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[ENG] Is tree-planting good for climate and ecosystems?

Recent press articles are questioning the idea that planting trees is a good way to fight climate change and biodiversity loss. They are often using very specific science papers from which they draw general conclusions, so we felt important to clarify a few things around that topic. 

First, in climate science, there is no doubt and a large consensus exists within the IPCC’s scientific community that forests and natural carbon sinks have a key role to play in climate change mitigation. Natural carbon sinks are indeed clearly identified in the last IPCC 1.5°C report, as a way to balance unavoidable carbon emissions in the long term.

This does not mean that it has the capacity to solve the climate crisis, and everybody recognizes that decarbonization of our economy is the main challenge we face if we don’t want to see massive disasters and species extinctions occurring in the next 30 years. However, decarbonizing our economy involves lots of individual behaviour and social changes that cannot happen very fast in liberal and democratic societies. Decarbonization of our economy is a bit like fighting an addiction (to fossil fuels): it must start now but will necessarily be a progressive process that will last for decades. In that transitioning period, it is necessary to use all means to reduce and sequester carbon, and only trees can proactively sequester carbon, while other carbon mitigation methodologies only reduce emissions.

The recent trend to align National (Paris Agreement) and Corporate (SBTi – Net Zero) climate strategies with science is a good thing, and many actors participating in that movement are considering as a priority natural climate solutions such as reforestation, forest conservation or regenerative agriculture, in order to increase carbon sinks globally.

IPCC’s 1.5°C scenarios: Agriculture, Forestry and other Land Use strategies (in brown) are required as a solution to limit global warming under 1.5°C in all scenarios.

How is tree planting a relevant solution to mitigate climate change?

As it grows, a tree captures CO2 from the atmosphere and transforms it into biomass through the process of photosynthesis. Carbon is then stored in the tree trunks and branches (aboveground biomass), but also in the roots (around to 30% of aboveground biomass). This way, a hectare of tree plantation has the capacity to sequester 11tCO2 to 45tCO2 per year (Source: PUR Projet calculation using assumptions from “2019 Refinement to the 2006 IPCC Guidelines for National Greenhouse Gas Inventories”) depending on tree species planted and climatic conditions. In tropical humid climates, carbon sequestration tends to be higher than in cold and/or dry climates. 

Moreover, carbon not only is stored in the tree-biomass but also in soils:  at a global level, there is 3 times more carbon stored in the first meter of soil than the total amount of carbon stored in biomass. Trees being in constant interaction with soils, tree planting activities can impact the soil carbon sequestration in different ways: i. generating a flow of organic matter to the soil (when branches, leaves or fruits are falling on the ground) ii. enhancing incorporation and decomposition process thanks to tree roots that oxygenate and de-compact the soil*. Interestingly, as opposed to tree biomass, soil carbon sequestration related to tree planting happens to have more potential in boreal and temperate forests than tropical landscapes.

Finally, tree plantation can also accelerate the decarbonisation of the economy: a good example of it is the potential substitution of cement by timber products in the construction sector. Sustainably managed tree plantation (if 1 tree cut is compensated by at least 1 tree planted), has the potential to produce timber products with limited emissions. Knowing that the production of one ton of cement emits around 1tCO2, shifting from cement to timber has the potential to significantly reduce the carbon intensity of our buildings.

Enhancing carbon sequestration through tree-planting 

Increasing natural carbon sinks through tree-planting activities is not an easy task either and cannot be industrialized at large scales, like the recent article of ETH-Zurich/Crowtherlab suggested. Working with nature and ecosystems cannot be standardized and requires a deep understanding of the context and landscape before acting. Of course, converting massive grasslands (that have been there for thousands of years) into an artificial mono-species forest is not good for the planet. Neither is restoring part of the deforested Amazon rainforest with non-native tree species. By trying to cut corners to save time and money without integrating the project into local ecosystems and communities is a huge mistake that can generate tremendous negative side effects on biodiversity, communities and even annihilate expected climate impacts. So YES to natural climate solutions, but not at any price! Pricing carbon at 5-6 euros per ton will never allow the development of qualitative and impactful reforestation projects. That is why at PP we advocate for a higher price for carbon.

