This seems like something that they could have and should have found out hundreds of years ago?
i'd read several state Ag agencies that have known this. It's why you keep watering newly planted trees even after their leaves fall, until the ground freezes. The roots will keep growing.
Not until its published? Have not read the article but I thought this was common knowledge.
Winter root growth is widely-observed amongst people who work with tree roots annually (bonsai hobbyists and professionals, commercial ornamental tree growers, especially propagation-adjacent roles) in milder temperate climates (eg: Pacific Northwest).
In my time studying bonsai (past 7-10y) in every winter (Jan-Mar) I've repotted (anywhere from partial to full bare rooting, partial to full root structure editing) many PNW-native deciduous trees (alders, bigleaf + vine maples, cottonwoods, etc) as well as non-PNW-native deciduous (birches, beeches, elms, maples, hazels, hornbeams, stewartia, bald cypress, cherries/plums, quinces, snowbells, etc). In somewhat-mild-and-milder climates there is always some root growth going on. Such winter root growth is much more aggressive in conifers, particularly pines, but also in spruces, cypress-family species (junipers).
Here's a diagram from a paper showing scots pine and rowan/ash adding either root or vegitative growth in various parts of the year:
Temperate trees collect or spend carbon in the warm parts of the year, i.e. between bud-break and mid-summer. In cooler parts of the year, they do various things: store it in the wood, move it around (redistribution + retranslocation), spend it (future-season buds + cambium + root expansion) or carefully avoid spending it (dormancy).
Even in these periods they're still collecting sunlight if they can, quite a few deciduous species can photosynthesize at least a little bit directly through their bark -- young twigs have much thinner bark even in trees that get very rough bark (eg: black cottonwood). And evergreens are collecting sunlight any time mild-or-warmer conditions are in play.
Trees are active in some shape or form any time they are able to be. If you live in USDA hardiness zone 7 or warmer and have trees/shrubs outside you can notice this more easily than in colder climates (where the grow/no-grow seasons are more sharply bounded). Roots are not the only thing expanding in winter. Take a picture every day of a branch on a deciduous or evergreen tree and you'll see bud expansion.
Yeah but they aren't making this clear and see people misinterpreting this not realizing that it is in temperate forests only. I don't see how any tree roots can be formed when frost line is feet below the surface.