We Can’t Plant Our Way Out of the Climate Crisis

Facing the realities of reforestation for carbon management

Climate change is a topic that confounds and increasingly divides us, yet one proposed solution—planting trees—enjoys widespread support across political affiliations and nationalities. And why not? At first glance, planting trees appears to be a win-win solution to the climate crisis as well as one that requires minimal sacrifice. The notion is undeniably appealing: We can plant trees instead of taking on the complex and difficult task of decarbonizing our economy. However, while trees play a vital role in our climate system, planting trees is not a panacea for climate change. There are significant misconceptions in the general public’s understanding of this issue, coupled with well-publicized tree planting initiatives that not only overpromise and underdeliver, but are inherently flawed, despite the best of intentions.

Planting trees to mitigate climate change is a straightforward concept. Trees absorb and store vast amounts of carbon, with more carbon sequestered in trees and forest soils than is present in the atmosphere.¹ Since the Industrial Revolution and the corresponding rise in greenhouse gasses from human activities, forests have captured 30 percent of the additional carbon emissions.² If we could increase the amount of human-generated emissions sequestered by forests, it would significantly lower the amount of carbon in the atmosphere. One way to seemingly achieve this goal is to increase the number of trees on Earth by planting more.

Planting a Forest

Humans have been planting trees for food and timber for thousands of years, but the idea of widespread reforestation is relatively recent. The concept of planting trees to restore forests gained momentum during the Industrial Revolution as rapid deforestation occurred, accompanied by an increasing awareness of, and concern about, the loss of trees. But reforestation efforts have met with varying degrees of success. Just as a city is more than a collection of people, a forest is more than a collection of trees. Forests are complex ecosystems that include bacteria, fungi, invertebrates, grasses, wildflowers, vertebrates, and of course, trees. However, not just any tree will do for any forest. The species of the trees are crucial, as individual species have co-evolved with other native organisms in specific locales. Genetics also play a significant role. Trees native to a particular area are adapted to local conditions. For instance, aspen trees planted in the upper Roaring Fork Watershed and sourced from lower elevation nurseries often can be easily identified in spring because they leaf out several weeks earlier than the native trees.

For a reforestation effort to succeed, a range of factors must be considered. The process involves studying the species composition of nearby intact forests, collecting seeds from mature trees that grow in similar environments, germinating these seeds in a nursery, and then planting them. However, even with meticulous planning and execution, success is not guaranteed. Mother Nature may not cooperate. Some years might be too dry, leading to the death of seedlings, while other years might be too wet, causing seedlings to be washed away. Additionally, herbivory from animals or insects can prove fatal for newly planted seedlings. Whether due to unpredictable forces of nature, bad luck, or inadequate planting practices, research has found that 50 percent of planted seedlings die within the first five years.³

Clearly, reforestation is not the blindingly simple carbon solution that is often portrayed. When executed properly, reforestation is complex and challenging, in part because its goal is to restore a forest ecosystem so it can provide the services only an intact natural system can, which is very different from the goal of sequestering carbon for a few hundred years. Still, many difficult endeavors are viable climate solutions. To determine whether tree planting is an effective tool for climate mitigation, we need to assess its potential to cool the planet and evaluate the efficacy of trees as a carbon sink.

Do trees cool the planet?

Trees undoubtedly sequester carbon. Roughly half of a tree’s biomass is carbon absorbed from the atmosphere. However, there are concerns that increasing forest cover in certain areas could actually contribute to global warming. Studies have shown that expanding tree cover in regions that are snow-covered for a significant part of the year can lead to net planetary warming rather than cooling.⁴

To grasp how trees might warm the planet, it’s essential to understand the concept of albedo, a measurement of how much light an object reflects. High albedo surfaces, like snow, reflect more light, whereas low albedo surfaces, such as dark forests, reflect very little. Planting trees in snowy areas replaces highly reflective surfaces with darker, less reflective ones, leading to more solar energy being absorbed by the Earth’s surface. This effect is primarily in conifer trees, which keep needles through the winter. Deciduous trees like aspen shed their leaves, which allows the snow on the ground under them to reflect sunlight. Additionally, trees only sequester carbon while they are photosynthesizing or growing. In colder regions, trees grow more slowly; even coniferous trees enter a state of dormancy in winter, resulting in less carbon being sequestered.

A 2021 study by Williams et al.,⁵ explored the combined effects of carbon storage and changes in albedo, finding that increasing forest cover in the Rocky Mountains and the Intermountain

West would lead to net planetary warming. In contrast, increasing forest cover east of the Mississippi River and on the West Coast of the U.S. would likely result in net planetary cooling.

Permanence

A critical question in all efforts to remove carbon from the atmosphere is the longevity of the carbon storage. Carbon emitted from fossil fuel combustion can last for hundreds or even thousands of years. If carbon is removed from the atmosphere today only to be re-emitted 5, 10, or even 50 years from now, it is not an effective climate mitigation strategy. This issue is particularly complex when it comes to trees. Trees can live for incredibly long periods, with many species used in reforestation projects having lifespans greater than 300 years. However, the carbon stored in trees is quite vulnerable. Forests can be cut down, or trees can succumb to insects and disease outbreaks. Natural disasters such as wildfires, droughts, wind events, or flooding can also destroy forests. When trees die in these events, much of the stored carbon is released back into the atmosphere relatively quickly. Furthermore, climate change increases the likelihood of these disturbances, with higher temperatures making events like wildfires and insect outbreaks more probable.

