A major conversation in the realm of climate change is on the ability of trees and forests to fight it. Many postulate that restoring the abundance of trees could have “mind-blowing potential” to deal with climate change.

Environmental organizations throughout the world rely on planting trees and responding to mass deforestation as a way to ensure a better future for humanity and nature. The idea of regenerating our world’s forests has also influenced transnational programs, such as REDD+, through the United Nations Framework Convention on Climate Change. 

Basic environmental science has certainly backed up this notion. It is widely accepted that through the process of photosynthesis, trees are capable of pulling in atmospheric carbon dioxide, thus decreasing the potential for global warming.

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The idea is, if enough trees are present in the world, this process on a grand scale could extract enough carbon dioxide from the atmosphere to reverse the grim future associated with climate change. In early 2019, Thomas Crowther, a British scientist, posited that if 1.2 trillion trees were planted across the globe, they could potentially soak up a greater amount of carbon than humans emit year by year. 

Recent research by a team of dendrochronologists, including five people from the University of Arizona Laboratory of Tree-Ring Research, may be a bit of a reality check though. The gathered data shows that in the race between warming and carbon sequestration, warming may be stronger because it could be more prohibitive to tree-growth than previously thought.

Of course, this recent research does not mean that trees cannot be used as a tool to combat climate change. Margaret Evans, a dendrochronologist from UA who participated in the research, believes that to fight the climate issues that face us, no tool should necessarily be disregarded. 

“Forests can play a really important role and we need to have a whole portfolio of things that we’re doing,” Evans said. “To solve this problem, we’re going to have to throw everything at it.”

With that being said, the scientists involved in this research have found that the ability for trees to act as carbon sinks during ensuing climate change may not be as significant as once thought. 

Stefan Klesse, a postdoctoral fellow at the Swiss Federal Research Institute for Forests and Landscape in Switzerland and a participant in the research, said that the team’s goal was to try and predict future effects of global warming on tree growth using space-for-time substitution. 

The method of space-for-time distribution is used by analyzing characteristics associated with one area affected by certain environmental factors and using that to predict what another area may experience amid those factors in the future. For example, in climate research, one may examine the width of tree rings in an area with climate-induced stresses and use findings to assume the stresses to come in another area. 

In simple terms, Klesse described, “you use a tree in Arizona to tell someone how a tree in Montana will grow in the future if you extrapolate that Montana will get as warm as Arizona.”

According to Evans, the team enacted a space-for-time substitution by examining the Douglas fir tree across its geographic distribution in North America. They chose to research the Douglas fir because of its major presence. In the lower 48 states of the U.S., it has been measured to have the most biomass out of any tree species, Evans described.

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In the Douglas fir tree, the team was examining tree rings. By examining the width of tree rings, dendrochronologists can track the growth of a tree during a given year. The wider the ring, the more growth that occurred. So, this also allows for dendrochronologists to see how productive a tree was in response to weather variations from year to year. 

“Dendrochronology, for a century, has been looking at the relationship between the width of these rings and the climate conditions that the rings were formed under,” Evans said. “What was the weather like in the year that that ring was formed — was it a wetter than average year, a warmer than average year and what does that do to the width of the ring?”

For this study, the team pulled from a rich collection of tree-ring data on the International Tree-Ring Data Bank. With this data set, they were able to examine the average growth rates of Douglas fir trees across its range and assess the vulnerability to climatic trends and variations in specific areas. 

Klesse said that tree-ring data showed the highest growth rates in northern coastal California, coastal Oregon and coastal Washington. In these areas, there is very little climate variability so there is little to no variation between tree-ring widths year by year. Klesse and Evans both described the observed tree-rings as “train-tracks.” They look the same from tree to tree, year to year. 

Arizona’s Douglas fir trees were shown to have medium growth rates. Here, it is much drier and there is more variability in climate than on the northwest coast.

The area with the lowest growth rates was in the very cold, dry areas. Areas of Montana and the eastern edge of the Rocky Mountains were shown to be quite unproductive. These trees were also shown to be extremely sensitive to climate variation. 

So, what does this all mean? Klesse and Evans explained that this means two things. First, the space-for-time substitution they used could not actually be used to predict growth rates as the climate warms. Through the data, it was actually shown that the warmer areas like northern Arizona and coastal California are actually much more productive than in the cold, dry continental areas.

This means that if they took Arizona’s larger productivity and used it to predict Montana’s future productivity in response to warming, it would show that Montana would actually have higher growth rates as global warming persists. This does not make sense in light of what else the team discovered. 

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Their second discovery, which ended up being the key one, was that the Douglas fir trees across the board were less productive in years that were warmer and drier.

Overall, space and time differed in regard to whether warmth was a good or a bad thing. In warmer areas, it was shown that productivity is greater, which indicated a “positive gradient” across space with temperature. On the other hand, it was shown that warmer years caused less productivity which indicated a “negative gradient” across time with temperature, according to Klesse. 

In the case of global warming, this negative gradient across time is much more significant. Warming is causing the trees to become less productive and also more sensitive. 

“As it gets warmer and drier the trees are getting more and more sensitive,” Klesse described. “They are not keeping the same sensitivity as they have had in the 20th and 19th Centuries.” 

As the climate becomes more unfavorable for trees and imposes more stresses such as drought and wildfires in Arizona and more temperature variation on the northwest coast, this will likely lead to less potential for trees to sequester carbon out of the atmosphere. 

Klesse described how this could lead to a worrying positive feedback loop. In the natural sciences, positive feedback loops are when “loops enhance or amplify changes” and can lead to a path further and further from natural equilibrium, according to the Carleton College Science Education Resource Center. 

In the case of this research, it can be predicted that as climate change persists, trees will become more sensitive to climate variations and warmth. As this happens, there will be less capability for trees to pull carbon dioxide out of the atmosphere.

As a result of this, climate change could become more difficult to combat and more intensely persist. It then all comes back to the trees. More intense climate change will only exacerbate tree sensitivity and inability to sequester carbon dioxide, leading to further climate change. 

On a smaller level, Klesse also described that this could be harmful to our tree-reliant economies and it could greatly change the landscapes so valuable to those who spend time outdoors. 

Evans said that this new research is not meant to fully discount the ability of trees to protect our future, but instead it could “chart a path between excessive optimism and excessive pessimism.”

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