Assessing the Mechanical, Physiological and Carbon Accounting Effects of Different Pruning Regimens

2022, Stella Dee, University of Massachusetts, Amherst, Co-PIs Dr. Brian Kane.

Although pruning is one of the most common arboricultural practices, the mechanical, physiological, and carbon management costs and benefits are not yet fully understood. Many benefits of shade trees, including temperature mediation, pollution capture, carbon reduction, and intercepting precipitation depend on maximizing leaf area, even as greater leaf area increases drag, lowers sway frequency, and perhaps increases likelihood of failure. Meanwhile, the carbon costs associated with commercial tree care are significant; pruning a particular tree often involves large trucks, aerial lifts, chainsaws, chippers, tub-grinders, and stump grinders, all equipment that uses significant quantities of fossil fuels. Prior research suggests that the arboriculture industry is responsible for seven times the carbon emissions of other industries of similar size (Luck et al. 2014). However, Petri et al. (2016) describe pruning practices as making the “single greatest” difference in industry carbon emissions. Much of the past research into the
effects of pruning has been limited by its short duration, with trees being pruned and examined in time periods as short as a single day. This project will attempt to reconcile some of these competing objectives to determine costs and benefits of different pruning regimes with respect to tree health and carbon accounting.

In this project we will subject at least two species to six pruning treatments, including different severities of reduction and removal cuts, as well as different styles of removal cuts. We will quantify mechanical and physiological response to these treatments by assessing change in sway frequency, damping ratio, sprout growth, wound occlusion and leaf area for two years following pruning, measuring these response variables once each year over the duration. We will quantify management-related carbon emissions of the different treatments using updated versions of the equations developed by Nowak et al. (2002).

The outcomes of this work will help guide commercial arborists in understanding the effects of their pruning cuts, in thinking through which pruning cuts might be more appropriate in which situations, and in guiding municipal arborists and urban foresters to better understanding the carbon emissions and leaf area implications resulting from arboricultural practices.

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Results:

Our results suggest that low to moderate intensities of pruning have significant effects on the invertebrate community associated with the tree, and that the nature of these effects can be dependent on both pruning type (removal versus reduction) and species. The effect of low to moderate intensities of arboricultural pruning on biomechanical sway properties appears to be statistically absent, while the effect of such pruning on leaf area index (LAI) is minimal. There appear to be species-dependent differences in resource allocation, causing some species to prioritize epicormic regrowth while others prioritize wound occlusion. These differences between responding to pruning via epicormic regrowth versus wound occlusion appear to have ramifications for the ecological response as it relates to the invertebrate community; in some species, reduction pruning appears to cause more epicormic regrowth and attract a higher quantity of herbivorous invertebrates. Higher LAI and greater quantities of deadwood in the tree may also be associated with greater quantities of herbivores. Pruning has the potential to decrease the ratio of predatory:herbivorous invertebrates associated with the tree, especially closer to the time at which pruning took place. These effects occur even when pruning takes place in the dormant season and can last for at least two years following pruning.

With respect to carbon accounting, the greatest usage of fossil fuels comes through chipping, and the chipper run-time is exponentially longer during the process of transporting brush to the chipper rather than during the chipping of brush itself. Reducing the distance between brush and the chipper has the potential to greatly reduce chipper run-time. The amount of carbon removed from the tree during pruning is significant, and since the benefits of pruning for tree health are as yet unproven, the carbon costs of pruning may not justify the benefits in all cases.

Although our current results are limited by small sample sizes and a single location (Amherst, MA), they suggest that practitioners may underestimate the degree to which arboricultural pruning impacts the community of arboreal invertebrates associated with trees. Moreover, they call into question the prevailing sense among practitioners that reduction cuts are “lower impact” on trees compared to removal cuts. Our results suggest that the opposite may be the case for some response variables, particularly when it comes to the effect on herbivorous invertebrate abundance which may negatively influence tree health in following pruning.

 

 

Year: 2022

Funding Duration: 2 years

Grant Program: Hyland Johns Research Grant Program

Grant Title: Assessing the Mechanical, Physiological and Carbon Accounting Effects of Different Pruning Regimens

Researcher: Stella Dee, Co-PI Dr. Brian Kane

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