Over the past 10 years, PUR Projet has developed a deeper understanding of how to restore ecosystems and activate natural carbon sinks while participating in local development and improving livelihoods. When designing such projects, focus is put in priority on: i. a deep understanding of the landscape ii. a careful selection of tree species iii. creating the conditions for the trees to grow well, iv securing the permanence of trees over the long term.

FIND THE RIGHT PLACE 

We plant trees only to restore degraded lands. We mainly develop agroforestry projects in regions and landscapes where agriculture has caused deforestation, and slowly degraded fertility generating simultaneously more incentive for the farmers to go find new parcels and deforest more. By reintroducing trees in agricultural parcels, we activate all kinds of ecosystem services such as soil regeneration, regulation of temperature and humidity, biodiversity regeneration, plus all the tree products that bring revenue diversification and improves the livelihood of farmers. Not to mention the deforestation avoided by sustaining decent yields of the farmers’ parcel over the long term.

We are also considering soil carbon impacts that tree-planting might have. As mentioned previously, even if planting trees generally has a positive impact on soil carbon, it is not always such a straightforward statement, especially in afforestation projects. If land use and practices before our plantation were already making the soil rich in carbon in the first place, planting trees is then not so incremental to the enrichment of carbon in the soil. In very specific cases like afforestation in desertic areas, the Soil Carbon stock can even decrease (as shown in Northern China in a recent study). In agroforestry, such negative side effects are very unlikely to occur, but we have identified potential risks when shifting a parcel from a pastoral system (where a lot of organic matter is added to soil naturally from cattle) to silvo-arable systems (Agroforestry with crops). For that reason, we tend to avoid such land-use change in our designs and focus tree-planting on parcels where crops are cultivated, often with intensive and soil-degrading practices. This way we make sure impacts on soil carbon are positive or in the worst case neutral.  

FIND THE RIGHT SPECIES

In forestry projects, and more specifically in Agroforestry, it is key to select carefully what tree species are to be planted. Successful agroforestry requires tree species: i. adapted to local climate conditions ii. that can participate to the local ecosystem equilibrium iii. that match with agronomic needs of the parcels iv. that maximize ecosystem services delivered to the farmer and communities living there. Our project design includes a large diversity of tree species to be able to deliver all these impacts. Over the past years and various projects, we have launched over the world, over 600 species have been planted, with an average of 22 species per project, always prioritizing native species to make sure ecosystems won’t be damaged and participating in the conservation of local biodiversity.

Trees are a natural habitat for many animal species. Promoting diversity of trees also contributes to bringing back biodiversity on the farm and the parcels we operate and thus enhance many more ecosystem services. We know that moving from monoculture to complex and diverse shaded systems can increase the population of pollinators on a farm (for example in Mexican coffee farms pollinators species diversity was increased by 25%**). Increasing the number of pollinators and their diversity in agricultural landscapes can help establish a better-balanced ecosystem and plays a key role on crop yields: it was estimated that differences of yields from one farm to another can be explained by 20 to 30% from the presence of pollinators.

In the same way, appropriate selection of tree species can bring alternative tree-products (like fruits or wood) to the farmer and communities. This contributes to improving livelihoods, diversifying incomes and making farmers more resilient during droughts and periods of food insecurity. 

Ecosystems services that a tree can bring are numerous (we have identified 46), and the main challenge to design an impactful reforestation project is to activate them as much as possible.

Planting trees is not the end-goal: making sure that they grow and live long is.

