We’re all in this together: Climate-forest connections mean local tree deaths have widespread impacts

Reference: Swann, A.L., Laguë, M.M., Garcia, E.S., Field, J.P., Breshears, D.D., Moore, D.J., Saleska, S.R., Stark, S.C., Villegas, J.C., Law, D.J. and Minor, D.M., 2018. Continental-scale consequences of tree die-offs in North America: identifying where forest loss matters most. Environmental Research Letters13(5), p.055014. https://doi.org/10.1088/1748-9326/aaba0f

Large scale tree death caused by Mountain Pine Beetle infestation in a forest in British Columbia Source: UBC Micrometeorology (CC BY 2.0)
Breaking free: How forests can have impacts beyond their bounds

“We are all connected.” It’s a mantra you’ve probably heard repeated so many times that it may have lost all meaning. While the saying may be somewhat tired, the sentiment remains very much true when it comes to the earth’s forests and the role they play in the climate system. At the local scale, trees not only take up carbon dioxide, the main greenhouse gas driving climate change, but also have a big impact on how water, air, and energy move through a landscape.

Trees draw water from the soil and release it through their leaves as water vapor in a process called transpiration. Trees absorb some of the incoming energy from the sun, but also reflect some of it. Vegetation also impacts how wind moves over the land. The local climate impacts of trees are even something you can experience for yourself if you want to cool off on a hot day – try going somewhere with lots of trees! Exchanges of water, carbon, and energy between trees and the atmosphere not only have an impact on a forest’s close surroundings, they can also have large scale effects.

Atmospheric circulation, the movement of air across the globe, plays a big role in determining the local climate of a given location. Whether a place is hot, dry, cold, or wet depends in part on where its air is coming from. This is why when you watch the local weather forecast on television, the meteorologist will point out pressure systems and air streams, which determine how air is moving in the atmosphere. Atmospheric circulation is influenced by the exchanges of water, heat, and energy, such as those that occur within a forest. In this way, the forest is connected with the larger climate system and can have an impact far beyond its boundaries.

One way these interactions have been studied is through measurements of the movement of water, wind, and carbon dioxide on towers in forest canopies like this one in British Columbia. Photo by Andreas Christen, UBC. (CC BY 2.0)
Start of something new: A changing climate brings more dead trees

Because of the interactions between vegetation and climate, there is a lot of concern over the potential impacts of what could happen if we lose forests. In addition to deforestation, forests can be lost through large tree die-off events caused by drought, disease, or pests. While we’ve already experienced some instances of large tree die-offs in places across North America, such events are predicted to become more common with climate change. Knowing how forests can be connected to far away areas, the question remains of what are the potential impacts of these massive die-offs could be.

A team of scientists led by Dr. Abigail Swann, from the University of Washington, set out to investigate whether some of these connections might be more important than others. In other words, would losing certain forests have a larger impact on other regions? In order to do this the scientists used a climate model. They simulated what would happen if there were massive tree die-offs in different parts of North America, and then analyzed how temperature and carbon uptake would change in other forests across North America, which could potentially amplify these effects of forest loss on climate change.

What I’ve been looking for: A better understanding of forest connections

The researchers found that there were a wide range of potential impacts of forest loss in different regions, with both positive and negative impacts on overall carbon uptake observed. Loss of trees in the mid-Atlantic had the greatest positive impact on carbon uptake on average. This is likely a result of loss of forest in one region leading to a change in atmospheric circulation across the continent, which in some cases means better conditions for photosynthesis and therefore more carbon uptake overall.

Changes to carbon uptake caused by forest loss in the Pacific Southwest (left – yellow region) and the Mid-Atlantic (right-yellow region). Brown represents areas where carbon uptake will decrease, white indicates no change, and green indicates an increase in carbon uptake. Source: Swann et al., 2018 (CC BY 3.0)

On the other end of the spectrum, loss of trees in the Pacific Southwest region (largely in California) had the greatest negative impacts on carbon uptake across the continent. In this case, the shift in atmospheric circulation resulted in conditions across the continent that were less favorable for photosynthesis. Loss of trees in this region resulted in large decreases in carbon uptake and increases in temperature in areas spread across North America. This result is particularly concerning considering that California is one area that has already experienced some significant drought-induced tree die-offs.

Impacts of massive die-off in Pacific Southwest forests (shown in yellow in first map) include changes in water uptake, atmospheric circulation, temperature, and carbon uptake (GPP) in other forested areas across the continent
Source: Swann et al., 2018 (CC BY 3.0)

While there is still a lot to be done to fully understand these forest-to-forest connections and the drivers behind these interactions, the results can help to inform some of the decisions we make about our forests and how to best conserve them. This study showed that rather than just the size of forest loss, the location of the forest can be even more important in determining what are the large-scale impacts of that loss. As interest grows in using forests for carbon sequestration, knowledge of these cross-continental connections can help target which areas may provide the most benefit and avoid which areas may pose the most risk.

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Jeannie Wilkening

Jeannie Wilkening

I am currently a PhD student in Environmental Engineering at UC Berkeley where my research focuses on ecohydrology, which means I look at interactions between ecosystems and the water cycle. Before coming to Berkeley, I did my undergraduate in Chemical Engineering at University of Arizona and an MPhil in Earth Sciences at University of Cambridge, where my research focused on biogeochemical cycling in salt marshes. When I'm not in the lab, I enjoy knitting, hiking, watching too much Netflix, and asking strangers if I can pet their dog. Twitter: @jvwilkening

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