Carbon Sequestration
Plants have the potential to save the planet through the process of photosynthesis removing carbon dioxide from the atmosphere, reversing the build-up from burning fossil fuels which is causing climate heating. Perennial woody plants such as trees and shrubs store the carbon long term, especially if they are part of permanent ecosystems where soil disturbance is minimised, so that carbon is also stored long term in soils. The planet saving potential of trees and soils is becoming widely recognised, with tree planting and rewilding schemes being promoted for carbon offsetting.
Plants to Save the Planet
Plants For A Future (PFAF) started the project we now call ‘Plants to Save the Planet’ (PSP) three years ago. We realised that the plants information and search facilities we provide could be part of a global movement to shift public focus from issues concerning people and animals onto recognising the power and importance of plants. Since we started PSP we have extended our database of 8000 useful plants, adding around 700 species suitable for tropical situations. We have also introduced search terms, ‘glossary items’, to enable designers to identify plants which are most suitable for designs with the following aims: Carbon Sequestration, Carbon Farming and Food Forests.
Carbon Sequestration in Plants and Soil
Understanding the carbon cycle on our planet is crucial to combat our
climate crisis. In our gardens, and on our farms, we can make choices that
increase the rate of carbon sequestration over land. By increasing carbon
sequestration in plants and soil, we can work to regain balance in our
planet's carbon cycle. Carbon-conscious gardening and carbon farming are
crucial tools in climate change mitigation.
Carbon is present in the atmosphere, in the ground, in oceans and in
living organic materials. It is exchanged between these different
reservoirs through a wide range of natural processes. Without human
interference, the natural flow of carbon would keep levels reasonably
stable, and the carbon cycle would remain in harmonious balance. According
to the Salk Institute, every year plants and other photosynthetic life
capture 746 gigatons of CO2 and then release 727 gigatons of CO2 back.
Unfortunately, human activity has dramatically increased the amounts of
carbon (approximately 37 gigatons) that is released into the atmosphere
annually. This imbalance in the carbon cycle is one of the main
contributing factors to the greenhouse effect and global warming.
We can help to redress the balance by:
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Choosing the right plants – to sequester as much carbon as possible, for
as long as possible.
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Caring for the soil to boost its carbon capture capability.
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Protecting and restoring existing carbon sink ecosystems around the
globe.
In this article, we'll delve into this topic in more depth, to example,
what exactly gardeners and growers can do, and what we can all do to make
a difference.
Trees: Natures Most Efficient Carbon Sinks
Most people are aware that planting trees is a good thing in climate
change mitigation efforts. Trees take carbon from the air through the
process of photosynthesis and store it as lignin in their trunks, roots
and branches. This means that trees are able to turn CO2, from the
atmosphere, into a very stable form of elemental carbon.
Determining how much carbon an individual tree can sequester each year,
and over its lifetime, is a complex business. Carbon sequestration rates
are influenced by the species of tree, its size and age, temperatures and
many other environmental factors. Even the best figures are only
estimations, and it is difficult to get accurate figures. These figures
are always extremely variable and will be accurate only for the very
specific area where they were taken.
Generally speaking; however, all trees have an astonishing capacity to
store carbon while in growth. Research has shown that all tree species
absorb CO2 from planting to old age (200 years plus). However, they reach
their peak in terms of carbon sequestration in their 'teenage' years (from
10-45 years after planting).
https://www.farmcarbontoolkit.org.uk/toolkit/sequestration-farm-biomass
Trees are particularly useful for carbon sequestration due to their size,
large root structures, and because they are perennial, and will not die
back each year. But it is important to remember that the benefits will be
lost when trees are chopped down, and the CO2 will return to the
atmosphere if the material is then burned.
So trees that are planted for carbon sequestration purposes must remain in
place. Permanency in plantations is a crucial component of climate
mitigation.
The Best Trees For Carbon Sequestration
Which trees will be best for carbon sequestration will definitely depend
on where exactly you live, and the options best suited to your situation
and climate. The best options to look for are those that grow quickly and
live for a long time. Of course, trees generally don't have both of these
attributes at the same time.