How many large scale tree-planting projects have failed due to poor design and lack of a long-term vision? Large-scale planting projects have flourished in recent years. In India, Ethiopia or Turkey, millions of trees are planted in very short periods, but after planting, what actions are taken to make sure the trees will survive? Nearly none, unfortunately… What is the point of planting trees if 90% of them die within the next 12 months?

The first years after planting are critical because trees do not have a significant root system yet, which makes them very vulnerable to droughts or diseases. When trees are small they are also very vulnerable to animals who might come and eat their leaves and cause irreparable damages. 

At PUR Projet, we have developed strategies engaging the farmers we work with to take care of the trees after planting and make sure they grow properly. We train them on when and where to plant in order to maximize survival, on how to recognize disease or lack of water, identify if fertilizer is needed, as well as provide them with materials to protect seedlings against animals. But most of the work consists of regular visits to the farmers to keep them involved, answer their questions, reminding them of and showing them all the benefits of trees. Farmers we work with are essential to tree survival, taking good care of them, listening to their needs and answering to them day by day. They are the reason why today we can proudly disclose a survival rate of over 75%. 

In the long term, we also ensure that this sequestration is permanent. “Non-permanence”, is a well-known issue in the forestry carbon sector, and covered by all main carbon quantification methodologies (UNFCCC, VCS and Gold Standard). Non-permanence is the term used to qualify the risks that the carbon removals achieved by tree biomass at a given time may be reversed in the future from natural or artificial causes. Main examples of non-permanence risks are: i. fires ii. pests & diseases iii. forest cuts

From a climate change perspective, fires are the worst case of non-permanence as the carbon stored in trees is immediately and integrally released in the atmosphere. When trees die from pests or diseases, non-permanence is progressive and partial as biomass is slowly decomposed on the ground: part of it can be stored in soils but a significant part of it is released in the atmosphere during the decomposition process. Finally, when trees are cut down for wood, non-permanence can be either immediate if the wood is used as energy supply, or delayed for decades if the wood is used as timber (furniture, buildings, etc.). In both cases, carbon stored in the wood will end up in the atmosphere.

It is the responsibility of organizations that claim to develop programs enhancing carbon sinks with forest and tree planting to make sure they will remain in the long term. Non-permanence risks described above must be analyzed carefully and mitigation activities implemented over the long term. At PUR Projet, we do such analysis considering periods of up to 80 years and put strong safeguards and mitigation plans around non-permanence risks, notably:

  • Our carbon calculations always include a discount of 10 to 20%, to cover the risk of non-permanence. Discounted units are mutualized in a sort of “assurance fund” that guarantees that even if a dramatic event occurs in a project, carbon removal claims will remain true. 
  • Our project budgets include long-term provisions reserved to finance non-permanence mitigation activities during the entire duration of the project. 
  • Our projects are designed with and for the local communities, who ultimately became the guardians of the non-permanence challenge. For example, our projects’ design focuses a lot on:
    • Revenue diversification (fruits, beekeeping, etc.): decent livelihoods are a condition to reduce the pressure on forests and natural ecosystems at a landscape level. 
    • Empowering communities in tree-nursery management in order to develop their capacity to replant trees if non-permanence issues occur in the long term. 

 

Aurélien joined PUR Projet in 2018, with a strong experience in carbon certification, impact measurement, and MRV process. As a senior manager, he works on the development of innovative tools and solutions to support projects monitoring and to maximize impacts of PUR Projet activities. 

 

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* Trees also tend to increase general soil health by reducing erosion: 1. tree foliage protects soil from erosion caused by raindrops and soil projection caused when drops hit the ground 2. tree roots increase water absorption potential of soils which reduces surface water flows in case of heavy rains that wash surface soils and take away all nutrient and organic content to rivers and oceans 3. trees are a natural windbreak and reduce erosion effect of wind

** See “Pollinator diversity increases fruit production in Mexican coffee plantations: The importance of rustic management systems” Carlos H. Vergara *, Ernesto I. Badano 2008 

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