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Pruning: A Case Against Routine Thinning
By Bruce W. Hagen
Current pruning practices for mature trees emphasize thinning and dead branch removal rather than correcting structural problems, e.g., maintaining a single leader by removing or shortening (reducing) competing leaders, reducing or removing branches with included bark, keeping branch size to less than 50 percent of the trunk diameter, improving branch spacing, etc. Obviously, pruning dead branches is a valid practice because it reduces hazard potential, facilitates more rapid closure of the branch collar, while making it easier to monitor changes in health. The case, though, for routine crown thinning, is not as easy to make.
Crown thinning, is the selective removal of small, live branches throughout the crown to reduce the leaf surface area. The majority of branches should be removed from the outer third of the crown. Because only small branches are removed, there is no change in the to the tree’s size or shape. ANSI A300 Pruning Standards state that “not more that 25 percent of a tree’s foliage should be removed within an annual growing season.” Furthermore, it states that the percentage of crown thinning must be adjusted to account for inherent tolerance, age and condition and environmental factors. Tree workers often overlook this last point and view the 25 percent as a goal, rather than an upper limit. Another problem is that estimating the percentage of foliage removal is tricky. They typically underestimate the percentage of foliage removed, and often wind up thinning more than 50 percent of the foliage.
Thinning is not necessarily a problem unless too much of the foliage is removed. Health, vigor, age, species characteristics must be considered when determining a tree’s tolerance of foliage loss. The objective of pruning will also determine how much pruning is needed. At any rate, removing more than 25 percent of the crown is seldom warranted and is usually counterproductive. Excessive thinning of trees with dense crowns to reduce wind-sail and potential for toppling may actually increase potential for limb breakage by allowing thinned branches to sway more freely. Removing more than 20 percent of the canopy often stimulates vigorous growth, increasing foliage density to a level greater than that prior to pruning. The typical response to excess thinning is increased shoot elongation and epicormic sprouting along branches that were previously shaded. In this manner, excessive thinning creates additional work because these dense sprouts must then be managed. Many tree care companies, though, simply remove the sprouts during the next pruning cycle. In general, sprouts should be selectively ‘thinned’ and shortened rather than removed, because they help to compensate for lost foliage.
It should be noted that moderate thinning may be useful to reduce wind-sail in trees in windy areas and those growing in shallow or coarse soils. Crown thinning may also help to reduce damage due to snow loading. Trees weakened by decay and those with girdling roots or other serious defects may also be candidates for moderate thinning. Selective thinning to reduce end-weight, or shorten (reduce) branches can reduce limb weight in order to compensate for structural defects. Trees plagued by certain leaf diseases may also benefit from moderate thinning.
Lion’s tailing, another common pruning practice, is problematic. It involves the removal of many of the inner and lower laterals, which shifts growth and the load-bearing surface toward the top of the tree, placing greater stress on the scaffold branches and trunk. Lower laterals are vital because they help to distribute wind loading more evenly and dampen the effects of strong wind. Furthermore, they increase branch taper, reducing the potential for branch breakage. Lion-tailed branches, which typically grow vigorously, add little caliper below the foliage. One further complication is that the branches are often weakened by decay due to the number, size and quality of pruning cuts made to strip out the laterals. So, lion’s tailing weakens branch structure, increases limb breakage potential, stimulates water sprouts and can lead to sunburn injury. Of greater concern, though, is that this practice leaves few options in the future to shorten overly long branches or reduce end-weight. In many cases, removing critical, outer canopy foliage, cabling heavy branches or removing large branches are the only options left to maintain safety.
Pruning does not improve health as commonly claimed–it’s a wounding process that can lead to decay and cause stress. More importantly, a reduction in leaf surface area impairs a tree’s ability to feed itself. Although, most trees survive excessive pruning, they pay a price in terms of reduced growth, longevity, appearance, structural stability and resistance to decay and other serious pests.
