The caller suggested that ideally the dowel would be within 1/16" of the inside diameter of the conduit. But if the "average" amount of total annual seasonal movement is 1/8" per foot of width, a 5 1/2" diameter dowel would have an "average" amount of total annual seasonal movement of almost 1/16", the desired tolerance.
I'm thinking that if the dowel is turned from a billet containing the center of the tree, almost any species will do.
Average radial shrinkage -- green to dry -- is 4%. At EMC annual radial movement is possibly well under 1%, or around 25 thou. I sat "possibly" as I am not that familiar with North American woods.
A quarter sawn billet should also be satisfactory.