Carbon capture heroes: Why we urgently need more trees
Amidst all the noise surrounding capturing carbon, can planting trees really make a difference?
In recent years, planting trees has increasingly been acknowledged globally as an essential, cost-effective undertaking in the fight against climate change. Leaders in many countries are attempting to utilise their function as carbon sinks in efforts towards reaching net zero, but often without any clear plan or research into whether this is even possible. Despite this, it is widely acknowledged by environmental researchers that forests can and will play an integral role in efforts to capture carbon; trees mitigate and provide adaptation to the worst effects of climate change - primarily due to their large biomass that can moderate atmospheric pollution whilst also regulating temperature and water. This was emphasised in the IPCC’s 2022 report findings that reported how we can halve global emissions by 2030.
In turn, fresh impetus has been put on tree planting campaigns as part of carbon-offsetting schemes, from governments and companies down to individuals - all hoping to adjust their ways of operating and living - towards a greener and more sustainable future. However, the reality of tree planting is often more complex, with a risk of box-ticking or greenwashing; organisations (from conservation groups down to industrial timber merchants) who have committed to tree planting as part of these efforts can potentially - and in some cases, deliberately - mislead supporters or donors. For example, large companies (such as Shell) have pledged huge tree planting schemes to offset emissions; however, an analysis by Oxfam found that Shell would need to plant trees totalling up to an area the size of Italy to offset just 35% of their emissions.
The ability of trees to capture carbon can take many years to initiate - much slower than the potential for carbon release and biological damage (e.g., through felling or fires), and it can be incorrectly assumed that the sequestration process is instantaneous (and permanent) after planting. Therefore, it is important to consider several key things - including forest growth trajectory, the emissions generated from planting, the re-release of carbon if trees perish, what sequestration can be confirmed, and the unpredictability of future climate conditions.
In addition, planting on historically unforested land can have an adverse effect if not chosen prudently (as emphasised in the 2022 IPCC Report), with a further challenge of maintaining the multiple and varied other ecosystem services forests provide. Ultimately, through resilient new planting projects, we can both slow global climate change, and - crucially - maximise our ability to adapt to it.
Planting in the UK vs equator
Tropical forest lands have the greatest potential for sequestering the largest quantities of carbon. By the 1970s, the total area of tropical forests globally was 1,628 × 106 hectares, with a total storage of organic carbon amounting to 393.50 × 109 tonnes; although these figures are not 100% reliable, it remains that tropical forests play an enormous role to sequester carbon, totalling at just under half of the world's living terrestrial carbon pool. As has been witnessed by ecologists over decades, any form of destruction or removal of forests (such as clearing for agriculture) leads to significant increases in carbon dioxide in our atmosphere.
However, one study in 2020 published results of an investigation that tracked 300,000 trees over 30 years, concluding that the ability of tropical forests to absorb carbon from the atmosphere was lessening, driven by carbon losses from trees dying. This study emphasised that tropical forests, although still vital, have a limit to how much carbon they can absorb, and must be protected. In addition, tropical climate change hotspots (such as south Asia) face enormous political and land use pressures (i.e., rapid population growth, food security issues, and climate unpredictability). Despite these barriers and limitations, the tropics have undeniable value to the world for a range of ecosystem services.
Regardless, as the world becomes more conscious of the limited time we have left to mitigate the worst effects of global climate change, in the UK we are looking closer to home. As such, there are concerted efforts in temperate climates to plant more trees, and en masse.
The potential for adaptive forests in the UK
Historically, Great Britain has struggled to regain the thick forest cover that once covered the island; thousands of years of clearing land and subsequent cultivation caused many of its forests to vanish. In 1919, forests in the United Kingdom were at their lowest recorded coverage, around 5 per cent of land area. The creation and progression of the Forestry Commission up until the present day has (admittedly slowly) turned these lows around, with economic and strategic objectives that developed into economic, political, sociological, and scientific concerns. However, tree cover is still one of the lowest in Europe, and in 2020, the Woodland Trust recommended a significant increase (19%) in woodland if the UK is to be carbon neutral by 2025.
“In light of global climate change that is already beginning to occur, preparing for an uncertain climatic future is now paramount.”
Thus far, there is observable evidence of changes in the composition, structure, and functioning of forests as a result of climate change, through extreme weather events such as drought and heat stress. In the face of a changing – yet unpredictable – future, there are compelling arguments for the management of temperate forests as adaptable ecosystems that are prepared for uncertainty. If temperatures continue to warm as predicted (based on changes so far), the UK must be prepared for increased threats to our forests (e.g., drought, fires, pests, and diseases).
