aside from a low hanging canopy which might effectively shield snow from accumulating near a truck; a major contributor to tree wells are the tree trunks themselves.
Tree trunks absorb the sun's energy and build up heat, greater than the surrounding area, in their surface. This is especially true where the trees have large trunks and sunshine in winter is common - the Western States, Canada, etc. The heat buildup in the trunk melts the snow closest at a faster rate, hence the funnel shapes. There is additional heat radiation which helps the well become broader.
Tightly bunched trees which shield the sun more - like many of the heavily managed European forests, allow less sun intrusion down to the trunk snowline interface, hence less melting, so less well development. Areas, regions, climates which have less sun during winter will also not develop as strong wellling.
Deciduous forests, even tightly forested ones, will develop more welling because there is no winter canopy.
Latitude has a lot to do with this also - high altitude but lower latitude forests like the Western US develop strong wells because of the more direct sun rays in winter coming thru less atmosphere..
Lower altitude and higher latitude forests like Europe, Scandinavia, Russia and the like, receive less direct sun rays coming through more atmosphere.
Predominant winds also have an affect on how a well develops.
For large trunked trees like Ponderosa Pines with high branching the well can be suprisingly symetric - the shady side of the truck still seems to pick up heat from the sunny side and have almost as much welling.
Last season I stared down a well of a particularly large Ponderosa and the well, which easily stood off from the trunk some 8 feet - I swear I couldn't make out the bottom which was at least 25 ft down.
When I'm skiing on the sunny side of a hill, in glades of large trees, I'm particularly wary of wells, because they can be broad and deep and not easy to spot until you;re almost upon them.