Tree tubes protect young trees from deer predation, and, if installed correctly, against damage from voles and rabbits (eating bark along the base of the tree).
The tops have a mesh covering to prevent additional predation.
The tubes stay on the tree until the tree has grown substantially so the terminal ends are above the tubes (the mesh needs to come off when the tops of the saplings reach beyond the top of the tubes, otherwise they can cause deformation of the tree).
The tubes are necessary to ensure better survival. (They may be mandated, depending on whether the plantings are done as part of a federal or state cost-share program. This helps protect the landowner and the tax-dollar investments in conservation.)
While PennDOT may be doing some of the planting noted, numerous similar plantings are done for riparian buffers and for tracts that have highly erodible characteristics. I suspect the plantings are done by private landowners.
(PennDOT however has been in the news lately because it has been cutting more trees along the roads it services to ensure better views for its traffic cameras.)
Depending on funding, programs may be available through the USDA Natural Resource Conservation Service, or through non-profit groups dedicated to environmental and water quality improvements, such as T.U., Pheasants Forever, etc.
If you are an agricultural landowner with erosion concerns, and an interest in possibly participating in a program, check with a local conservation district or NRCS office for program activity and program sign-up availability.
Otherwise, a numer of tree nurseries offer them for sale for general public use.
While the tubes are something of an unnatural sight, they shouldn't be on the trees any longer than 10 years, and most likely will be off the trees in 5 years. It all depends on the growth of the trees, and the attention to responsibility by the owner/participant.
What the tubes do indicate is that the landowner is participating in a conservation program or is otherwise concerned about ensuring tree growth on those acres.
Hope that helps.