Mechanism of root graft formation

Root grafts appear to be formed as a result of the continuous pressure that develops as adjacent roots grow and increase in diameter {[318],[760],[765-767]}. Eis {[764]} studied root graft formation in Douglas firs. According to his theory, root graft formation starts when two roots meet by chance. After establishment of firm physical contact, the cambium cells rapidly enlarge and produce pith-like callus tissue. Just before the onset of latewood formation, cell division occurs. At the end of the growing season, the growth of callus mounds (up to 2 mm in diameter and 1 mm in height) can be detected on both roots simultaneously. On young roots, these mounds grow directly opposite each other. Subsequently, the mounds break through the bark of the young (or small) root, and almost immediately the union of the two cambium layers will occur.

The well-developed bark of older roots hampers the rapid union of the cambium tissues. In old roots, it can take up to 4 years before union is established. During this period, the callus continues to grow as pith-like cells. Finally, xylem tissues produced by expanding cambium will enclose the callus mound and terminate its growth. Rapidly growing roots appear to form root grafts most frequently. The thick layer of bark at the base of a tree probably prevents the formation of root graft connections. Seedlings growing close to the base of an old tree do not graft with the main roots. However, seedlings are able to form root grafts with smaller roots at some distance from the trunk {[764]}.