Branch architecture and its relation to shoot-tip abortion in mature Fraxinus pennsylvanica

Abstract

The architecture of branch complexes, 10 to 14 years old, from mature male and female green ash trees (Fraxinus pennsylvanica var. subintegerrima (Vahl) Fern.) of two crown shapes was analyzed. Shoot length (annual increment of extension growth) declined with increasing order of branching and 47% of the shoots were less than 30 mm. Although variable, shoot length also tended to decrease along an axis. There was a consistent positive correlation between lengths and numbers of daughter shoots and lengths of parent shoots, but for a given parent length the predicted values declined proportionately with increasing order. An index of apical control comparing the lengths of terminal shoots with the longest lateral shoots revealed weaker apical control with increasing branch order in the broad, rounded crown shape class compared with the narrow shape class. Elevation angles of terminal daughter shoots were highly correlated with that of the parent shoot. When a shoot tip aborted, the terminal was replaced, but a weakening of apical control resulted in the modification of branch development and the formation of forks in certain axes. There was a reduction in the angle of divergence of lateral shoots in response to shoot-tip abortion, the terminal replacements being the most acute. Key words: crown architecture, shoot-tip abortion, green ash, Fraxinus, tree form, branching pattern.