Ed Gilman poses this thought experiment at the end of Chapter Four in his book, An Illustrated Guide to Pruning: Describe how starch can get trapped in the xylem, making it unavailable to the tree.

The process of photosynthesis in trees yields sugar as a by-product, which is translocated to different parts of the tree through the phloem and stored in the xylem of trees. Starch is an important resource for the tree because, according to Gilman, starch is readily available for growth and defense. Trees need access to starch in order to continue to function economically. Gilman explains that we can think of stored starch like hard-earned money that’s deposited in the bank.

When we prune trees, we expose both the phloem and xylem tissue to oxygen, which can invite infection from fungi and bacteria, as well as pests that are carriers of fungi and bacteria. Depending on how severe the pruning dosage is, we disrupt the translocation process of water and nutrients on either a small or large scale, proportional to the size of the cut, by removing the protective layer of bark and introducing oxygen to the exposed tissue. Depending on several other factors such as tree species, environmental stressors like drought or flooding, and disease or insect infestation threats, the pruning dosage and the timing in both the season and in the specific tree’s life stage could be detrimental. Large cuts that remove significant amounts of wood like topping cuts can be particularly harmful.

Trees have a defense mechanism that arborists commonly know as CODIT (compartmentalization of decay in trees). There are benefits and disadvantages with the CODIT mechanism. One of the major benefits is that as a natural defense, CODIT can be effective for helping trees defeat infection introduced through wounding. One of the major downsides of CODIT is that it takes large amounts of energy to be effective, diverting key resources in the process.

Although there are four walls that we know of in CODIT, for this discussion I’d like to focus on the fourth wall, known as the barrier zone, which is the strongest wall, but perhaps also the most costly in terms of growth and resulting structural compensation.

Gilman writes “the barrier zone forms from axial parenchyma and cambium along the edge of the outermost growth increment present at the time the tree was injured…the barrier zone is distinctly different from normal wood, and more energy is required to produce it…Wall 4 resists organisms associated with discoloration and decay from penetrating into the wood produced following injury,” (Gilman, 70). Wall four is very effective at blocking decay causing organisms from moving from the heartwood of the tree into the newly formed sapwood to the outside of the barrier zone.

But every rose has its thorn, and the barrier zone is no different. Because it is so effective at not allowing chemicals and organisms to breach it, Wall 4 also makes it difficult for starches in the xylem to move across the barrier zone through the symplast connecting the sapwood and phloem in order to be allocated by the tree for growth and defense. In cases of serious wounding like large topping cuts, most of the starch stored in the xylem will be unavailable because the barrier zone will encompass the entire circumference of the tree, locking off all of the storage space between the cambium on each side of the tree. Not only will the tree lose it’s access to the energy bank that came with such a high price to create initially, but it will have a smaller storage capacity afterwards because the infrastructure must be rebuilt with the newly formed sapwood after wounding occurs (Gilman, 70).

Wall 4 can also have high structural costs as well, because it is structurally brittle, according to Gilman. He writes that “the separation or delamination that occurs (after Wall 4 forms), called a ring crack, may follow Wall 4 all or part way around the trunk…One or more secondary cracks, called radial cracks, can form along a ray from the ring crack…This is a serious challenge for the tree because fungi now have a direct pathway to new wood formed after the injury,” (Gilman, 70). Gilman also reminds us of Shigo’s charge that cracks can be a bigger problem than decay as a result of tree injury.

With large pruning cuts like internodal or topping cuts that remove large portions of wood, there is huge taxation on the tree system. This type of malpractice denies a tree the energy it saved through huge physiological investments, by initiating CODIT’s barrier zone and complicating the ability for sugars to flow through translocating tissue and be utilized effectively when they are needed, such as after losing large amounts of leaf tissue in heavy pruning dosages. It not only exposes the tree to large areas of decay, it also introduces the potential for structural weaknesses associated with Wall 4 like ring cracks and radial cracks, which can form a vicious cycle of continuous decay beyond the barrier zone into fresh sapwood. These structural issues can lead to higher risk of failure of entire tree parts. And through the inhibition of the tree to carry on with its normal physiological processes, coupled with huge energy loss, tree death is possible as well.

Every tree will eventually face a time of stress induced from biotic and abiotic factors, and the energy reserves that it has worked so hard to save will be extremely helpful in aiding the tree in these difficult times. As arborists, we shouldn’t place added stress on trees through poor cultural practices out of ignorance of tree physiology. It’s our job to first understand trees on a cellular level, because this is where our best management practices are born out of. Unfortunately, if we ignore the critical biology that drives tree health and longevity, this is where quality arboriculture will also go to die.

You can take that to the bank.