The precise temperature at which root growth begins varies with species, but it's a sure bet that MOST roots on MOST of our plants begin to grow by the time the soil gets to this temperature. Bud swell begins shortly before, simultaneously with, or very shortly after root growth begins. In the phenology of the highbush blueberry, the roots were growing when the leaf buds swelled. Initial root growth, budswell, and early shoot growth depend for energy upon nutrients stored in the plant from the year before. If plants were deprived of proper water and nutrients last year, you can bet that early growth will be feeble this year.
Cambial activity begins early in the season also, at the time of, or just before, budswell.
Remember that the xylem and phloem don't themselves move up or down, but things move up or down in them. Also remembering that nothing is cut and dry in botany, there is some bi-directional movement in both xylem and phloem, but the primary flow is in the direction indicated above. If the mice girdled your tree trunk last winter, or if the wire trimmer chewed it up last fall, a ring of phloem and possibly cambium has been destroyed. The xylem remains intact and can transport water from roots to the tree top. The buds break, the top leafs out and maybe the flowers on the crabapple bloom.