Are thistles randomly distributed when they grow in a fairly uniform field?

Question

Thistle - http://en.wikipedia.org/wiki/Thistle

Answer 1

Thistles are not clonal and spread by seed (they generally don't reproduce asexually).  Their seeds are dispersed by wind and can travel a long distance.  They also have a very high seed output and tend to establish in disturbed areas.  So when you see a clump of thistles, they are growing in a disturbed spot available for them establish well (along roadsides/trials, on a pile of upturned dirt, a spot where other vegetation was killed off, etc.).

Answer 2

They have wind borne seed something like a dandelion's. They are usually dispersed in a fan shape depending on the direction of the prevailing wind. They will definitely take over in bare ground the same way that sand burrs and nettles will, but Canadian Thistle can germinate and grow in areas of thick grass which makes it a particularly noxious weed.

Ironically, the flowers are some of the sweetest smelling of all!

Answer 3

My experience has been that they are clumped or aggregated.  Thistles asexually reproduce so often in a field when you see a cluster, you are looking at clones that are growing from the same root system.  This would lead to an aggregated dispersion pattern.

Edit:

Absolutely thistles reproduce asexually.  From the USGS site on invasive plants about Canada thistle: "Horizontal roots may extend up to 15', and may grow 6-15' deep in soil. By asexual reproduction, it is possible for a colony of entirely male plants to maintain itself. Generally, reproduction from root system contributes to localized spread."