Creating a blueprint for a greener future: Why fighting climate change is not as simple as planting some trees

On a sunny, mild day last November, a group of volunteers planted 250 trees on a plot of land in the western edge of Hamilton. But at least one of those trees had particularly special significance.

“I lost my wife this summer,” explained Jim Quinn, a biology professor at McMaster University who volunteered with the tree planting. “They made a tulip tree available for me to plant in her memory. I’ve always liked tulip trees, and I have a tulip tree growing in my front yard planted in memory of my father. Tulip trees are pretty special to me.”

There’s just something about trees, isn’t there? Along with the obvious gifts they provide us  —  wood, fruit, shade…oxygen  —  research has shown trees can have some surprising benefits too, including improving our mental health, and even helping us heal faster. When you factor in the potential positive impacts on the environment, such as improving air quality, sequestering carbon, and helping clean water and soil, it seems like there’s simply no down side to planting trees. It’s no wonder governments around the world (including Canada, which has pledged to plant 2 billion trees over the next decade) are including tree-planting as part of their climate change battle plans. And trees’ positive impact on climate change is undeniable, but there are some serious caveats to that statement that might surprise you. It turns out reaping the benefits of our leafy friends takes more than sticking a sapling in the soil, and if certain factors are properly considered, planting trees isn’t always a net positive for climate change.

“If we want to expand tree cover, where will we have the most climate-positive effect?” said Christopher Williams, the director of environmental sciences at Clark University in Massachusetts.  “Unfortunately, we can’t just put them anywhere, because there are some places where an expansion of tree cover might actually have little or no benefit in terms of cooling the global climate system.”

Consider Quinn’s tulip tree. That particular species wasn’t chosen at random: the tulip tree is native to just a few places in Ontario, and the Niagara Peninsula happens to be one of them. The 250 trees were the first plantation of an innovative project from McMaster University’s Centre For Climate Change that’s the result of a wide breadth of research into what makes a forest an efficient climate-fighting tool. On just one hectare of land, the Centre will plant 1,000 native, climate-adapted trees to create a model of a carbon sink forest: a forest that stores more carbon than it releases. It will show that, with careful planning and expertise, we can build forests that provide the myriad benefits we know and are continuing to discover, while also making a meaningful impact on climate change.

Let’s take a step back, way back, probably to elementary school, where you first learned about trees’ role in the carbon cycle. You probably had it explained something like this: humans breathe in oxygen and breathe out carbon dioxide, while trees breathe in carbon dioxide, and breathe out oxygen. The ever-so-slightly more complex explanation is that tree leaves absorb carbon dioxide and, using water and energy from sunlight, convert it into the chemical compounds the tree needs to live, in a process known as photosynthesis. Considering human-generated carbon dioxide (from our cars and homes and factories, not our lungs) is the most prevalent greenhouse gas contributing to climate change, the carbon-eating power of trees sounds especially handy right now.

“Trees are good! And on a planet as a whole they have a net positive impact,” said Alex Guenther, an Earth system science professor at the University of California, Irvine.

But trees are also complex  —  much more complex than the basic “carbon in, oxygen out” concept we learned in Grade 3  —  and that means that when we’re prioritising their impacts on climate change, we have to consider a number of factors. Guenther, for example, studies volatile organic compounds, or VOCs, a type of gas that are emitted by trees, some of which can actually contribute to global warming.

“Trees have a vast difference in their capability of emitting these volatile organics, which do impact the short-term climate forces,” Guenther said, adding that the impact of these emissions also depends on the surrounding environment — even high-emitting trees have less of an impact in a clean, natural environment, compared to an environment that’s already suffering from air pollution.

Guenther said that in a natural environment, there is a mix of a few high-emitting trees and many low-emitting trees, so that the total impact of a forest is moderate. But in a plantation setting where you might be planting a lot of the same type of tree for harvesting, that can create a ton of added emissions.

“A lot of the plantation species — fast-growing, sun-loving trees — tend to be the high emitters. So if you only plant something at the high end, you have the potential to throw things out of balance.”

Guenther said the takeaway of this evolving research is not to stop planting trees, or even to stop planting trees that are high emitters of VOCs (those are an important part of their local ecosystems, he explained). Instead, he said the lesson is to consider all aspects of a tree’s impact on climate, both locally and globally, and be thoughtful about how we go about creating new plantations  —  which species we include, and where we place them.

Williams at Clark University studies the carbon dynamics of forests in North America, and in particular what happens when the makeup of those forests change  —  when they’re cut down, or burn in fires, are infested with destructive insects, or when new trees are planted.

“One thing the average person might not be aware of is that the presence or absence of forest strongly impacts how reflective the planet is,” Williams said. “As we expand tree cover, we tend to make the surface of the Earth darker and that causes the surface of the Earth to reflect less sunlight out to space. What may be counterintuitive to most people is that, at least for this effect —  albedo —  alone, the presence of a forest actually warms rather than cools the planet.”

Like Guenther, Williams stressed that this is not an argument against planting trees, but that it requires those who are planting trees with the aim of mitigating climate change to ask hard questions about where and how to go about it. Just planting any tree anywhere won’t necessarily achieve the goals we’re setting out to reach.

