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Why is pruning crucial to London's trees?

Justin Morgenroth, B.Sc. M.F.C.
Urban Forestry Consultant

Urban trees are expected to perform not only ecological roles, but also aesthetic and cultural roles. As a response to these expected roles and the complete integration of humans into the natural environment, the practice of tree pruning has developed. There are three primary approaches to pruning: crown thinning, crown raising, and crown reduction. The primary benefits of tree pruning, which include minimizing safety concerns, maximizing tree health, and improving aesthetic values can be realized through a combination of these three techniques.

Safety
Ornamental and shade trees must fill their ecological, cultural and aesthetic roles without becoming hazardous to humans or human constructs, such as buildings and utilities. So, for the purpose of minimizing trees that are hazardous to humans or property, pruning has become a necessary and integral part of managing the urban forest. This practice includes, but is not limited to, removing dead or broken branches, preventing branch conflicts with hydro wires, removing roots from sewers and water mains, and removing branches that interfere with lines of sight along roadways and walkways. These issues can cause serious injury and even death, hence the importance of safety pruning, especially in densely populated areas.

To minimize these risks, all three pruning techniques are utilized. Crown thinning is practiced to remove dead or broken branches, crown raising is practiced to maintain sight lines, and crown reduction is practiced to prevent branches from interfering with hydro wires. These examples reveal that safety pruning is typically a reaction to a known issue. Conversely, proactive pruning eliminates these issues before they become problems, thus reducing the need for future safety pruning. The primary goal of proactive pruning is typically related to tree health.

Tree health
Pruning is used to promote tree health in two ways. The first is related to crown thinning which emulates a natural process called self-pruning. The result of this natural process is branch death, which occurs after its net photosynthesis production decreases beyond a minimum threshold. This process is a physiological response which helps trees to improve overall health by focusing their energy reserves on branches which continue to provide a positive energy balance through elevated photosynthesis levels. Crown thinning through pruning emulates this process by removing heavily shaded branches found at the bottom and inner portion of the crown as these branches will inherently receive less sunlight, the catalyst for photosynthesis and energy production. By preemptively removing these branches before self-pruning occurs, a tree’s energy reserves can be utilized more efficiently while preventing the incidence of hanging, dead branches in tree crowns which is especially hazardous in populated areas.

The second way that pruning increases tree health is related to the size and severity of wounds left on a tree following branch removal. The importance of this factor is that wounds are entry points for pathogens or insects. Since open wounds invite infection, a quickly healing wound is better for tree health than a slowly closing wound. Wounds result from pruning, vandalism, or natural causes such as branch death or wind-related branch breaking. Though wounds are inevitable, slow healing rates are avoidable. Pruning, when done properly, leaves a relatively small, clean wound that will begin closing within a growing season. Conversely, broken branches or branches torn from trees during acts of vandalism require numerous seasons to close and thus trees are susceptible to pathogens for longer periods of time. So, by pruning low branches and removing poorly attached branches, pruning minimizes pathogen entry via large, slowly-closing wounds.

Aesthetics
In addition to pruning for practical purposes such as human safety and tree health, pruning for aesthetic purposes such as desirable crown form or flower production is practiced. Though trees grow naturally according to specific genetic traits, their morphology is influenced by environment – factors such as light and moisture influence growth rates and form. These environmental factors result in epicormic branching, vertical branching and downward branching. Pruning is used to correct these unwanted traits and promote a desired form.

In fact, distinct styles of aesthetic pruning, such as topiary, espaliering, and pollarding, promotes unnatural forms. Topiary modifies the natural form of shrubs and trees into any desired form, most often geometric shapes or animals. Espaliering is used to train trees and shrubs to grown flat, usually along a wall or trellis. Pollarding removes all annual growth; the following growing season results in an abundance of new branches growing outward from the end of a select few branches.

Thus, pruning is especially important in urban areas with respect to its positive effect on human safety, the minimization of property damage, the promotion of tree health, and the improvement of community aesthetics. Though pruning has often been a reaction to trees outgrowing their planting space or become hazardous, proactive pruning is a valuable option that can minimize the occurrence of negative scenarios. This option is likened to preventative maintenance with a motor vehicle. Remember, a small pruning job done properly today will prevent a large pruning job from being necessary tomorrow!

Related article: Pruning methods

Justin Morgenroth is a consulting urban forester stationed in London, Ontario . He received his Master’s degree from the University of Toronto ’s Faculty of Forestry. Justin’s work is motivated by the belief that trees have crucial aesthetic and cultural values, and that their systematic elimination from densely populated urban areas is an avoidable disaster. He can be reached for comment at justin.morgenroth@utoronto.ca.