One eventuality that many predict - but may provide an advantage - is that trees will grow faster and capture more carbon in a warming climate with increased Co2 levels (for example, this study in 2017). This potentially means more trees will thrive and sequester carbon more rapidly in the UK. Preparation for the unknown is vital but complex, so many tree species that hold carbon quickly could be crucial. This could be achieved by planting trees that do well already (as well as trees that are expected to, by observing their traits and projecting how they might cope with different climate situations. Watch this space for more info on this topic!). 9Trees have been preparing for this scenario by projecting different outcomes for different sets of species. Table 1 further below in this post details a range of ‘native’ and ‘non-native’ tree species that we believe can build resilience into the UK ecosystem. For example, the silver birch is a native tree that research has found to be a species that has acclimation mechanisms making it adaptable and resilient to various future climate change outcomes. These include its rapid growth rate, relatively high herbivore resistance, ability to adapt to different climates, and recolonisation traits.
Contrastingly, one example of a tree species that has seen a decline is the iconic English oak tree, due to increasing pests and diseases. If oak trees suffer, we must be prepared with tree species that can be resilient to climate change and various other threats. This may include looking further afield for species that have a range of attributes that can prepare us better (see Table 1 for more info).
It is projected that many current tree mixtures in the UK will potentially be unsuitable in the future, with the ranges, mortality and growth rates of some species being altered. For example, many trees that historically have thrived in Great Britain may suffer from the worst effects of temperature increases, in particular drought stress - such as beech, small-leaved lime, and hornbeam. Therefore, reducing risk through species diversification in the UK may be key to adapting to potential future climate change.
More research is needed to assess which tree species will thrive in a warmer (currently temperate) climate; however, trees that are tough growers, resistant to drought and heat stress, pest, pathogens, storms, and flooding are examples of the attributes that will be favourable for resilience.
Prudent species choices for an adaptable future
How can we build more resilience into our forests?
It may be appropriate to evaluate the tree species that are currently thought to have an excellent future in the UK, summarised in Table 1. In their guide to species selection for the UK, Barcham Trees note that, regardless of their carbon credit score or potential for storage, the species evaluated here are all excellent environmental contributors, and that the storage figures could store up to three times the amounts shown here if planted in rural locations compared to urban ones.
Overall, native broadleaved trees are viewed by many as some of the most beneficial trees to plant to build more resilience into Britain’s ecosystems and mitigate some of the worst effects of climate change. For example, it is projected by some that increasing mixtures of native broadleaved species could help to reduce some detrimental local impacts caused by hot temperature extremes and improve carbon sequestration, whilst damage from increased pests and pathogens (due to warmer weather) could be reduced in severity using native mixtures.
Iconic native species contribute greatly - not only to carbon capture, but also to enhance biodiversity, wildlife habitats, and overall forest health. It is hoped planting more of these species will bolster resilience and prepare us for uncertainty. Oaks, for example, are projected to grow faster and thrive in a potentially warmer climate. Similarly, other iconic species such as hazel and rowan are both widespread with wide ranges throughout Europe, meaning they are adaptable to differing temperatures. Each species offers different attributes that will be advantageous when planted in mixtures; for example, alder is relatively tolerant of waterlogging;
But what about non-native trees? Technically, ‘native’ means anything that grew in Britain since the end of the last ice age. However, nature is much more fluid and changeable, and does not recognise borders (including oceans). If we take into account both the unpredictability of our future climate and the adaptability of trees, non-native trees could be extremely valuable to prepare us for uncertainty.
“A range of species with different attributes can prepare the UK with an arsenal of woodland, carefully selected to match each site, that can bolster our natural defences against a range of disturbances.”
The ‘assisted migration’ of species (or, improving numbers of ‘non-native’ trees) is seen by many as a method of improving resilience in mixtures of trees for future scenarios (for example, in this research by the Canadian Council of Forest Ministers). However, in the UK, many of these ‘non-natives’ have already begun to be introduced. For example, the genus Prunus originated in warm climates (tropical and subtropical in Asia, Africa, South America and Australia), but is also successful in temperate climates, making it a potentially ideal candidate for planting in the UK. This is particularly true for the German (or European) plum (Prunus domestica Hauszwetsche) - which has thrived in continental Europe, and stores large amounts of carbon.
