Role of axis reversal from the short-shoot to long-shoot habit for crown maintenance in slow-growing Betula maximowicziana trees.

摘要

Premise of the Study: Branch growth and its spatial arrangement determine crown architecture, leaf display, and, thus, the productivity of trees. Branch axes elongate by the sequential production of shoots with differing morphology and function, such as short shoots and long shoots. This study investigated ontogenetic changes in axis growth in Betula maximowicziana and quantified the role of axis reversal between the short-shoot and long-shoot habits, particularly reversal from the short-shoot to the long-shoot habit. Methods: From 8 trees with varying levels of growth vigor, 716 branch axes forming the basic crown architecture were sampled. Past growth of the branch axes was reconstructed from leaf and bud scale scars and compared between slow-growing and vigorously growing trees. Key Results: Branch axes reversed more frequently between the long- and short-shoot habits in slow-growing trees than in vigorously growing trees. Short-shoot-origin axes that reversed to the long-shoot habit lived for longer periods and grew larger than axes that remained in the short-shoot habit. Short-shoot-origin axes reversed as they grew away from branch apices, typically >6 yr after they had originated. Conclusions: Reversal of short-shoot-origin axes to the long-shoot habit is an endogenous growth process of trees with reduced vigor. Like epicormic branching, the reversal may contribute to the maintenance of productivity of large old trees by prolonging axis longevity and filling the inner part of the crown. This study presents an ontogenetic change in branch growth, which broadens our perspectives on the growth and survival of long-living trees.