Another misconception is that thinning interior branches is beneficial because it increases light and wind penetration into the canopy. Despite relatively low light levels, interior foliage in many trees is productive. Sunflecks are short duration points of light that penetrate a tree’s crown as the sun moves. These bursts of full sunlight play an important role in temporarily increasing photosynthesis. Additional light enters the crown as leaves and branches move in the wind. The level of light intensity, though, varies greatly by species, and is dependent on crown shape and foliage density. Little interior foliage forms in trees with very dense canopies e.g., cypress. In most trees, though, foliage extends well into the inner crown where diffuse light or full sun for short periods enters. Interior foliage (shade-leaves) can function at a relatively low light intensity. Branches that are no longer productive are shed. This is a natural process and generally not a concern. Because density of interior foliage varies by species, moderate thinning may be useful to stimulate and retain inner branches to aid in developing branch taper in large, wide spreading trees. The need for thinning then is justified on species characteristics, failure patterns, age, size, environmental factors, objective, etc.
Young trees are fairly tolerant of pruning in excess of 25 percent of the foliage, while older trees are sensitive to the loss of even 10 percent of their foliage. Excessive thinning may cause older trees to decline or predispose them to secondary pests due to the stress. For this reason, pruning of mature trees should focus on developing a stable structure rather than randomly thinning foliage.
Excessive pruning is a significant problem for trees because it reduces photosynthetic output, ultimately impairing health. It is important to remember that there is a functional balance between a tree’s root system and its crown. This balance develops in response to environmental factors, e.g., light, soil moisture and other soil conditions. For instance, trees growing in favorable sites typically have a large canopy and relatively small root system. The root systems of trees growing in less favorable sites are comparatively larger than their crowns. In healthy trees, there are enough roots to supply water and nutrients to the shoots for normal growth. When there is a water or mineral shortage, more carbohydrate is directed to the roots to gather needed resources. Conversely, trees experiencing a carbohydrate shortage, produce more shoots at the expense of root growth. In this manner, excessive pruning, which decreases photosynthetic output, stimulates vigorous shoot growth. It may take several year’s for a tree to reestablish the root:shoot balance that existed prior to pruning.
One of the most visible effects of excessive pruning is the release of latent (epicormic) buds– sprouting due to the exposure of previously shaded bark to sunlight. Carbohydrate reserves are tapped to produce these sprouts. Reserve energy is also diverted to wound sites to resist decay (CODIT). The overall effect is stress and weakened natural defenses. Other impacts include an altered architecture with increased hazard potential. In addition, excessive pruning done at the wrong time can lead to sunburn damage to the bark of newly exposed branches. The bottom line is that excessive pruning effects health, appearance, structure, and increases maintenance costs.
The pruning of large, older trees that have never been structurally pruned, and those that have been lion-tailed is a challenge. Over-mature and declining trees are particularly prone to failure due to increasing mass and slow growth. Such trees are unable to produce enough reaction wood or annual increment growth to offset increasing branch weight and/or developing decay. Weight reduction in older trees should be considered, but leaf surface area is critical. Emphasis must be placed on reducing excessive end weight and/or reducing the crown to prevent branch failure. Crown reduction–the selective pruning of the outer crown to decrease height and spread, can significantly lessen the risk of branch failure or uprooting. Adequately reducing the crown of large trees often requires making relatively large ‘reduction’ cuts–greater than 4-inches in diameter. Such cuts, though, are more likely to develop a column of decay, because there is no natural protection zone to resist pathogens. Branch protection zones occur only within the branch collar of lateral branches that are relatively smaller than the trunk or parent branch. Smaller reductions cuts are more likely to close without developing appreciable decay. The goal should be to remove as little foliage as possible, while meeting the objective of weight reduction. Work on declining trees is best done over several years to reduce stress. Unless there are no other options, large pruning wounds should be avoided. Trunk decay is often the result of removing large scaffold branches or codominant branches. Such wounds usually close slowly and branch protection zones, should there be any, are typically weak. Large pruning wounds that cut across heartwood can result in significant decay.