This increase in disturbance affects the carbon stored in forests significantly. William Anderegg, Ph.D.’s research (see “The Future of Colorado’s Forests: A Conversation” in this report) focuses on the future of forests and other ecosystems in a changing climate.⁶ His findings suggest that many current predictions about the carbon sequestration potential of forests do not fully account for the latest science on the permanence of carbon storage, indicating that our estimates may be overly optimistic. Predicting how climate change will amplify these disturbances is extremely challenging. Dr. Anderegg’s lab has modeled the impact of increased disturbances on forest carbon storage in the U.S., finding that by the end of the 21st century, forests will likely contain the same amount of carbon as today. In other words, any carbon gained from additional growth or regrowth will be cancelled out by increased disturbance.

Restoring forests by planting trees that are more resilient to the disturbances likely to occur in their area due to climate change may be a way to strengthen those ecosystems. Yet again, this approach represents a different goal and a different process than planting trees to mitigate carbon.

Space, Scale, and Time

Tree planting as a climate mitigation solution also suffers from certain issues of feasibility. First, there is the issue of space. Planting one trillion trees, as some suggest is the best way to address the climate crisis,⁷ would take between one and two billion hectares of land area. That is equivalent to the area of Canada and the U.S. combined. While some scientists believe there is sufficient land not currently used for agriculture or urban areas, many argue that much of this space consists of grasslands and savannas, which are important native ecosystems in their own rights.

Second, the process of tree planting struggles with scalability. Collecting seeds from mature, seed-producing trees involves manual labor, as does germinating seeds and planting seedlings—tasks that require significant human effort and do not easily benefit from economies of scale. Although several startups are experimenting with planting seeds using drones, the effectiveness of these efforts remains limited; a recent study reported a seed survival rate of between zero and twenty percent.⁸ Low survivability rates are particularly problematic as our seed supply is limited and labor intensive to replenish. The problem of scalability comes into focus if we look at One Tree Planted, one of the larger nonprofit global tree planting organizations. In 2022, the organization planted 52 million trees, which by most metrics is a lot of trees. Unfortunately, at that rate, to reach one trillion trees would take approximately 19,000 years (assuming they all survive).

Finally, there’s the factor of time. Planting enough trees could eventually sequester significant amounts of carbon but eventually implies a very long-term perspective, and climate change is a problem that needs to be addressed now. Research by scientists from MIT’s Climate Interactive showed that planting a trillion trees over the next decade would reduce global temperatures by only 0.15°C by the year 2100, which would have only a minor impact on expected warming. This small impact is because it takes time for trees to mature to the point where they can sequester significant amounts of carbon.⁹

Seeing the Forest for the Trees

There are good reasons to plant trees, but carbon mitigation alone is not one of them. The end goal matters more than the short-term effort. If the end goal is to reduce the amount of carbon in the atmosphere, then tree planting does not make sense. In fact, tree planting projects solely focused on sequestering carbon can have negative environmental impacts. Between 2021 and 2023 the number of plantation projects listed on voluntary carbon markets doubled in both number and size.¹⁰ Tree plantations may look like natural forests from the outside, but they are as much a forest as a wheat field is a grassland. Both are necessary for human civilization, but we need to be honest with ourselves as to what they provide. They provide resources, not habitat or functioning ecosystems.

However, if we measure success by the area of restored native ecosystems, or by the number of species brought back from the threat of extinction, then tree planting can be an important part of the puzzle. Over the past 20 years, the environmental movement has become increasingly focused on addressing climate change. Science is clear that climate change, unmitigated, is an existential threat to humans and much of life on Earth. But climate change isn’t the only environmental threat that needs addressing. Between 2010 and 2020, Earth has lost 47 million hectares of forests (almost twice the area of Colorado).¹¹ Similar statistics can be found for other ecosystems. Globally, habitat loss to make way for agriculture, development, and resource extraction continues at a blistering pace. As a result, we are seeing record numbers of species going extinct. By some estimates, species are going extinct at over 1,000 times the expected rate.¹² These environmental threats require us to take many more and different actions than simply measuring carbon.

Planting trees to slow habitat loss and restore ecosystems can help prevent extinctions. The right species of tree, restored in the right place and with the right method, can help stabilize soils and retain water by protecting the snowpack. If we have these larger goals in mind when considering tree planting, rather than focusing on the number of trees planted to sequester carbon in the short term, we have the potential to successfully address some of our greatest environmental challenges. We might even sequester some carbon along the way.

If our goal is to reduce global warming, planting trees in the Roaring Fork Watershed isn’t a good use of our limited time and money. Trees native to the Roaring Fork Watershed are generally slow growing. At the top of the watershed, the long winters and short summers prevent fast growth. At the bottom of the watershed, relatively arid conditions result in slow growth. As the Roaring Fork Watershed is snow covered for much of the year, changes in albedo from increasing forest cover will potentially cause net warming, or minimal cooling at best, if the trees planted are aspen.

But trees aren’t just carbon, and there are other important reasons besides global warming that might warrant planting trees. Forests are the largest ecosystem in the Roaring Fork Watershed and much of our native wildlife depend on them for survival. Reforestation to restore ecosystems can be incredibly beneficial. On Independence Pass, for example, decades of tree planting has helped restore native forests and stabilize soils. We can’t consider tree planting to be a silver bullet for carbon sequestration. Yet, tree planting in our watershed may help us achieve habitat restoration, native species regeneration, and water conservation if it is done carefully and done well.

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