pictures link

While there are plants called tumbleweeds, actually tumbleweeding is something diverse plants do.

Looking up tumbleweed in plant books creates confusion. In the few of my many plant identification books that have tumbleweed in the list of common names, it is paired with a whole array of plant scientific names. For example tumbleweed is Salsola kali, Salsola tragusSalsola iberica, Salsola collinia, Salsola australis, Kali tragus and Amaranthus albus. These are all members of the plant family Amaranthaceae (into which the formerly independent family Chenopodiaceae has been merged) so there is some uniformity. But in fact, the names are confused. The genus Salsola is from Eurasia, the ones in North America are widespread weeds that arrived in the 1800s. Several of the plants listed above are the same plant species, reported in books of different ages. Salsola kali is commonly called Russian thistle as well as tumbleweed and S. kali has been revised to be S. iberica, then S. tragus and now Kali tragus. But while the USDA website uses the name Salsola kali, it gives it as only found on the East and West Coast of North America with S. collinia the one found throughout the Great Plains. Earlier works on the Plains, such as Kirkpatrick 1992 Wildflowers of the Western Plains, list S. kali as the common Salsola of that region. So both sequential name revision and the recognition of multiple species where before only one was seen, have been occurring.

The source of the problem is undoubtedly the existence of many hard-to-separate species in Eurasia which are only now being studied (but there are war zones where it is unsafe to study plants), some of which were introduced to North America and may for various reasons be different in North America as compared to their ancestors in Europe or Asia.

Salsola sp., Russian thistle or tumbleweed
Salsola sp., Russian thistle or tumbleweed
However, I must point out that "tumbleweed" is listed as a plant name in the minority of my plant books. For the various Salsola species given above, most are primarily called Russian thistle. (A misleading common name because generally thistles are in the sunflower family, Asteraceae, while the Russian thistle is quite different in structure and classified in the amaranth and goosefoot family, Amaranthaceae).

But, the best thing is, tumbleweeding is a lifestyle shared by a variety of plants.

Plants, being rooted, do not easily send their seeds away from the parent plant. Different plant species have different and complex mechanisms for spreading (dispersal). Some wrap a sugary fruit around the seeds so that animals will eat them and carry them away, to be defecated in a distant location. Others put barbs or glue on the seeds, so they stick to the outside of passing animals. Still other seeds have attachments that help them fly on the wind.

wind-bourne seeds (milkweed, Asclepias)
Tumbleweeding is one of those mechanisms. The plant breaks off at ground level and the wind blows the whole aboveground plant away. As the plant turns over, it drops its seeds.

Periodically, scientists make lists of seed-dispersal mechanisms and compare regions. When they do that, tumbleweeding is very rare. It is virtually nonexistant in the world's forests. That makes sense, there is no wind under the trees and trees themselves don't tumble well. Since most of the scientific community has always been from forested regions, whether it is western Europe or the U.S. East Coast, tumbleweeding is rarely mentioned.

Thus, the observant Texan or Coloradan is puzzled to see different plants blowing by. One of the places one notices tumbleweeds in on fences. Once, the windswept plains were barrier-free. Now dispersing tumbleweeds hit the fence.
scurf pea on fence
scurf pea on fence
There is a tumbleweeding plant caught on the fence in the photo above. It is a legume (pea family, Fabaceae), the scurf pea, Psoralidium tenuifolium. There are no leaves and it is dark-colored, so to see it, click on the picture and enlarge it. Most peas are not tumbleweeds. But this one is.

scurf pea tumbleweeding
Another view of a scurf pea tumbleweeding
(apologies for faded slide)
For tumbleweeding to be effective, there has to be wind. In addition, the vegetation must be sparse enough and low enough for the tumbleweed to bounce through. And, it works better where there are no large pockets of unsuitable habitat: tumbleweeds that are blown into lakes almost certainly perish.

Grasslands (prairies, steppes, pampas, desert grasslands, a variety of names) are where tumbleweeding works. And in those places, plants of quite different ancestry tumbleweed. The scurf pea above is one example. In the American plains there are also tumbleweeding morning glories (the bush morning glory, Ipomoea leptophylla), several other legumes (silverleaf Indian breadroot Pediomelum (Psoralea) argophylla and gopher weed (Baptisia lanceolata), and a variety of native amaranths (for example winged pigweed (Cycloloma atriplicifolium) and Amaranthus name those I know about. (A list of another 22 genera in 10 plant families is given in Baker et al., 2010).

scurf pea tumbleweeding
That scurf pea from a distance

Good tumbleweed habitat.
In addition to the native tumbleweeds, tumbleweeds from around the world find central North America suitable habitat and have prospered, in particular Russian thistle and its relatives (the Salsola species given the name tumbleweed) and prostrate pigweed (Amaranthus albus).

winged pigweed, Cycloloma atriplicifolium
winged pigweed, Cycloloma atriplicifolium

What makes a good tumbleweed? Two plant characters seem important. First, a round shape. The winged pigweed above is a classical example. This plant will roll, dropping seeds as it goes.

The second is to let go of the roots. All tumbleweeds started out rooted to the ground. Even if the plant is an annual and died with the first frost, roots will keep it anchored.

But most tumbleweeds are perennials. Every year the top blows away, leaving the roots behind. This is not accidental. When the plant has pulled all the nutrients it can into the roots and the seeds are ripe, an abscission layer forms. An abscission layer is a double row of cells that are not connected to each other, and consequently easily allow a break to occur. (This is the same basic process as the release of leaves in autumn or the fall of ripe fruit. Links: explanation; photo ) On a windy day, the tug of the wind pulls the plant top free of the roots and away it goes!

Loveland, Colorado
Abscission layer on tumbleweeding plant
Loveland, Colorao
Again, click to enlarge the photo
The western plains and intermountain grasslands are a tumbleweed-friendly part of the United States. Where there is low vegetation and wind, they are found. These days, tumbleweed-friendly places across the world probably share the same weedy species. Here are tumbleweed pictures from Australia: link
young Russian thistle
young Russian thistle
Tumbleweeding works for the plant, but for humans, mostly they clog up fences and bounce along the road. 

But wait, here, in Yulara, Australia, they turned tumbleweeds into art.

Yulara, Australia
hanging from ceiling, decorative, painted tumbleweeds
Yulara, Australia
Lined up along the ceiling by the window, those are tumbleweeds, each spray-painted a different color, mainly silver and gold, but also red: 
red painted tumbleweed as decoration
Red-painted tumbleweed used as decoration.

When life gives you  lemons  tumbleweeds, make art!

Comments and corrections welcome.

Article from the Answer Girl, Casper (WY) Tribune link
Baker, D. V., J. R. Withrow, C. S. Brown and K. G. Beck. 2010. Tumbling: use of diffuse knapweed (Centaurea diffusa) to examine an understudied dispersal mechanism. Journal of Plant Science and Management. 3:301-309. 
Kirkpatrick, Z. M. 1992 Wildflowers of the Western Plains, University of Texas Press, Austin.

Kathy Keeler
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