Figure 3: The Power to Heal. There is an old mantra about
trees often seen in older arboricultural texts: "We heal,
trees seal." In reality, trees and people can both heal. We
humans have the advantage that many of our tissues can
heal independently of each other—our bones can heal, our
internal organs can heal, our skin can heal, and so on. In
contrast, a tree can only heal from its meristematic tissues—the
cambia and bud initials. These near-surface tissues are where
a tree's "stem cells" initiate primary and secondary growth.
In the case of the wild cherry (Prunus avium) pictured,
vandalism has occurred to the tree's base but the
damage has not gone deeply, allowing the remaining
vascular cambium to produce new tissues to replace
the stripped-off outer bark. If cambia had been fully
stripped away by this attack, the damaged area could
not have healed in this way; however, only being able to
heal from peripheral cells could seem like a fundamental
weakness of trees, depending on your perspective.
This is a trait of trees that has occurred repeatedly in
evolution because it successfully deals with key physical
threats to a tree: browsers stripping the bark or chewing
off twigs, another tree falling and bashing against it, or
boring insects. Trees have yet to evolve to accommodate
modern physical threats such as the digger, the powerline,
or the chainsaw. However, it is wrong-headed to think of our
healing powers as superior; it depends on the context. If
you were to plug ten bullets into my body, I am unlikely to
recover; fire ten bullets into the trunk of a mature Douglas
fir (Pseudotsuga menziesii), and it will just occlude them. In
my opinion, the mantra needs updating—something along
the lines of "We heal one way; trees heal another way."
"If you were to plug ten bullets into my
body, I am unlikely to recover; fire
ten bullets into the trunk of a mature
Douglas fir, and it will just occlude them."
32 CityTREES
