How? By snipping them off. Or turn the taproot into a trunk. What else is a trunk other than a long taproot above soil level? If all the feeder roots are below it, it doesn't make a difference to the tree. Roots can bark up when they're exposed to air and light.
What point? When there's fine feeder roots, enough of them to support the tree.
This is a known field growing issue, and why people avoid collecting trees from sandy locations.
When I sow pines nowadays, I keep them in very shallow containers to prevent a taproot from forming. And in organic material as opposed to coarser inorganic, the high water content really helps to prevent taproots from forming because there's a dead zone on the bottom of the container. That practice means that when I put them in the soil, either of these two situations happens:
- One root takes over and becomes a taproot.
- All roots produce feeder roots and no taproot ever forms.
The longer you wait and the sandier your soil is, the bigger the chance of the first situation occurring. In damp forest floors, this rarely happens.
I dug 30 red pines this spring (sown in shallow pots in 2018 or 2017), and they all had a serious tap root digging half a foot into the ground. I think I've read it somewhere but I'm not sure, my own experience would say that it's a good idea: Dig them up every single year for the first couple of years and keep the roots maintained. After some 4 years, you could expect that they'll stop trying to make that tap root.
I don't know if girdling could work, or if they might bridge it.
I'm doing the first couple of years in a pot now. Ground growing is faster, but it also means that I can have 30 nice looking pines out of which just 10 survive after a serious root cut. I'd rather spend that extra year, keep 10 nice ones and sell the other 20.
One technique that occurs naturally in pot grown spuce, is that the tap root coils up around the edge of the pot. Eventually it will start sending out finer feeder roots. This could take years, decades even. It's a possibility to keep in mind; just bend the thing upwards and hope for the best.
Grafting seedlings (root grafting) into the base is another possibility. But I've never done that.