Senior Research Associate, School of Ecosystem and Forest Sciences, University of Melbourne, Burnley Campus, 500 Yarra Boulevard, Richmond, Victoria, Australia, 3121

When considering tree morphology and anatomy, it often comes as a surprise that branches evolved first and that leaves are highly evolved and modified branch systems. It seems counter intuitive. Branches evolved from leafless photosynthetic stems called telomes and in the earliest land plants, the vascular system consisted of a single, central, elementary strand of xylem. This strand conferred support allowing plants to grow taller. Phloem tissues evolved later.

The connection of the vascular trace to branches in primitive plants was easily achieved as the core could split into two at the apex and supply both stems as there was no phloem tissue surrounding or exterior to the xylem tissue to impede the connection. Dichotomous branching is a primitive characteristic displayed by some plants still and may provide insight into the differences between branches and co dominant stems in modern trees. Once phloem tissue had evolved the situation became more complex. For the xylem tissues to retain continuity, woody plants had to evolve a way of allowing xylem to pass through phloem tissues. Hence the branch trace and gap. These have a significant impact on the nature of modern tree branch attachment.

There is diversity in the way branch traces and gaps have evolved. The simplest form of connection is via a single trace and gap, in which a single strand of xylem tissue passes through a single branch gap in the surrounding cambium and phloem tissue with the xylem in the upward-facing (adaxial) position. This form of vasculature evolved early, but has subsequently been modified in plant evolution. The branch bark ridge (BBR), stem bark ridge (SBR) and branch collars are morphological manifestations of the anatomy of the respective unions.

A single branch trace and gap fits early models of attachment and the observation that xylem tissues run downwards under the branch with little attachment above. Multiple branch gaps and traces may lead to more complex forms of attachment and perhaps differences between species of trees. We know the anatomy of branch attachment is an important aspect of natural branch shedding and we know some of the reasons and causes of shedding, such as shedding of self-shaded branches and branch losses during storms. However, others such as sudden limb drop remain poorly understood.

We know that as branches die and are shed, they go through a number of progressive changes, which are known to arborists and are commonly used in branch health evaluation. The way trees naturally shed branches has informed modern approaches to pruning. Understanding and using the morphology and anatomy of branches, such as the BBR, SBR and branch collars combined with observations of branch shedding has seen the elimination of flush cuts and the development of targeted and specialist habit pruning techniques.