Deep furrows separate thick, corky ridges on mature eastern white pine.
Because the periderm protects the tree from outside elements before it breaks apart, a new, active periderm forms beneath the old one, within the active phloem tissue. The cells outside of this new periderm then become isolated and die. This process, which can occur once or multiple times depending on a tree’s age and species, results in alternating, non-living layers of old periderm and old phloem tissue called the rhytidome — the Greek word for wrinkle.
My remaining, uncategorized photos all seem, at first, to display the same rough, thick, multilayered rhytidome — the iconic representation of bark that I held in my mind’s eye before starting this project. Slowly, though, as I closely observe my photos and get my hands on living specimens, differences begin to emerge.
On some bark, gaps appear between the outer rhytidome layers, where one could easily catch an edge and pry pieces away from the trunk. Bark with this characteristic can further be divided into three new bark types: scales of bark like on a black cherry; thick, irregular plates like on a black birch; and vertical strips like on a red maple. Other trees have rhytidomes with more tightly adhered layers, creating an additional three bark types: intersecting ridges like on a white ash; ridges broken horizontally like on white oak; and uninterrupted ridges like on northern red oak.
For the first time in months, I now have a clear walking path through my office thanks to 10 neat piles of photos representing each of my 10 bark types, which become the foundation for an identification key. I enjoy months in the woods searching for my next set of descriptive clues to differentiate the species within each bark type. At a particular stage in the growth of white pine, for example, I discover fine, horizontal cracks that are evenly spaced, like writing paper. I spend weeks looking at nothing but sugar maple trees before noticing that the surface of their bark is crackled, like old china.
After almost three years, I complete the bark key and hand in my thesis. But my field guide is still missing an important piece: I have yet to address the environmental influences behind bark’s grand diversity. Drought, fire, temperature extremes, limited growing seasons and interactions with other organisms all have influenced the evolution of different bark characteristics. I begin to research this missing piece by investigating the functions of paper birch’s bark features, which I have been wondering about for some time.
The habitat and range of the thin-barked paper birch includes high altitudes and the far northern regions of North America — places where temperate fluctuations are most extreme. Trees can be damaged by sunscald and frost cracks, where abrupt transitions from sun-soaked warmth to cold, or the reverse, can crack or kill sections of bark and open pathways for insects, fungi and other harmful invaders. Yet, this species only maintains a thin outer bark as it matures, even though thick, multi-layered rhytidomes generally provide the best thermal and structural protection.