The combo of feeder roots and mycorrhizal fungi are able to operate so long as soil temps are roughly 50F. So I would say it's not quite correct to assume these structures are not present in the fall. No, more likely, during a mid-summer dry spell, the feeder roots would die off, and then be regenerated once moisture returns. The fungi live on, albeit, they too are able to coast for a while.

The combo of feeder roots and mycorrhizal fungi are able to operate so long as soil temps are roughly 50F.

That's interesting, do you know what species of tree that is? I read a science report about tree roots once, and according to that the roots of ulmus glabra is active down to 41F. I guess there is difference between trees, but that single species is the only one I ever found info about.

Feeder roots dying in winter, is it the cold that kills them or the ground freezing and ripping them apart? If the latter, would they survive in a climate where the ground never really freezes?

The two main times of root activity are spring and fall. Spring for each kind of tree is whenever the winter buds open and new shoot growth starts. So spring can be March or it can be June, depending on the species of tree. Fall is when new shoot growth stops and winter buds are set. New roots are generated in spring, 60% of the annual increase in length of existing roots occurs in fall. These two facts are why bare-rooted nursery stock dug in November is warehoused until spring - the roots cut back or dried out when the plants are dug will not begin to be replaced until the following spring.

Hug, my comments were generalized, not specific to any one tree type. And yes, I'm near-certain that species would vary to at least some degree. Cats and dogs both have four legs, one nose, two ears, etc.....but the veterinarian that therefor decided they could be treated exactly alike would soon need another line of work. So it is too with trees I suspect.

The feeder roots are simply not permanent structures, growing, expanding, then dying all through the growing season. The structures embo speaks of above are the more permanent parts of the root system, not these very fine roots with fungi embedded within and without. And too, once one dips their toes into the world of mycorrhizae, things become bewilderingly complex in a hurry. Helper bacteria that set the stage for fungal symbionts, trees releasing messenger chemicals into the rhizosphere to "call" out to various fungal partners.....it is anything but simple.

Incidentally, we often lament this lack of "feeder roots" on stock we may receive, but it probably matters a great deal less than we may have been thinking, again due to the less than permanent nature of these structures. The tree can make more! One thing's for sure-if you want to see lots of these very fine roots-what we've been calling feeder roots-withhold nitrogen fertilizer. Nothing shuts this whole process down faster than fertilization. We bury bare-root trees upon receipt in a mixture of old wood chips and a bit of incidental soil. It is nothing less than amazing to see the amount of fine root regeneration these plants create in this very low-N situation.