Selecting Against Early Leaf Emergence
Two trees at at a chestnut orchard in Landrum, South Carolina are pictured to illustrate early leaf out in one but not the other. The early leaf out can be detrimental in growth of hybrids and can be selected against in breeding programs. Click on any photograph to enlarge it.
Leafing out appears to be a continuous trait (that is trees leaf out over a gradient of time). Leafing out is strongly under genetic control rather than environmental control as is the case in many tree species. The early leaf out characteristic, which can genetically be imparted by the Chinese chestnut tree originally crossed with an American chestnut tree, can show up in later backcrosses to American chestnut trees. Early leaf out needs to be selected against when breeding chestnut hybrids.
Early leaf out can cause the terminal bud to be frozen back in late frosts. One way to select against early leaf out is to note its occurrence in orchard records. Another way is to select for trees that have a strong central leader. If the tree does not have a strong central leader, it may not be present because a late frost nipped it back. Pure, native American chestnut trees rarely are effected by late frosts since leaf out occurs so late in spring.