it will be hallow roots, the reason why is that the roots are able to be as close the base of the plant and pull as much nutrients as possible while the permafrost is usually an inch into the ground the roots can suck up as much as it can from what little room its terrain has to offer, with most of the soil being frozen all plants have to pull nutrients any means possible, kinda like how a cactus needs its roots to be miles long to get as much water as possible these plants have adapted specifically for their biome