What is Earth’s tree carrying capacity and how might it mitigate climate change?
Author: Samantha E Andres
September’s 2019 global climate strike was a huge success. Millions of people gathered together in cities all over the world to advocate for change. This strike caused a significant uproar among global communications, industry, and political relations, but an imminent question still remains: what’s next? Climate change is a global crisis that is multifaceted, and deeply interwoven with so many aspects of our daily lives, many of which are hard to avoid. But what if I told you it was as simple as planting trees?
The building blocks of life
Let’s begin with Carbon. This is one of the most essential elements for life on Earth. It forms the building blocks for sugars, DNA, proteins, and fats that sustain life. It is also involved in the maintenance of our atmosphere. When Carbon binds to Oxygen, it forms Carbon dioxide (CO2), which is responsible for holding the energy our planet receives from the sun in a manner that it prevents it’s escape back into space. In the face of our current climate crisis, CO2 tends to get a bad rep, but it is actually more necessary than we think. Without the emission of these greenhouse gases, our Earth would be frozen solid, and lifeless, especially for plants. However, it is when these emissions begin to rise too high that global temperatures, storm frequency and severity, sea levels and ocean acidification begin to rise. For example, as CO2 is absorbed from the atmosphere into the ocean, it interacts with Calcium (Ca), decreasing the concentration of Carbonate ions (H2CO3) and increasing the concentration of Hydrogen ions, thereby decreasing the pH (making the solution more acidic) of the ocean and subsequently leading to the acidification of the ocean. This process affects many marine ecosystems, with direct impacts observed in shelled organisms and corals as this acidification leads to decreased capabilities to produce and maintain shell structures. The downstream effects of this process result in disruptions in food webs, and decreased productivity for fisheries. However, the dominant impact from rising CO2 is warmer temperatures, which has many downstream effects for terrestrial ecosystems. Presently, CO2 emission levels are far higher than historical evidence suggests. But just how much longer can this continue?
Figure 1: Illustration of the carbon cycle Photo by: UCAR (Center for Science and Education
Reducing atmospheric CO2 with forests
Carbon sequestration is the process by which CO2 is captured from the atmosphere and held somewhere in the earth in either a solid or liquid form. This is where trees come in. Forests are by far one of the greatest carbon sinks, in which CO2 is an essential component of their growth system (e.g., photosynthesis). They trap CO2, keep it out of the atmosphere and oceans for many years in the form of wood, leaves and roots, and form the basis for ecosystems, and biogeochemical cycles.
By putting more effort into reforestation, a recent study suggests that trees have the capacity to store up to 205 gigatonnes or 2.05×1011 tons of carbon. To put that into perspective, that is the equivalent of forty-one billion elephants.
The random forest
Estimating suitable habitat for reforestation on a global scale was not an easy task. These scientists used a combination of satellite derived images, and photo interpretation measurements, paired with global environmental software to add layers of soil, climate, population, and topographic variables (characteristics of terrain) to build a model to essentially estimate Earth’s present “tree carrying capacity”. In order to achieve this, the researchers produced a random forest machine learning model. While this is a fitting name given that we are talking about trees, it is also a statistical technique. Random forest is essentially a model built using a subset of artificial intelligence that constructs a series of random decision trees based on the data it is given. These decision trees are the foundation of the random forest, which can then be used to understand patterns and trends in the data.
Figure 2. The global potential tree cover available for restoration (0.9 billion hectares of canopy cover) Figure 2.C from Bastin et al. 2019.
The Future
“This places ecosystem restoration as the most effective solution at our disposal to mitigate climate change.” (Bastin et al. 2019)
The random forest model also proved capable of predicting future trends in losses of canopy cover across different land management regimes. These results are novel in that they highlight the opportunities for climate change mitigation through global reforestation initiatives, but also the urgent need for action. They estimate that if left under the current climate and land management trajectory, potential tree canopy cover may shrink by ~223 million hectares by 2050, with a large majority of loss occurring in tropical ecosystems.
This research comes at an unique time in forestry and conservation, as past projects associated with the remediation of deforested land have focused primarily on planting species that provide direct “ecosystem services” to people such as timber production. However, our changing world is in the midst of a major paradigm shift, with less economic weight being put on the forest resources and more being put towards the benefits associated with efforts to conserve them. Such a change has the potential to restore an equivalent of 25% of the current atmospheric carbon pool, which could reduce a substantial amount of the present global anthropogenic carbon burden. Although these effects will not be immediate, as forests need time to grow and mature, this study points out the long-term ecosystem benefits of increasing global reforestation efforts.
Reviewed by: Luiza Aparecido 
