City Trees is a premier publication focused on urban + community forestry. In each issue, you’ll learn how to best manage the trees in your community and more!
Issue link: https://read.dmtmag.com/i/1386860
The Pith The pith of a young twig is best considered as cheap packaging material. Some species produce a very large pith in proportion to their twig diameter—for example, elderberry (Sambucus nigra). As such it risks forming twigs that fail in compression because this species has taken the concept of cheap packaging very far indeed. As with other aspects of tree growth, there are some species that are structural risk-takers because of the competitive advantages this can give them in the woods. Piths vary in their nature; there are piths that are filled with a sponge-like material, those that are vir- tually empty of content, and piths that have multiple chambers. Pictured here are two shots of the cham- bered pith of a common hazel. Note the extreme scale of the pith tissue compared with the surrounding wood cells; it saves a lot of construction costs to have almost nothing in the center of this twig. Oddly, due to its emptiness, the pith can be highly resis- tant to the ingress of fungal decay. (You will often find decay stopping at the edge of a stem's pith.) The Fracture As all my ESEM images of wood are based on looking at wood that has been torn apart, let us consider fractures in greenwood and note some common micro-features. Once you have initiated a crack in a branch or stem of a tree, it is usually quite easy to pull the two pieces further apart. As you do so, you will probably hear a crackle or a 'kck...kck...kck' noise. This is the sound of wood fibers being broken, telling you that the wood grain is not absolutely straight and that there is some resistance to the crack propagating further. In these ESEM images of fracture surfaces in English oak, the cracking has taken a path of least resistance in places, where you can see flat, exposed ray parenchyma cells. However, in other parts of the scan, you can find twisted and broken wood fibers in small clumps—these are the cells that gave rise to the 'crackle' as you prop- agated that crack through the wood by bending it. Wood fibers are strong: try pulling apart a live twig— even of only 8 mm diameter—in a straight line. I bet you cannot do it. So, by having some interweaving along the wood grain of a stem or branch, the sapwood is made more resistant to cracking, and slightly less effec- tive as a conduit for sap. A classic trade-off scenario. 28 CityTREES