Carbon capture schemes often favour younger trees, which grow more
quickly. But slower growing trees can sequester much more carbon over
their considerably longer lives. The best strategy, therefore, from an
environmental standpoint, is generally to plant a number of different tree
species, to maximize shorter and longer-term sequestration.
Different studies have identified different trees that are especially good
at sequestering carbon in different bioregions. For example, a 2001 study
identified horse-chestnut, pines, oaks, plane trees, sweetgums, and bald
cypress as good choices for urban trees in the US.
https://www.nrs.fs.fed.us/pubs/jrnl/2002/ne_2002_nowak_002.pdf
Method for Calculating Carbon Sequestration by Trees in Urban and Suburban
Settings can be found on the EPA.gov website.
https://www3.epa.gov/climatechange/Downloads/method-calculating-carbon-sequestration-trees-urban-and-suburban-settings.pdf
Setting and environmental conditions are very important. But so too are
human considerations. The best trees to plant will often be those which
serve other benefits to humanity. Quite frankly, this limits the
possibility that these trees will be chopped down. Fruit and nut trees are
therefore often good choices since they can serve multiple functions at
the same time.
Forest or Woodland Management For Carbon Sequestration
The key thing to remember when planting trees for carbon sequestration is
that afforestation is about more than just trees. While trees are
important for carbon sequestration, they are only one part of the complex
and balanced ecosystems that help to maintain natural cycles on Earth.
There is a danger that we do not see the forest for the trees. A forest is
a complex web of life, each element of which is required for maximum
carbon sequestration.
Careful management of forest or woodland can result in systems that are
far more than just tree plantations. As well as increasing biodiversity, a
true forest or woodland system can have layers of planting and complex
systems which increase the amount of carbon sequestration overall.
Traditional orchards are often laid out with grass sward beneath the
trees. The grass ground cover protects the soil while trees absorb carbon.
However, while this cover can be beneficial, a mixed ground cover with
nitrogen-fixing plants and guilds of beneficial planting in layers around
the trees will increase carbon sequestration overall and create a more
dynamic and stable ecosystem. Orchard or woodland management that
incorporates carbon trapping trees with layers of other beneficial plants
is usually referred to as forest gardening, or, on the larger scale,
agroforestry.
In woodland and forest systems managed by humans for human benefit can
also include coppicing. Coppicing can be used to maintain systems which
sequester plenty of carbon whilst also allowing humans to harvest timber
of various uses. In such systems, less land must be given over to trees
for the same carbon benefit.
Carbon Sequestration in Bamboo
Trees are not the only plants of interest when discussing carbon
sequestration. Bamboo is another plant which is believed to have
impressive carbon capture potential. Bamboo has received a lot of
attention in sustainability circles in recent years.
Giant, woody bamboos share characteristics with trees and have impressive
carbon sequestering capabilities. Again, there is a range of factors that
can influence bamboo's rates of carbon sequestration, and notes of caution
have been raised about putting too much trust in the results of individual
studies.
But living bamboo has been shown to store a similar amount of carbon to
tree plantations: from around 100 to 400 tonnes of carbon per hectare. And
like wood, it has the capacity to replace other carbon costly products
and, when managed and used effectively, can keep carbon from the
atmosphere for a significant length of time.
https://www.inbar.int/understanding-bamboos-climate-change-potential/
Shrubs and Hedges
Often overlooked in favour of larger carbon-capturing trees, shrubs and
hedgerows can also play an important role in increasing the carbon capture
capabilities of a piece of land. As permanent, perennial features of an
agricultural landscape, or a garden, shrubs and hedges are vital to carbon
conscious land management. The wider and higher a hedge, the more carbon
it will generally sequester. But careful design and management of mixed
hedgerow systems can also help to improve overall carbon sequestration.
Deep Rooted Perennial Plants
All plants will help is sequestering carbon to some degree. But those with
deep tap roots will store that carbon more effective within the soil.
Other perennial plants, especially those with deep, thick roots,
therefore, are also effective in land management for carbon sequestration.
Since those deep, woody roots will generally remain in place for quite
some time, they should help keep that carbon locked up below the soil for
years to come.
