As strange as it seems, the 1970s to the 90s may have been a Golden Age for arboriculture in America.

The concept of ‘Urban Forestry’ was introduced in 1965. Alex Shigo, the father of modern arboriculture, was conducting his groundbreaking research. And most importantly, the International Society of Arboriculture (ISA), formed in 1976,1 had not yet asserted complete dominance over tree maintenance practices.

In 1991 the Chicago Urban Forest Climate Project2 counted every leaf in Chicago, and even came up with the estimated amount of leaf surface area. This project offered the first proof that trees cleaned the air, sequestered carbon, and combated climate change.

Today it is practically heresy to differ with ISA accepted practices. But remnants of independent thought can still be found. One of the most incisive articles, A Case Against Routine Thinning, is by Bruce W. Hagen.3 The Fundamental Concepts are from that article.


  • Pruning removes parts that serve a useful purpose (foliage, branches, buds ie: 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 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 growth 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.


1. 1924 is often given as the founding of the ISA, and they are sure to celebrate their centenary in 2024. 1924 was the year the Connecticut Shade Tree Conference held their first annual conference. The ISA was formed 52 years later in 1976.

2. Chicago Urban Forest Climate Project

3. Mr Hagen served for years as an Urban Forester with CalFire. Among his many publications are Living Among the Oaks and Oaks in the Urban Landscape (University of CA), as well as Tree Structure and Function.


A Case Against Routine Thinning, by Bruce W. Hagen

Thinning the Crown, Urban Tree Foundation

Thinning the Canopy; Pruning Shade Trees in the Landscape; Edward F Gilman and Nathan J Eisner; University of Florida.

Chicago Urban Forest Climate Project