Energy and Pruning

Energy Production, Energy Flow, and Energy Storage as it Relates to Pruning
Pruning live tissue from a tree is probably the most impactful procedure we as arborists can perform to keep a tree healthy and structurally viable over the longest period. Pruning must be done judiciously with purpose, precision, care, and careful consideration. Pruning should not be undertaken without a solid background in tree biology and physiology. Removal of live tissue is permanent; the part cannot be reattached. Removal of live tree tissue, by definition, makes a smaller tree and reduces the amount of energy that can be manufactured in addition pruning is an injury that will initiate the decay process.
Where does energy come from? Energy is only produced as a result of photosynthesis and most commonly occurs in the leaves of a tree. Leaves are the terminus of a complete vascular pathway comprised of both Xylem and Phloem vessels. Xylem brings in the water, dissolved elements and compounds that are transformed by the Sun’s energy into starches known as photosynthates. These starches move out of the leaf into the Phloem where they move toward the tree’s trunk and down to the roots. A large portion of the tree is made up of tissues that are not leaves. Since leaves are the only place energy is made that means the rest of the tree must consume photosynthates to stay alive. To be as efficient as possible trees are able to produce much more energy than, under normal circumstance, they can consume. The excess energy is stored away. When the Xylem is examined closer it can be seen to have four components one of which is the Xylem Parenchyma and is the only living part of the Xylem. This tissue can remain alive for years. A cross sectional look at a limb or trunk may reveal growth increments also known as growth rings. The Xylem Parenchyma may live for a few years or many years and is partially determined by tree species. For example, oaks could have eight or more living growth increments to store excess starches in addition to other chemicals. Specialized Parenchyma called Ray Parenchyma tie into the cambium where the current year’s phloem is transporting the photosynthate downstream toward the roots. The Rays draw off starch from the phloem as needed to keep local tissue alive and to replace stored starch reserves that might have been consumed.
How does this relate to pruning? Trees grow, at least a little every year, and that growth occurs over the previous year’s growth plus the new tissue that results from primary growth. Secondary growth develops morphology in response to external stimuli such as gravity, persistent winds, or sunlight. This development is how a branch can grow horizontally and grow longer without collapsing under its own weight. Branches also support leaves. The amount of energy these leaves produce directly impact the branches development. Pruning leaves off the branch reduces the energy available to the Parenchyma. Studies have shown that leaves produce about 66% more energy than is needed to keep the tree alive. Therefore, pruning more than 66% of the leaves off a branch places the branch into an energy deficit and if the branch cannot replace the lost leaves then the stored reserves will become depleted and the branch will die. Energy stored in the trunk will not move out the branch to save the branches life. Since trees grow in autonomous compartments as a collection of branches, leaders, and trunks it can be seen that pruning more than 66% of the leaf baring surface area off branch will lead to the decline of the whole tree.

Let’s take a more complete look at how a tree uses its starch resources. Staying alive is the most basic of goals but the tree also needs to reproduce, grow in size, and protect itself among other demands. These demands can total 66% of the total photosynthates leaving only about 33% in excess of demands. Keep in mind the phenology and the rhythmic consumption of starches over the course of a year. Spring places a demand on the reserves as it produces fruit, primary growth, secondary growth, and leaves all after staying alive all winter. Midsummer the tree is back to storing loads of excess starches and then fall comes around and new energy production comes to an end and a switch to living off reserves begins.

This is a simple look at pruning as it pertains to energy production, flow, and storage. Future articles will take a closer look into where and how to make pruning cuts.