Both researchers also noted that forests provide a lot of benefits beyond their climate change-fighting capabilities, and with careful planning we can reap everything a forest has to offer.

“We need to be thinking about managing and valuing forests not just for their carbon sequestration benefits, but across the whole portfolio of benefits they provide,” Williams said.

There are a number of key elements to consider when designing a forest that has a net positive effect on climate change, while also benefiting the local environment, including humans. Species and location are two of the most important factors, according to Rob Keen, a professional forester and the CEO of Forests Ontario, a nonprofit that supports forest restoration, stewardship, awareness and education. When planting a new forest, or replanting in a managed forest, Keen said there’s a lot of modelling that goes into deciding which species and how many are planted. That’s because you want to have a good mix of native species that grow at different rates to ensure the sustainability of the forest long term. Sometimes that means planting different trees than may have been there. Some forests in Ontario looked different prior to European settlement, and there’s efforts to restore to the pre-settlement makeup of trees.

But there’s also a need to consider resiliency in the future, both against invasive species like the emerald ash borer (a small green insect that lays its eggs in ash trees, killing the tree from the inside out), and against the impacts of climate change. “There are going to be areas that are much wetter than they currently are and other areas that are going to be under severe drought,” Keen said. “Even just that moisture regime change will change the kind of species that are going to grow and survive in those conditions.”

Careful management of the forest in the long term also influences its climate change impacts, including harvesting trees, Keen said. You might think cutting down trees sounds counterintuitive, but clearing some trees helps encourage growth of others and keeps the forest healthy. And when it comes to carbon sequestration, turning trees into long-term use products, like telephone poles, allows the carbon they store to be locked away for even longer than if they were left in the forest. In the forest, a tree can be knocked down or die (or worse, catch fire) and all of that carbon is released through decomposition.

Alanna Bodo, a sessional lecturer in climatology at McMaster, said it’s a careful balance to find the “sweet spot” of harvesting that benefits a forest without harming it. Some of Bodo’s PhD research (which partly inspired the development of the carbon sink forest) looked at harvesting techniques at a research forest at Turkey Point, on the shore of Lake Erie. The forest is a former plantation  —  a forest of one species planted for high volume of timber production  —  and Bodo found that the right amount and style of harvesting not only benefited the forest’s health, but also helped in its transition to a more natural, diverse forest ecosystem.

“Thinning allows for the successional species to grow and develop so a plantation forest naturally gets converted into, here in Ontario, a Carolinian forest. It’s almost like restoring it back to its natural habitat,” Bodo said.

Effective thinning even promoted the carbon sequestration of the trees that remained, when compared to a control forest that was left untouched.

All of these factors  —  tree species, environment, and management  —  went into the careful planning of the carbon sink forest project. The forest was the brainchild of Altaf Arain, the director of the McMaster Centre For Climate Change, along with Wayne Terryberry, the university’s coordinator of outdoor recreation and natural lands, and Myles Sergeant, a physician, McMaster instructor, and the president of Trees For Hamilton, a local nonprofit. Inspired by the forest research model at Turkey Point, the three came up with the concept of building a forest that would demonstrate a protocol for carbon sink. The forest is located on private land that has been leased to the university (for the grand fee of $1) and will eventually be donated. Around a dozen different native, climate-adapted tree species were chosen, and are being planted gradually over the next year. They’re also being protected while the trees get established, including building an 8-foot fence around the forest to prevent it from becoming an all-you-can-eat buffet for local deer.

Once the forest is established, it will create a number of important opportunities for research and education, Arain explained.

“There are huge research questions we can answer with this forest,” Arain said. “The number one question we want to ask is how quickly will it be a net carbon sink? What’s the survival rate and the growth rate of the trees? Down the road, biologists can look at the impact of biodiversity. It will be like a learning lab for students.”

It will also serve as a model for how to build a carbon sink forest in Ontario: if private companies or organizations want to achieve this goal, they’ll be able to use McMaster’s forest as a blueprint that can be scaled up to any size, hectare by hectare. And along with the environmental benefits for the community  —  providing shade, and helping to clean the air and water  —  Arain said he wants the forest to be a place for public education and enrichment, when people can come to learn about the project and take in the soothing benefits of forest immersion.

That’s something that, as a family doctor, Sergeant was particularly enthusiastic about.

“There’s actually been studies that show planting trees decreases the risk of cardiovascular death in the areas that have more trees, improved immune function, and all of the mental health stuff I think people are aware of now. Stress, anxiety, anger, violence, mood all benefit from having trees,” Sergeant said. “I love my patients and I’m quite happy to put a stethoscope on someone’s chest and figure out what’s going on. But sometimes I look out my window and it’s super hot in a place I grew up in, and I know it’s not normal, and I think to myself ‘am I better off planting a tree?’”

With the right plans and research to figure out which tree, and where, he just might be.

With science communication as a strategic priority, McMaster University’s Faculty of Science is proud to support long-form science journalism. This is the fifth in a series of eight Long Reads. Read the previous four stories about the importance of developing teaching skills in future faculty, the physical and mental benefits of exercise, the link between music and the mind and a McMaster collaboration with NASA.