The most compelling reason to prune mature landscape trees is to establish and maintain a more stable form. A single, dominant leader, with well-spaced scaffold branches comparatively smaller in diameter that the trunk is the optimal tree form for wind resistance. The key, though, is to begin pruning when trees are young and to continue pruning throughout the life of the tree.
Structural problems in older trees with multiple leaders and branch defects are problematic, but can be minimized by using proper pruning methods and a long-term strategy. Competing leaders and those with weak unions should be subordinated (reduced) to minimize potential problem. This helps avoid making large wounds. Most pruning should consist of removing as few living branches as possible and keeping cuts few in number and small in size. Prune to meet your objective, but understand that some objectives may not be compatible with tree health.
Arborists must start using a more prescriptive approach to pruning than applying the same treatment to all trees. Pruning must be based on more than minimal pruning standard, aesthetics or outmoded ideas. What is needed is a more comprehensive guide reflecting the current science-based knowledge about tree structure, biology and response to pruning. One publication: An Illustrated Guide to Pruning, Second Edition, authored by Dr. Ed Gilman’s of Florida State University– meets that need. This book is the most detailed, comprehensive and reasoned publication now available. Arborists would do well to study it.
Pruning tips:
- remove or shorten (‘reduce’) competing branches and those with included bark
- keep branches to 50% or less than the diameter of the trunk, parent branch by thinning or reduction.
- direct growth, control branch size through branch removal or reduction (‘drop-crotch pruning’)
- remove dead and low vigor branches
- manage end-weight
- reduce crown of large, old or unstable trees
Fundamental concepts:
- Pruning removes parts that serve a useful purpose (foliage, branches, buds (potential growth) and contain stored energy.
- Pruning reduces photosynthetic capacity and depletes stored energy.
- Pruning creates wounds which can lead to decay
- Energy is critical for growth, maintenance, reproduction, and defense
- Pruning interrupts the normal hormonal balance between the roots and shoots.
- Pruning can interrupts the normal hormonal balance along a branch, releasing suppressed buds.
- Pruning alters root: shoot ratio.
- The response to excessive pruning is vigorous regrowth (replacement of lost foliage)
- Root growth slows so balance can be restored.
- Water and mineral shortages may result, thus heavily pruned trees are under greater stress, this can lead to dieback, decline, increased susceptibility to pests or even death
- There are two seeming opposite effects of pruning:
- overall ‘dwarfing’ of tree–it is smaller and has less mass
- invigoration of individual branches–new growth is concentrated in fewer growing tips, thus greater elongation of each of the remaining shoots.
- Lower lateral branches along the trunk or parent branch contribute energy, resulting in greater stem diameter growth (‘taper’). Taper increases resistance to wind-loading and end weight. Thus, the removal of lower laterals reduces stem taper.
- Trees are most stable when ½ of their foliage grows is distributed in the lower 2/3 of the tree.
- Lion’s-tailing concentrates weight and wind-loading in the outer canopy. Wind loading is more evenly distributed in unpruned trees.
- Live-crown ratio should exceed 60 percent
- Excess thinning changes aerodynamics, which may destabilize branches by eliminating the wind-buffering effect that neighboring branches provide.
Professional pruning standards ANSI A300 and ISA BMPs– Key points:
- Topping and lion’s tailing are unacceptable
- Protect the branch collar when pruning
- Remove no more than 25% of the foliage (this is the max not the goal!)
- When reducing the length of a branch, cut to a lower lateral, one that is at least ⅓ to ½ the diameter of the branch being cut.
- Pruning cuts that reduces the length of branch to a lower lateral should be made from the top of the branch bark ridge to a point directly opposite the bottom of the branch bark ridge.
- Reduction cuts should not exceed 4 inches in diameter.
