Optimizing tree responses to root pruning in mature trees

2021, Justin Morgenroth, PhD, The University of Canterbury, Co-PI Andrew R. Benson, PhD

Roots are important tree structures and have a key biological function in tree health and survival. Despite this, root pruning is a common practice. The roots of trees are removed, usually for buried utility maintenance, sidewalk repairs or other civil construction projects. When roots are damaged, or removed, the wound sites can become points of entry for decay-causing organisms such as fungi, which can lead to secondary tree health problems. Similarly, the removal of roots, particularly large roots, affects tree function and growth by reducing the surface area of nutrient and water absorbing roots. New roots need to be grown by the tree for healthy tree function and growth to continue. It is therefore necessary to understand how to optimize tree responses to root injury, by applying correct pruning techniques in the same way as branches are pruned from the crown. To achieve this, we will undertake different root pruning regimes on mature trees to see their response to pruning position (internodal cut versus cut to a lateral root branch) and seasonality (pruning in summer versus winter). Pruning cuts will be made on a variety of root sizes to see whether the responses depend on root size.

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In this study, we investigated how different root pruning methods and seasonal timing affect the responses of mature Platanus × acerifolia trees. While branch pruning has been widely studied, there is far less research on how trees respond to root pruning. Our goal was to improve understanding of the biological responses of tree roots to different pruning strategies, particularly in terms of root regrowth and wood discoloration (as a surrogate for decay).

To explore this, we selected thirty-two mature Platanus × acerifolia trees in Christchurch, New Zealand. To these trees, we applied four different root pruning treatments. We pruned roots either just before the summer growing season or just before winter dormancy. Additionally, we made cuts in two different locations: either internodally, leaving a stub with no side roots, or at a junction, where we retained a subordinate side root. This created four distinct treatment groups: internodal pruning before summer, internodal pruning before winter, junction pruning before summer, and junction pruning before winter. Each tree had three roots severed, and after three years, we analyzed new root formation and measured wood discoloration at the pruning sites.

We found that roots pruned internodally produced more new roots than those pruned at a junction. Among all treatments, roots pruned internodally just before summer produced the highest number of new roots and recovered the greatest proportion of the severed root’s cross-sectional area. Most new roots emerged within ten millimeters of the original pruning site, which aligns with previous research on root regeneration. Interestingly, the time of year when pruning was conducted did not have a significant effect on root regrowth, suggesting that seasonality does not play a major role in the ability of Platanus × acerifolia roots to regenerate, at least in Christchurch, New Zealand’s temperate oceanic climate.

However, when we examined wood discoloration as a surrogate for decay, we found that both the timing and location of pruning cuts made a difference. Roots pruned just before winter exhibited significantly less discoloration than those pruned before summer. Among all treatments, roots pruned at a junction before winter displayed the least discoloration, which suggests that they were better able to compartmentalize dysfunction. We also discovered that smaller roots showed more extensive decay than larger ones, revealing an inverse relationship between root diameter and discoloration.

Our findings have important implications for arboricultural practices. If the primary goal of root pruning is to maximize regrowth, then pruning internodally just before the summer growing season is the best approach. However, if minimizing decay and wood discoloration is the priority, then pruning at a junction before winter dormancy is the most effective strategy. Our research provides valuable guidance for arborists and urban forestry professionals, helping to refine best practices for managing tree roots in urban environments.

Year: 2021

Funding Duration: 2 years

Grant Program: John Kimmel International Grant

Grant Title: Optimizing tree responses to root pruning in mature trees

Researcher: Justin Morgenroth,PhD

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For more information on this project, contact the researcher via TREE Fund at treefund@treefund.org.