Ever since I was little I would go around peeling off the smooth, pastel-coloured bark from the birch trees that surrounded my house. Every time, I would excitedly give the bark back to my mum, exclaiming how easily it came off and how beautiful it looked. Since then little has changed, except my curiosity to what exactly was going on.
Despite bark being a nontechnical term, we can define it as the part of a tree which overlays the wood. Specifically, bark consists of two main tissues. The secondary phloem, is the inner tissue, this part is alive and important in transporting organic molecules around the tree. An important feature of the secondary phloem is that it contains bands of fibre which prevent the splitting of the bark. The periderm is the secondary outer layer but again this consists of many layers; generally there is an inner layer which restricts water and nutrient movement from the outer layer, thus causing it to die. This dead outer layer prevents gas exchange, therefore lenticels are present. These are loose areas of cells allowing gas exchange to occur. Bark has a huge number of functions, it reduces water loss, provides protection from insects, birds and disease; so why would some trees go through the effort of constructing this vital protective layer just to go ahead and loose it?
The process of particular trees shedding their bark is known as exfoliating, and us girls are fully aware that exfoliating has major advantages. So what are the advantages of exfoliating for a tree, if there are any? What seems to be such a simple question to begin with becomes, like most Biology, incredibly complex as we delve deeper in.
The generic answer that most rangers will go for when posed with the question from some bright eyed tourists or locals, is simply that the tree grows faster than the bark and so causes the bark to rupture. The mind can flow back to a similar image of an insect undergoing ecdysis to illustrate the idea. Simply, the tree grows too big for its shoes and needs to replace them. Trees grow from the inside meaning the outer layers must expand, and outer bark being dead means it can’t expand and so must be shed to make room for new more elastic bark, which can expand with the growing tree.
Yet this answer seems to fall through when we are confronted with slow growing trees which equally undergo exfoliation. The Shagbark Hickory, is a common tree in Eastern United States and Southeast Canada. This tree is incredibly slow growing, only normally reaching half its potential size, and usually accompanied with a shroud of cheat sheets to increase growing speed to the enthusiastic gardener- and yet it peels.
So perhaps there are evolutionary adaptions to why trees peel, rather then it being a default to an imbalance between growth rates. For if natural selection has taught us nothing it’s that things are the way they are for a reason.
By looking at River Birch, we are confronted with a tree with plenty of water supplies and so reducing water loss becomes a distant priority in the trees mind. Exfoliation occurring in this species exposes the thin inner bark layer which can undergo gas exchange. Therefore exfoliation allows a higher amount of gas exchange and transpiration to occur. This would increase the trees metabolic rate as carbon dioxide and water no longer become such limiting factors as they were before. And so in this case exfoliating seems to allow River Birch to grow faster, and so more successfully.
While this could be a selective advantage which caused River Birch to undergo exfoliating, it does not provide us with an answer for all trees. Many other trees that exfoliate do not live in water-abundant habitats and also have thick bark. Another theory suggests that exfoliating may have a protective function to the tree. The shedding prevents a build-up of parasites, fungi, mosses and lichens, whilst the thick cracked bark is much less attractive to herbivores then smooth bark which grows with the tree. This therefore gives us a highly probable evolutionary adaptation for thick bark shedding.
Linking in to the prior idea there may be a third reason why some trees exfoliate. Birches are able to photosynthesis through there bark. Therefore the peeling of bark may allow the removal of a lichen light-blocking layer to expose the live inner bark. This way the tree can take advantage of sunny winter days to create carbohydrates even with no leaves yet available.
Though the true answer of exfoliating lays a little foggy, we must take into account that species diversity correlates just as much with bark diversity. Therefore whilst one answer is simpler and easier to satisfy the curious mind, in reality the answer must be taken into account with regard to each individual tree. In addition it must be understood that a combination of reasons for exfoliating are more likely to be true then a singular one. Birch may shed to not only get rid of parasites and potentially harmful organisms but also because this shedding allows an increase of its metabolic rate from increasing gas exchange, transpiration and light absorption. Overall we are left with a much higher photosynthetic rate despite the tree having no leaves. With regards to other trees, whilst convergent evolution is very much true it’s essential that each individual species should be taken as a case on their own to determine the evolutionary reasons for why that tree undergoes exfoliation.