This article was first published in Castanet Feb 20, 2024
Image courtesy of Sophie L Wilkinson, McMaster University: Wilkinson et al., 2018; Global Lakes and Wetlands Database.
Trees are climate change miracles. They take in carbon and give up oxygen. The region around them benefits from their shade and the tree puts moisture into the air. A mature tree can absorb 20-30 kilograms of carbon annually. However, trees make lousy carbon credits.
Let’s begin with age: a tree starts as a seedling, a tiny plant. That seedling captures almost no carbon. It takes ten years (depending on the species) for a tree to become a carbon absorbing machine. When you invest in a tree-related carbon credit you are essentially saying “I will emit carbon today but I promise to make up for it ten years from now”.
A song by Malvina Reynolds goes “And if you think you'll love me for a long, long time, plant an apple tree.” The very length of trees’ lives make it hard to safeguard them. You can plant a tree today -- who is going to safeguard it over the next 100 years? Trees can be lost to forest fire (releasing all the carbon they absorb), they can be lost to development and they can be lost due to disease, such as the pine bark beetle.
We also aren’t very good at creating healthy ecosystems. Planting projects have chosen trees that are wrong for the region, which then became invasive species. The Fort McMurry fire in 2016 was made worse because planted trees lowered the water table, drying out the peat bog, making the peat highly flammable. The effort to replace hardwoods with sustainable bamboo has resulted in forest loss (ironically hardwood forests are cut down to plant bamboo), and ecosystem loss from monoculture (planting nothing but bamboo). We’ve certainly learned from these mistakes but they highlight how hard it is to get it right.
One of the biggest issues with tree planting is the tug of war between food crops and tree plantations. In Brazil, which contains 60% of the Amazonian rain forest, trees are cut down to create land for agriculture and areas for grazing livestock. Before we shake a finger, we need to acknowledge the settlers aren’t getting filthy rich destroying the rainforest -- they are fighting for survival. In other regions replanting projects have purchased agricultural land outright in countries with uncertain food production. In some cases Indigenous roles in determining land use have been trampled in the interests of replanting.
Planting trees is not a worthless endeavor: while they aren’t a high quality carbon offset they can play an important role in climate adaptation. The climate is getting hotter and unusual weather patterns such as heat domes are bad around the world. The 2021 BC summer heat dome was responsible for between 600 to 700 excess deaths. These high temperatures are bad across Canada but worse in the cities. Because cities are primarily made of concrete and asphalt they trap heat. Planting trees (“urban greening”) can go a long way towards mitigating heat waves.
On a super-local scale if you own your house you might take a look at your southern exposure -- would your house benefit from a shade tree? If you pick a deciduous tree you can have it both ways -- in the summer the leaves will produce shade and in the winter you will still get the light and heat from sunny days. A pergola on the south face can be designed to let in winter light and limit summer heat.
Tree planting is less powerful globally -- rich countries planting trees in developing countries -- than it is locally. Support urban forestry in your location. And if you are in the market for carbon offsets, look for projects that are measurable and permanent.
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