Don't you UK guys get Arborist News? Here's the article; should answer questions about definitions etc.
Spydey yeah callus is $$$. The whole point is to make cuts where it will cost the tree the least to close them and gain the tree the most in new, viable branches.
SELECTIVE HEADING CUTS AFTER STORM DAMAGE
December 5th, 2002 was a day that will live forever in tree infamy in Raleigh, NC. An inch and one-quarter of ice put a crushing load on the area¡¦s arboreal resources. Huge limbs dangled like Damocletian swords, grotesque ornaments greeting their owners. Contractors were told that in the course of cleaning out broken branches they had to cut all the stubs back to a substantial lateral. This rule is called ¡§Natural Target Pruning¡¨ or ¡§making Shigo cuts¡¨. They were also told, following FEMA guidelines, to remove trees with 50% crown loss. It was a Catch-23: obeying the first rule would remove so much more living crown, many trees would be removed that could simply be restored.
If arborists wanted to facilitate the coexistence of people and trees, they had to reread the directions, the ANSI Pruning Standards. In ANSI A300 4.20, heading is defined as ¡§Cutting an older branch or stem back to a stub in order to meet a defined structural objective.¡¨ 5.5.6 states that ¡§Heading should be considered an acceptable practice in shrub or specialty pruning to reach a defined objective.¡¨ Since restoration pruning is a type of specialty pruning, the standards seem to allow for leaving stubs in trees for the defined objective of preserving them.
Also, selective heading cuts are routinely made in vine, shrub and fruit tree pruning, bonsai, pollarding and other arboriculture. So how can all selective heading cuts in a big tree be considered improper? This exceptional storm challenged the simplified rules, which seem based on a Cliff¡¦s-Notes reading of the literature. The old ISA seal says, ¡§Science, Research, Preservation¡¨; good words to work by. Preserving trees is the goal; preserving branches is the way to reach that goal. This may mean cleaning the crown of damaged tissue only down to the first good node. As Dr. Alex Shigo said in A New Tree Biology, p. 458, ¡§Topping is done internodal; proper crown reduction is done at nodes, OR at crotches. So the first separation must be nodes¡Xgood, internodes¡Xbad.¡¨
NODES AS NATURAL TARGETS
Cutting to large laterals prevents natural regrowth and takes stored resources away from the tree. Restorative heading cuts are not random or predetermined, like topping cuts, but selected according to biological criteria. But what is a node, and what does it look like? In A New Tree Biology Dictionary, Dr. Shigo defines ¡§node¡¨ as ¡§the position on a stem or trunk that was occupied by the terminal bud and its associated buds.¡¨ Some nodes contain fully formed buds that have been carried along in the cambium as the branch grew.
These buds are connected to the vascular stream and often anchored by compacted xylem, as shown on pages 238-9 of ANTB. Due to their vascular connection, the growth from these buds can be well nourished, and due to the xylem connection it can be well anchored. This dominant growth contrasts clearly with weak growth newly formed on the surface of the bark from adventitious buds. Some botanists also define these growth points as nodes, but terminal bud locations offer a clearer target.
What do target nodes look like on the outside? A bulge just before a decrease in diameter can indicate reduced branch growth beyond a terminal bud. A cut just outside a bulge will also leave a smaller wound, and retain more symmetry and structure. Some raised areas may contain dormant buds visible to the naked eye. Some bumps and bulges may be due to pests, so the surface of the cut should be examined to ensure that is not the case. Wrinkles on branches can be the same swollen collars that once formed around the base of lateral branches. If a scar indicates that a lateral branch was shed at these locations, there may already be preformed lateral buds on the outside. They may also already contain what Gilman and Lilly called the ¡§unique chemical barrier called the branch protection zones¡¨ in Arborist News, August and October 2002. These articles are viewable online.
DANGEROUS DROP-CROTCHING
Locating nodes without laterals may seem sketchy at first, but consider the alternative. Reducing damaged branches back to the center of the tree can increase the danger of windthrow. In The Body Language of Trees, Mattheck and Breloer caution against removing more weight from the windward, storm-damaged side of the tree. ¡§The crown shape and the wind then combine forces to lift the pruned side of the crown, so reducing the normal stress and indeed perhaps transforming it into tensile stresses (i.e., lift!). When this happens, the effective sliding surface between the root-ball and the ground is so severely reduced that the tree blows over far more easily.¡¨
If drop-crotching exposes the remaining branches to more stress and strain, how is the tree safer than if heading cuts were made? The damping effect of limbs, for years thickened by torque, is altered while other branches thicken under the new load. The tree is vulnerable to disintegration while new reaction wood is formed in response to the new stresses. As Dr. Karl Niklas notes in the Tree Structure and Mechanics Proceedings, ¡§When exposed by the removal of neighboring stems, previously sheltered and mechanically reliable body parts may deform or break even under wind conditions that are ¡¥normal¡¦.¡¨
Avoiding decay is another good reason to make nodal cuts just below the storm-caused wounds. Large wounds on trunks are unlikely to close before they start cracking and become what Schwarze, Engels and Mattheck refer to in Fungal Strategies of Wood Decay in Trees as ¡§motorways for decay-causing fungi and bacteria racing into the heart of the tree.¡¨ Our strategy must be to minimize the infection courts we create. Retaining branches that Nature topped also avoids sun injury, defined by Shigo in ANTB Dictionary as ¡§¡Kwhen trees are suddenly exposed to direct sunlight¡KThe bark cambium is affected and the outer bark plates are flattened¡¨. These injuries are slow to seal because the tree¡¦s interior bark is very thin, and the sun dries the tissue at the edge. Big pruning cuts and sun-damaged bark may never seal.
Restore or remove? Where to make the cuts? It depends on:
„h Species- good sprouters and good compartmentalizers
„h Age and vigor of tree, which affects sprouting potential and wound closure
„h Size of wound ¡V smaller wounds = faster closure
„h Available laterals or other obvious nodes with sound wood
„h The need to retain a central leader and weight balance
THE TREE¡¦S RESPONSE
Retaining stems and scaffolds by making heading cuts can minimize sprouting by leaving, much higher in the tree, a smaller surface from which they will arise. Cutting deeper to a lateral may result in the attempted formation of more leaders growing more vigorously from a larger wound. The greater the dose of pruning, the greater the shift in the auxin/cytokinin balance. A part of the cytokinin effect in relieving apical dominance when applied to the bud may be the stimulation of vascular development connecting the lateral with the main vascular system. In The Formation and Development of Dormant Buds in Sugar Maple, Church and Goodman observed that "Epicormic sprouting below the live crown increased as additional amounts of the woody crown were removed...¡¨
When storms upset the balance between roots and canopy, the tree responds by sprouting to restore the balance. The more that is removed from the tree, the greater the imbalance and the reaction. At some point there will no longer be enough photosynthesis and the tree will decline. In The Practice of Silviculture, Smith notes that ¡§Diameter growth may suffer if the live crown ratio¡Kis reduced to 40 percent or less. Reduction in diameter growth slows wound closure.¡¨
say there is no such thing as a proper heading cut in a mature tree, that heading = topping, that all stubs are bad, etc. I'm trying to kick the door open to hearing any comments, broad or nitpicky.
