Re: Why some trees have bark or new bark that grows horizontally?

Hi Rocky,

Trees have structures known as lenticels along the branches.  Lenticels allow gas exchange 
between the trunk and the atmosphere.  All trees have lenticels, and in the case of trees with thin, 
smooth bark, they are quite apparent, appearing as numerous horizontal lines on the trunk and 
branches.  

Lenticels are not at all obvious on trees with thick bark.  Instead, the vertical fissures in the bark 
become the dominant feature.  Why do those fissures form?  

As trees grow in diameter, the new tissue is being added in the region between the wood of the 
trunk and the bark.  New wood is added to the outside surface of the existing wood, and new 
layers of bark are added to the inside surface of the existing bark.   As the diameter of the trunk 
increases, the older layers of bark (the outermost layers) must split to accommodate the 
expanding wood.  I should point out that the older layers of bark are dead, and cannot grow to 
accommodate the expanding trunk.  On trees that have thick bark, the resulting fissures are 
really clear, since they cut through several layers of bark.

On trees with thin bark, the splitting outer bark can result in the peeling bark seen on some birch 
trees.  The outer bark may also fall off in slightly thicker pieces, as in sycamore.  For other thin 
barked trees, such as beech, the old bark sloughs off in such small fragments that it’s not even 
noticeable.

With respect to your observation about older cherry branches, I can only speculate that the 
branches hold on to more layers of bark as they age, resulting in thicker bark.  This would lead to 
the obscuring of the lenticels and the development of vertically oriented fissures.

I can’t find any good explanations for the variation in bark thickness seen among trees.  Part of 
the difficulty in explaining it is the fact thin and thick barked trees are found in all the same 
environments, and there is not a clear phylogenetic grouping of either kind.  That is, both thin 
and thick bark can be found in diverse classifications of trees.

As far as the color of cherry bark goes, it is certainly due to some anthocyanins, but I can’t say if 
it’s the very same ones that make the fruit red.

Dr. Alex Brands