Tap Roots

Rather than going out and trying to look through research papers on the topic, I thought i would stop by here and see what you all might have to share.  

Recently I joined a conversation about raising trees from seed.  I had commented that my own preference is to direct seed if at all possible, as this minimizes damage to the very important tap-root, which ultimately helps the plant to become more resilient and able to get established more quickly.  

The response I got was:

Nurseries routinely prune tap roots and mainstream woody plant physiology describe tap roots as juvenile root structures, i.e. the plant gets rid of it anyway as it grows out lateral shoots. I know a lot of specialist plant sources on the internet say all kinds of things about how fragile tap roots are and pawpaw communities seem to be part of that myth making. I just haven’t seen the evidence and it doesn’t fit well with existing science. Bottom line for me is don’t worry about tap roots.

I know how Paul Wheaton feels about this after listening to so many of the podcasts, but I do not recall him providing any information on where we can dig deeper.

Im sure that the answer, as usual, will likely be "It Depends".  Such as which species, what the intentions are, etc  

Which perennial species would the tap root be more susceptible?  Which would we more likely not need to worry about it?  

To be honest, it is not something I worry about too much. In general less root disturbance is better, so sowing direct might be considered the best option. However, this isn't always practical, and as long as the roots aren't completely pot bound, I'm pretty sure the trees will recover.
I have heard that nurseries routinely chop the roots of bare root trees, so as to get a better amount of root growth for transplanting, so I assume it doesn't do too much harm to some trees at least.

The size of the tree, the season, the soil moisture and temperature will all have an effect. I like to plant small trees because they establish better here and grow away faster than bigger trees (and cost less, so if they do die I've lost less!). I do plant by seeds as well, but I expect a greater proportion of seeds not to make it, due to weed and pest pressure. Theoretically those that survive ought to be stronger, but I can't say yet one way or another - my seed grown trees are either native (so ought to be better adapted to my climate) or fruit trees (which are not native, so I don't expect to do well).

As regards other perennials. I think I would worry more about whether the plant was going to get diseased from the root damage, rather than the loss of the roots themselves. Many tap roots you can harvest and they regrow - if you don't get a rot set in.

One of the things I learned from a greenhouse/nursery course long ago - was that true firs (abies; or maybe just the abies concolor that the subject was) that have a natural (and quite beautiful) symmetrical form would grow unsymmetrically if their tap root was chopped.  Kind of like:  if the root had to branch or swerve from being cut before it could continue downward, the top would also.  I haven't seen this effect myself; only heard of it.  But the teacher I learned it from was a nursery business owner who had a mature specimen of this in his own yard.  So maybe it is something he has seen more than once.  Maybe it's the kind of thing only those who are doing the pruning would even be able to observe?

In my many hours in the forest I have yet to see any mature tree blown over in a storm with its root system exposed that had a taproot. I personally have cut taproots in half when transplanting Bur Oak during dormancy with no ill effects. Another point to note is trees grown in air prune beds that are known to naturally grow long taproots and they seem to do great despite restrictions on root growth.