Recently, I participated in a panel on the use of AI in arboriculture. During the discussion, an audience member asked how we felt about the added environmental pressures associated with this technology. Most panelists acknowledged these concerns but had justified the costs—at least to themselves—by calculating the time saved compared to using traditional online resources. That exchange got me thinking about the other environmental costs on display at the conference: the few hundred people who traveled to attend (thankfully, relatively few flew in), the massive ballroom kept at a temperature cold enough to store dairy long-term, and the tree care industry itself, which was the focus of the event.

Earlier that week, I had given a talk highlighting findings from one of my earliest studies—an assessment of the net carbon benefits of trees. The idea came from my friend, Jeff Edgar, who had generously donated plants and space at his nursery for a study on transplant shock (a kindness he would later repeat for other projects). After a long day of installing trees and taking initial measurements, we went to a nearby bar to unwind. As we talked, Jeff shared that he often looked out across his nursery, knowing how much had gone into the growth and care of those trees. Considering the additional inputs at the original liner nursery and later in the landscape, he wondered: At what point does a tree break even? When does it shed its environmental debt and begin providing net benefits?

To answer the question, we modeled a specific scenario in the Chicago metro area (our scope) that included the production, growth, care, eventual death, and disposal of a ubiquitous red maple tree (our “functional unit”). According to survey data, this species could be expected to live an average of 48 years. We estimated that for over 60% of the tree’s lifespan—roughly the first 30 years—the inputs associated with production and maintenance outweighed the environmental benefits. The key takeaway from the article was that every year beyond this break-even point contributed meaningfully to increasing ecosystem services, and that retaining existing trees is more effective than replacing lost ones if the goal is to combat CO₂ emissions.

While working on this paper, I came across a highly cited—but, in my experience, rarely discussed—study that had a significant impact on me since I first read it as a student. Led by Dr. David Nowak, this work is the focus of this week’s Tree Research Journal Club:

Nowak DJ, Stevens JC, Sisinni S, Luley CJ. Effects of urban tree management and species selection on atmospheric carbon dioxide. Journal of Arboriculture. 2002 28(3):113-122.

What was done?

This study aimed to explore how urban tree species vary in their capacity to offset carbon, even when subjected to identical management and disposal practices. However, the core of the study was a detailed analysis of a single species—the same Red Maple mentioned above—to illustrate how variations in maintenance practices, disposal methods, and energy-saving benefits (such as shading) can influence both the timing and magnitude of a tree’s overall environmental impact. For this review, we will focus on that portion of the analysis.

What was discovered?

In contrast to the work we did, Dr. Nowak and his co-authors used an urban tree planting location as their functional unit. This seamingly simple change has a profound impact on the outcomes of the analysis. Where we were talking about a break even point for our hypotheical tree, Nowak’s team was calculating how long it would take for a spot to hit its last positive point. 

Essentially, a tree planting space in a city has the capacity to grow a tree of a certain size, depending on the species, available space and soil volume, care given, and other site conditions. As the tree grows, it sequesters carbon from the atmosphere, locking it into its woody tissues. Carbon sequestration ebbs and flows as trees grow, age, are removed, and are replanted. Unfortunately, much of the resulting wood waste is ground into mulch and spread in urban landscapes, where it quickly decomposes—releasing that stored carbon back into the atmosphere. In this disposal scenario, there is little long-term buildup of sequestered carbon. Meanwhile, CO₂ emissions from planting, maintenance, and removal operations continue. Over time, as this cycle repeats—tree after tree, year after year—there comes a point when the cumulative emissions outweigh what a tree can sequester at that spot. From that point on, the planting space becomes a net emitter of CO₂. It has passed its last positive point.

Figure 2 illustrates this relationship. Over the course of 400 years, successive generations of trees sequester carbon, steadily increasing the cumulative carbon stored over time (Sequestration). However, this is almost completely offset by the rising carbon released through decomposition (Decomposition). The Net Effect represents the carbon stored by living trees minus the emissions associated with ongoing maintenance (Maintenance). Eventually, maintenance-related emissions accumulate to the point where they surpass the carbon that can be stored by a tree at that site—this marks the Last Positive Point (LPP).

If you’re feeling a little disheartened right now, you’re not alone. I remember feeling crushed when I first read this back in grad school. Fortunately, the authors didn’t just drop that truth bomb and walk away. While carbon sequestration is often touted as a key ecosystem service, it’s not the only way trees help reduce CO₂ emissions. Strategically placed trees can lower heating and cooling demands for nearby buildings. Recognizing this, the team conducted an analysis in which the tree reduced energy use for a nearby structure.

Figure 3 shows how powerful carbon avoidance (i.e., avoiding carbon emissions through reduced electricity consumption) can be when assessing the net carbon benefits of urban trees. Sequestration, Decomposition, and Maintenance remain unchanged from Figure 1. However, the cumulative impact of years of energy savings is enough to produce a positive Net Effect. In this scenario, there is no Last Positive Point (Phew!).

You may be thinking, “Well, that’s great—but a significant number of urban trees, especially those along roadways and in parks, aren’t located near structures that could benefit from reduced energy use.” Fortunately, there are other levers we can pull in these cases—particularly regarding how removed trees are disposed of—to avoid reaching a Last Positive Point.

Figure 2 assumes that removed trees are chipped and spread as mulch, which quickly releases stored carbon back into the atmosphere within a few years through decomposition. However, our industry is beginning to explore better options for urban wood. I once heard a speaker remark that it’s ludicrous we consider mulch the highest-value product of urban trees (apologies for not remembering who to credit for this idea). Using urban wood for buildings or durable goods like furniture can store carbon for additional decades—or even a century.

Alternatively, repurposing wood “waste” to replace coal in power generation or peat in horticulture helps avoid emissions by keeping these stable forms of carbon safely in the ground. The authors also model a scenario in which wood waste is simply buried in an anaerobic environment—i.e., a landfill (Figure 4). Devoid of oxygen, these environments can preserve everything from hotdogs to hawthorns by significantly slowing decomposition. While landfilling is often viewed negatively, from a carbon sequestration perspective, it could actually be a net positive. Deep, cold bodies of water might offer similar preservation potential for wood, though—as with landfilling—other environmental impacts must be considered.

What we like about this paper

We appreciate that this paper presents a somewhat provocative and surprising finding, but then quickly pivots to practical solutions. It shows that urban trees can serve as a carbon sink—if we choose to manage them, and the wood they produce, differently. As always, we want to give ISA a shout out for making all articles in their journal (now Arboriculture & Urban Forestry) open access. Also thanks to ISA for giving us permission to reprint these images in our blog. 

Conclusions

We often focus on the many benefits of trees while overlooking the environmental costs associated with their production, care, and maintenance. Most environmental models calculate only the gross benefits, but as this paper demonstrates, the story can change dramatically when net benefits are considered. If we ignore environmental costs, our well-intentioned efforts could actually cause harm. More importantly, assessing net benefits reveals that even simple tweaks to our operations can drastically shift the balance toward greater environmental gains

About this blog

Rooted in Tree Research is a joint effort by Andrew Koeser and Alyssa Vinson. Andrew is a Research and Extension Professor at the University of Florida Gulf Coast Research and Education Center near Tampa, Florida. Alyssa Vinson is the Urban Forestry Extension Specialist for Hillsborough County, Florida.

The mission of this blog is to highlight new, exciting, and overlooked research findings (tagged Tree Research Journal Club) while also examining many arboricultural and horticultural “truths” that have never been empirically studied—until now (tagged Show Us the Data!).

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