We all know what tree bark is and we take it for granted as part and parcel of a tree’s structure, but we rarely think about why it’s so crucial or the fact most tree bark is unique to the species and is actually quite a sophisticated substance.
Tree bark has two layers: the outer layer protects the tree by helping to keep out moisture in wet conditions and by preventing moisture loss in dry conditions. It insulates the tree against low temperatures and also deters insects and fungus that may cause damage. It can even provide resistance towards fire depending on the species. It helps photosynthesis by storing carbon from sunlight, and acts as a microhabitat for various organisms.
The inner layer – or phloem – allows nutrients to be passed to the rest of the tree. Short lived, when the phloem dies it tuns to cork which then becomes part of the outer bark.
The bark’s appearance for each species is determined by the way the cork patterns develop. This can range from an overlapping and discontinuous pattern producing scaly bark to a continuous pattern that develops in sheets to produce a smooth bark – and everything in between.
For example, oak has deeply fissured bark, beech is smooth, birch sloughs off its papery bark and cherry has bark with hoops. And besides having differing appearances, the bark of different species can have specialist functions that bring certain advantages and disadvantages to the tree.
Birch trees shed their thin bark to remove lichen and moss infestations, but it also leaves them prone to damage by adverse weather conditions and insect attack. Silver birch bark specifically reflects the sun’s UV rays – a helpful but more likely necessary evolution for a species that can grow in fairly isolated areas lacking shade from surrounding trees.
Thick Scots pine bark protects against fire and also exudes a sticky toxic resin to repel destructive insects while oak trees repel them by releasing tannin vapour as their deep cracks caused by rapid growth offer easy access to insects and a foothold for ivy.
Conversely, beech bark is smooth which prevents insects and ivy from taking hold – but because the smooth bark grows very slowly, the bark takes a long time to heal if it’s damaged.
Interestingly, some barks produce sap that animals instinctively eat for pain relief or to reduce inflammation. Although this may help wildlife and livestock, it doesn’t really protect the tree while a mature oak can be home to over seven hundred lichen species. Tree bark is also home to millions of insects and spiders which attracts birds – providing a food chain and ecosystem.
Each of these tree species has survived for hundreds if not thousands of years with their different barks, each with particular advantages and disadvantages, specifically suited to the species of tree, and they coexist with one another in the same environment. That is special.