9Trees is working on several agroforestry projects in Scotland, check them out! Agroforestry project - 9Trees CIC / Agroforestry Project - annual subscription — 9Trees CIC
Trees that grow fruit have good potential for agroforestry (where agriculture interacts with trees, including the agricultural use of trees). Agroforestry in the UK has been historically somewhat limited, but there is a growing recognition of its potential for climate resilience and mitigation, amongst many other benefits. The Malus genus (apple; Malus elstar and Malus Jonagold) are trees with significant potential as carbon sinks, a process that is enhanced by a reduction in water availability, making them more adaptable to warmer, drought-afflicted conditions. Of the species in Table 1, pear trees (Pyrus spp.) capture the least carbon; this may be largely due to their reliance on water availability during specific months of the year in the sequestration process. Despite this, they are adaptable to dry or moist warm climates and have some tolerance to wind.
Prunus domestica Hauszwetsche. Source: NC State University
Following Table 1, certain species of Acer have the highest potential for carbon sequestration, as hardy trees that survive multiple pressures. For example, the Acer campestre Queen Elizabeth thrives in rich, well-drained soils, but is adaptable to any soil type, and tolerates drought, soil compaction and air pollution. Box elder (Acer negundo) is a fast and tough grower and is tolerant to pollution, soil compaction, and flooding.
Acer campestre Queen Elizabeth. Source: Tree Sales
In conclusion, a range of species with different attributes can prepare the UK with an arsenal of woodland, carefully selected to match each site, that can bolster our natural defences against a range of disturbances.
Table 1: Summarising different species and their potential for carbon storage. Source: own table based on species information by Barcham Trees.
Summary
Environmental researchers and policymakers largely agree that in light of global climate change that is already beginning to occur, preparing for an uncertain climatic future is now paramount. Improving adaptation, building resilience, and mitigating the worst climatic effects to the best of our abilities is considered the most prudent action required. As well as protecting forests in tropical regions, reforestation using the diverse tree species on available and appropriate land in temperate regions may be our best chance to bolster our ecological defences for non-linear futures.
Why does 1 tree = 1 tonne of Carbon in its life?
At 9Trees, we plant roughly 1000 trees per acre at 2.4m spacing, aiming for mixed broadleaved woodland. Overall, most research agrees that the amount of carbon that can be absorbed by trees depends on species choice, biomass (weight, height, crown area), location, and site conditions for each tree. Through years of planting new woodland in the UK, our research and experience suggest that the annual CO2 offsetting rate of the trees we plant varies from 21.77 kg CO2/tree per annum to 31.5 kg CO2/tree per annum.
To compensate for 1 tonne of CO2, 31 - 46 trees are needed, with on average around 1000 trees per acre currently in Europe. Using an assumed rate of 24 kg CO2/tree per annum, our calculation is as follows:
1000 trees x 24 kg CO2/tree = 24,000 kg of CO2 offsets, i.e. 24 tonnes of CO2/acre. This equals to around 971.65 tonnes per acre (Read more on these figures at Encon.eu)
However, if anyone wishes to help us do a deep dive into these facts (and even a PhD) we would love to prove ourselves right or wrong! We pride ourselves on our adaptability to (and support of) exciting and ever-evolving new research.
Realistically, around 30% of all trees planted will likely perish or be out-competed, and new self-seeded trees will fill the space left behind. Some will live for 40-60 years, others will reach 100-200, and some may live on for 1000+ years. The figures we use are all averages and cannot account for all the intricacies of the full life cycle of a tree - as well as the added biodiversity and microbial activity (including detritivores, added humus, and increase in Flora and Fauna).
Here at 9Trees, we believe it is necessary to simplify the research to be inclusive and enable everyone in the UK and Europe to be part of the drastic need for biodiverse woodlands. Currently, we want to make this available to many people in the UK, and we also look at the longer-term objective - creating functional ecosystems that store carbon both in the trees and soil. We don't just plant trees to sequester carbon - it is also about enhancing biodiversity, promoting mental well-being, and supporting local livelihoods.
In November 2022, a climate change-related analysis that used a range of UK official statistics concluded that forested land is a major carbon sink, removing more carbon from the atmosphere in 2020 than any other land type.
From 1 tree to 9Trees (and even 900 trees for a lifetime subscription to get us ahead of the game), every tree counts - every donation, each will or legacy left to us will be utilised in the most beneficial way for wildlife, wellbeing, and green jobs. We also realise there may be data used which in years to come may be found to be incorrect - and hopefully for the better. That is why we plant 12 trees when you pay for 9 and we replant for the first 3 years if we monitor any failures. Despite this, we want to ensure that trees planted now reach maturity, which is why we enter into management agreements for 50 years to ensure as many as possible survive.
Check out our ‘Stages of Tree Planting Blog’ or our 50 years of management blog and let us know if you want to get involved!
Planting trees with 9Trees CIC: The stages involved
50 years Woodland Management plan from 9Trees
By Neil Insh - Researcher