Carbon Sequestration in Annual Crops
Annual crops typically have lower root biomass than perennials because
they do not need to store energy in the same way. On average, plants
allocate 76% of carbon stocks to shoots and only 24% to the roots.
However, this can vary considerably due to different climates and
environmental conditions.
https://www.fcrn.org.uk/research-library/what-crop-type-atmospheric-carbon-sequestration-results-global-data-analysis
What happens next to the biomass generated will determine how much carbon
is sequestered in annual arable agriculture or a fruit and vegetable
garden.
Interestingly, scientists are looking into how plants can be engineered
for larger roots that will store more CO2 and remain and gradually break
down to deposit carbon back in the soil. Getting these plants into the
global agricultural food chain could potentially contribute a 20 to 46 per
cent reduction in excess CO2emissions annually.
https://www.wired.com/story/the-plan-to-grab-the-worlds-carbon-with-supercharged-plants/
Short term, there is a lot that farmers and gardeners can do to increase
carbon sequestration when growing annual crops. First of all, it is a good
idea to incorporate perennial plants into annual food production, through
forest gardening or agroforestry, polycultures (mixed planting schemes
rather than mono-crop planting). And by maintaining crop cover throughout
the year through the use of cover crops and successional planting.
Carbon Sequestration in Soil
So far, we've looked at carbon sequestration in plants. But it is
important to remember that soils hold four times the amount of carbon
stored in the atmosphere, and more than is held in vegetation.
https://doi.org/10.3390%2Fsoilsystems2040064
Modification of agricultural and gardening practices is a recognized
method of carbon sequestration. Soil can act as an effective carbon sink,
offsetting a significant proportion of carbon dioxide emissions annually.
But how well it sequesters carbon depends on how land is used. Land used
for annual cropping is depleted in soil organic carbon, as the carbon-rich
biomass of the crops is harvested and removed. Tilling and cultivation
also reduce the carbon stored, releasing CO2 into the atmosphere.
Carbon Farming and Gardening Practices – Increasing the Soil's Capacity to
Store Carbon
Farmers and gardeners can do a lot, not only to maximize carbon
sequestration in plants but also in the soil.
Measures to increase soil carbon include:
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Chopping and incorporating 'green manures'.
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Adding compost/ manures (brown organic matter).
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Incorporating biochar (black organic matter) into the soil.
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Boosting microbial/ fungal life in the soil and protecting the fragile
soil web.
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Effective crop management, crop rotation and diverse planting schemes.
They can also do a lot to minimize losses of carbon from the soil. Losses
are minimized by:
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Minimal cultivation/ cultivation at shallower depths ('no till'/ 'no
dig' practices).
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Careful use of farm machinery, and reduction of foot and vehicular
traffic over the soil.
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Avoidance of overstocking of livestock and careful livestock management.
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Management of soil to avoid erosion and compaction at all times.
As well as through careful and appropriate planting.
The Importance of Ecosystem Protection and Restoration
So far, we've focussed on carbon sequestration in plants and soil in
human-managed/ cultivated systems. But of course, carbon sequestration in
wilder and more natural environments is also hugely important.
Reforestation, ecosystem restoration and tree planting will definitely
play a role in tackling our climate crisis. Both protecting existing
forests and restoring degraded wooded ecosystems is crucial to carbon
sequestration. Though not a panacea, reforestation and afforestation will
play an important role in combatting the climate crisis.
Ecosystem conservation is not only about forests and woodlands, however.
Wetlands store 14.5% of the world's soil carbon, and yet they cover only
6% of its total land area. Effective protection and restoration of
wetlands, therefore, is crucial to climate crisis mitigation and
adaptation.
https://www.sciencedirect.com/science/article/abs/pii/S0925857417303658
Carbon sequestration is a complex business. The exact levels of
sequestration and the best strategies for combatting our climate crisis
are under debate. But what is clear is that we need to work with nature,
rather than fighting it, if humanity is to have a sustainable and ethical
future on this planet. And understanding the carbon cycle and our role in
throwing it out of balance – and bringing it back into balance in future –
is a good place to start.