We often say there’s no greater tool to remove CO2 out of the atmosphere than a tree. But how does this actually work? How do trees capture CO2 and where do they store it? How do we know the amount captured and what can we do to optimize this? Our Head of Research and Development, Harrie Lövenstein, explains it all.
First of all, Harrie: Can you explain how trees actually capture CO2?
Similar to humans breathing air through their mouths, tree leaves are equipped with tiny, microscopical mouths (stomata) taking in CO2. When green leaves become energized by solar radiation, CO2 (carbon-dioxide) is synthesized into high energy sugars (carbo-hydrates) in a process also known as photosynthesis. During this process oxygen (O2) is formed as well, which is ‘exhaled’ through the stomata.
The formed sugars are subsequently converted into building blocks like proteins and fibers to form new leaves increasing photosynthetic capacity, to construct a stem exposing leaves toward the sun, and to form roots anchoring the tree in the soil to maintain an upright position for optimal light interception.
Additionally, sugars help roots take up water and soil nutrients, which are transported to the leaves to support photosynthesis. And when the right conditions are met, mature trees direct sugars for flowering, and after successful pollination, they seed formation as well.
Deciduous trees shed their leaves when ambient conditions become unfavorable to growth, such as lower temperatures in winter or drought, which helps to curb maintenance costs. The shed leaves become litter in the soil and thus contribute to the soil organic matter, storing carbon in the soil. The same holds true for roots, which die off. In addition, sugars transported to the roots partially leak to the benefit of soil microorganisms, which enables the root systems to capture nutrients.
“All these processes over the years add up to a stable below-ground carbon storage.”
When does a tree start to take CO2 out of the air?
The carbon capture starts as soon as a tree has leaves that are able to intercept sunlight. This capture can be graphically visualized by an s-curve. The growth starts quickly, as every new leaf contributes to CO2-capture, as long as they are fully exposed to sunlight. Once upper leaves start casting shade, lower leaves intercept less light, resulting in lower photosynthesis. Additionally, as trees grow bigger, they may mutually cast shade on each other, especially at higher planting densities, resulting in canopy closure. Forest floors in pine woods can be quite dark.
As the foliage continues to expand and photosynthesis is lagging behind, the balance also shifts because the increasing canopy requires more and more sugars for their maintenance, resulting in diminishing net growth. These factors will cause the curve to level off.
Does CO2 sequestration only depend on leaf area and sunlight?
Just like you and me, trees need both water and nutrients to stay alive. And in parallel to people’s performance degrading when it’s too hot or cold, the same goes for trees. If it gets drier, the growth will fall back. This has to do with the stomata that can be found on the leaves. As they are open as the sun is out to take in CO2, they inevitably lose water by transpiration. The upside is that this cooling effect protects leaves from overheating, which hampers photosynthetic activity.
If this loss of water is replenished by the water that is withdrawn from the soil by the roots, everything is in balance. A serious lack of rain however, can decrease soil water content to a point at which stomata close, comparable with sealing our lips preventing us from breathing (when our nose is pinched as well). Consequently, leaves stop sequestering CO2 from the air until moisture content of the rooted profile is restored.
Stomatal closure enables trees to adapt to drier periods, whereas leaves may be shed during prolonged periods of drought. We can actually trace back these periods in tree rings, which are visible when studying stem cores or stem discs.
Trees also require nutrients, like nitrogen, phosphate and potassium which are incorporated in a.o. proteins as part of the tree’s tissue. Trees grown in degraded land may thus experience nutrient deficiency, which can limit growth. Poor growth, however, may also result from surplus soil sodium, also known as salinity.
Trees are sometimes subjected to biotic stresses which reduce growth. Just like humans, plants may become sick, with viruses and fungi slowing down growth. Also weeds, competing with trees for sunlight, water and nutrients, can reduce growth. Weeding is an example to protect young trees during their establishment.
How do you measure the CO2 capture?
The more CO2 is captured, the heavier the tree.
Another option to measure CO2 capture is real-time forest monitoring. Air analyzing equipment in the field that continuously gauges the absorbed CO2 and transpired water vapor provides insights on the vigor and health of a planted forest.
What do you do at Land Life Company to optimize the CO2 sequestration of a tree?
In the many geographies where Land Life Company is active, we relate the performance of different planting compositions to local soil and climate conditions to help us develop accurate tree growth models.
Taking into consideration the inevitable climate change in the coming years, these models help us to analyze the performance of different planting compositions under different future climate scenarios in terms of CO2 sequestration as well as longer-term resilience.
We always take a holistic approach, looking at the complete system above and under the ground. Apart from capturing CO2, we focus on other ecosystem services as well, like nature restoration, i.e. habitat restoration for endangered fauna, as well as local job creation. This way, our positive contribution to climate and nature will last for decades to come.