There's collecting and COLLECTING. Anything under an inch in diameter can be collected with mostly intact roots, but mostly there's nothing worth collecting under an inch in diameter (collecting is mostly about getting an already developed trunk)...Digging a more substantial trunk requires removing most of the roots and large cut wounds. Early collection can be done, provided the ground isn't frozen solid and you have somewhere frost free to keep the collected trees.
Also, the longer you wait until collection in the late winter/early spring, the more energy the tree is pushing from its roots to its branches. That is why collecting JUUUST before bud break is the best way for deciduous trees.