Figure 3. The development of a bark-included
union in a birch (Betula pendula) over eleven
years. The research work I have published on the
anatomy of branch junctions and particularly the
causal relationship between natural bracing and
bark-included junctions (see article in this issue of
City Trees) keeps me busy, as a lecturer, for many
audiences want to know about what my experiments
have found and their implications for tree work. I have
quite a few TOT images of branch junctions, showing
how they have changed over time, which is probably
a unique aspect of my collection. One thing they all
show is that the top (apex) of the union rises over time.
Students tend to think of secondary thickening in only
two dimensions, with the trunk getting "fatter" as the
vascular cambium, orientated in the vertical plane,
produces new layers of xylem. However, of course,
there is vascular cambium at the top of a branch
union which can be orientated in the horizontal plane,
causing secondary thickening upwards, rather than
outwards. If you look carefully at the marks on the
outer bark of this birch, you will see that the union has
risen about 150 millimetres (~6 in) in the last eleven
years. The bark-included junction has also changed
category, from one with a seam of bark at the top
(wide-mouthed) to one that now would hold water
within it (a cup union), so it has gained in strength over
time, although it still represents a defect in this tree
(Slater, 2016).
34 CityTREES
