Photosynthesis and Carbon Capture

Photosynthesis and

Carbon Capture

Plants are a natural carbon capture technology created by millenia of evolution. Using light, air, water, photosynthesising plants capture carbon dioxide from the air and convert it into sugars. These sugars are used to build the roots, leaves, stems and wood that make up the bodies of plants. Trees, which have a lot of woody tissue, store large amounts of carbon, which is why we use timber for domestic fires. It’s also why trees burn in bushfires.

The waste products of photosynthesis are water vapour and oxygen. In the 4.6 billion years that plants have been photosynthesising, they have transformed the planet by increasing oxygen levels in the atmosphere and creating the ozone layer. Photosynthesis is the innovation that has made life possible for all living organisms by capturing atmospheric carbon and turning it into food and oxygen. On this planet, plants are life.

Carbon dioxide and water use – the trade-off

To capture CO2, air has to enter the leaves of a plant. Air passes into the leaf through tiny pores in their leaves called stomata. This is a Catch-22 for the plants though, because when their stomata are open, plants also lose water vapour from their leaves, which passes out far more easily than air enters. This means that to grow, plants lose water - and that means that photosynthesis is not very water efficient. On average, for every gram of carbon stored, a kilo of water passes through the plant, though plant species do differ in their water-use efficiency. While plants might like to grow without losing water, the water movement through plants is vital to the water cycle, replenishing the atmosphere with water vapour that falls later as rain. Plant cycling of water, known as transpiration, provides 99% of all evaporation on land, and is critical to humidifying the air and producing rain. The remaining portion coming from evaporation of water in exposed soils, rivers, dams and lakes. The total amount of water released to the atmosphere by plants and evaporation is called evapotranspiration.

Tree rings are a palpable way of understanding the link between carbon capture and water availability in trees. In deciduous trees in temperate climates, tree grow through spring and summer, and lose their leaves in autumn, so the growth rings there represent a year’s growth. In Australia, where the majority of species are evergreen, tree rings are usually associated with optimal wet conditions when photosynthesis rates go up and trees put on growth. So, while growth rings here are not necessarily annual, they still represent periods of increased or decreased growth. In dry seasons, or during heatwaves, when water is scarce or quickly lost, carbon capture is low, resulting in narrow growth rings. In wet, mild conditions, growth rings are wide. The amount a tree can grow depends largely on how much water it has.

Stressed and weakened trees are more susceptible to disease and limb break. When plant tissues decompose, it releases carbon back into the atmosphere, becoming a source of CO2 rather than a sink. However, if wood stays dry, timber is a material that can store carbon for hundreds of years.

Water stress slows tree growth

Plant growth is tightly tied to water availability. When water is scarce, the stomata seal shut, stopping transpiration. Function shifts from transporting water through the plant to the atmosphere to preserving water within the plant. Without top-up rainfall or sub-soil water sources, soils become dry and plant growth declines. In extreme dry events, plants under water stress cannot meet their water requirements resulting progressively in leaf, stem and branch loss and, if drought continues long enough, the death of the whole plant. Drought stops plant growth and can kill if it continues for long enough.

Soils and Carbon Capture

Soil Organic Matter (SOM)

Soil moisture is critical to plant health and growth. But soils are in fact a huge contributor to carbon storage in their own right. Soils high in organic matter can store the highest amounts of water and nutrients. Undisturbed soils usually have high organic matter and good soil structure. Permanently damp soils in wetlands have very high rates of carbon storage. Soils with higher moisture can store more carbon and cool, moist soils store the highest amounts of carbon.

As soils dry, are ploughed or over-exploited, the organic matter in them is destroyed. Soils become dusty and compacted, with poor structure and are unable hold as much water. Extreme bushfires can remove carbon from the soil, sometimes stripping soils to bare rock. This damage removes a resource that can take hundreds of years or longer to replace, leaving skeletal soils with poor water-holding capacity. Extremely hot fires destroy soil carbon and emit large amounts of CO2 to the atmosphere.

Maintaining soil health will preserve soil moisture and tree health, bringing resilience to stressors like drought and heatwaves, and increase carbon storage sinks in the soil and tree layer.

Soil organic matter is created by several pathways including:

Mycorrhizal associations: the ‘Wood Wide Web’

Mycorrhizae are specialised symbiotic fungi that grow on plant roots. Plants supply mycorrhizae with carbon directly from their roots to help the fungi grow, thereby storing carbon in form of fungi - and mycorrhizae supply plants with nutrients that they extract from the soil. Mycorrhizal fungi can connect the root systems of different individuals and even different species, allowing nutient and water sharing, and even assisting in passing stress event messages (such as drought, or insect attack) between trees. The network of root-like hyphae fungi also produces glomalin, a sticky protein that acts as a kind of glue. Glomalin improves the structure of soil and soil moisture retention. It is glomalin that gives soils rich in organic matter their sticky/greasy quality.

Importantly, the health of one species relies on the presence of other species. Planting a diverse range of trees and shrubs helps plants to be resilient – and benefits the fungi that store carbon in the soil, improving soil organic matter and water-holding capacity.

Fruiting bodies of fungi emerging in mild, wet conditions are evidence of a good below-ground fungal network.

Mycelium of a fungus decomposing wood to extract carbon