Climate crisis: what can trees really do for us?
By the power of sunlight, forests turn huge amounts of carbon in the air into food: sugars for themselves and leaves, bark and roots that feed animals and microbes. Respiration, which happens in the cells of all living things in the forest, releases energy from that food and carbon dioxide (CO₂) back into the air.
As the amount of carbon in the atmosphere rises, this eat-and-be-eaten cycle increases to keep up. Metabolically, trees are running just to stand still. In the course of all this cycling, forests are locking up the major part of the 33% of human-caused emissions removed from the atmosphere into the land each year.
I (Rob) work in a forest full of beautiful 175-year-old oaks. Global CO₂ levels were around 280 parts per million (ppm) when these trees were seedlings. Now global atmospheric concentrations exceed 415ppm and are rising rapidly. Should these oaks reach 200 years old (not old for an oak), they will be surrounded by air containing around 550ppm of CO₂. Can the world’s mature forests stand these changing conditions and continue to offset some of our emissions from burning fossil fuels?
Carbon current accounts
To find out, my colleagues and I at the University of Birmingham’s Institute of Forest Research use a free-air CO₂ enrichment facility. Imagine a dinosaur-free Jurassic park with 102, 25 metre-tall towers treating forest patches with CO₂-enriched air that replicates the mid-century atmosphere: 565ppm – 150ppm above present levels. Then we measure everything we can: the width of tree stems, the size, weight, and chemical make-up of leaves, the branching architecture of the roots and much, much more. In this way, we record changes in the forest’s manufacturing of stuff, and in its health.
Our first results are in. In the canopy, photosynthesis rates are up to a third higher on sunny summer days in the CO₂-rich patches. Over a growing season, the increase is about a fifth. These are big numbers: imagine if your annual income went up by a fifth. Photosynthesis is the forest’s carbon income.
Since we began this experiment in 2017, the forest patches exposed to higher CO₂ appear healthy and productive. That may seem unsurprising. After all, plants love CO₂ so much that farmers add it to greenhouses to supercharge fruit and veg growth.