Tree planting is regarded as an important part of climate change mitigation strategy, but it must be remembered that choosing the wrong species and the wrong planting sites can produce the opposite effect to the one hoped for. The priority objectives are to improve the condition of natural forests through active management, and to protect old natural forests.
The proposal to plant billions of trees over the coming years as a climate change mitigation measure occupies a prominent place in public debate, and it is included in European strategy (the EU Biodiversity Strategy for 2030). Tree planting should always be seen as a positive action, yet when we move from slogans to the analysis of specific problems, the international forestry literature highlights a number of objections (limitations) and trade-offs that must be taken into account. This document looks at some of the most significant issues that need to be addressed if afforestation is to maximise environmental benefits.
Do trees always have a positive effect on the climate?
The importance of trees is mainly linked to photosynthesis – the conversion of atmospheric carbon into organic carbon and its accumulation in plant tissues and in the soil. However, other mechanisms also exist, including water transpiration, a process that absorbs heat and thereby lowers air temperature. Photosynthesis and transpiration are limited in places where the availability of environmental resources (water and nutrients) is low, which reduces the efficiency of these processes. Other mechanisms have the opposite effect. Trees with dark foliage (Mediterranean conifers and evergreens) have a lower capacity to reflect light (albedo) and absorb more energy (and heat) than open ground, which creates a warming effect. It has been calculated that, globally, albedo reduces the climate effectiveness of trees by 20% (Hasler et al. 2024). Finally, many species emit volatile organic compounds (VOCs), which in atmospheric chemistry contribute to the formation of greenhouse gases.
Is tree planting an effective strategy against global warming?
By creating new forests and protecting existing ones, only a part of the surplus CO₂ can be removed and stored (Holl and Brancalion 2020). Planting trees across, for example, 900 million hectares (Bastin et al. 2019) could store 205 Gt of carbon over 100 years, which is roughly one third of the carbon emissions produced to date. Veldman (Veldman, 2019) considers these calculations too optimistic and proposes a more conservative estimate, suggesting that 900 million hectares could remove between 40 and 100 Gt of carbon by the time the trees mature. That amount, while considerable, would correspond to only 10 years' worth of emissions at the current rate. The calculations put forward do not always take into account the physical and ecological constraints that prevent or discourage the planting of new forests and slow tree growth, so the carbon storage potential is often overestimated (Green et al. 2022).
Are some species more effective at capturing carbon than others?
Fast-growing species such as poplars deliver higher rates of carbon sequestration, at least in the early stages of growth. That is not the only factor to consider, because the persistence of organic carbon over the long term matters just as much. Carbon can be retained in the form of forest products (in plantations and productive forests) or in various components of the ecosystem (in natural forests and old-growth forests). Ensuring the durability of carbon stocks over the long term requires the forest to be fully suited to the ecological characteristics of the site where it stands, not only at present but also in the decades to come, when it will face the changes projected by climate forecasts (Wessely et al. 2024).
Is all land suitable for afforestation?
Recent estimates indicate that there are between 9 and 23 million km² of potential forest land worldwide (Bastin et al. 2019), most of it in intertropical regions that were previously covered by forest. Between 141 and 322 million ha lie close to urban settlements (Francini et al. 2014). In temperate regions, including Italy, forests are expanding spontaneously. Large parts of the land are occupied by non-forest ecosystems such as scrubland, grassland, tundra, heathland and so on, which have an intrinsic value linked to the biodiversity they contain and the carbon immobilised in their soils. In such situations, planting trees not only threatens the biodiversity and ecosystem services these areas already provide, but also releases carbon through soil disturbance. Moreover, trees, which are far more demanding than grasses and shrubs, can alter water and nutrient cycles. Avoiding tree planting in unfavourable conditions and instead directing efforts towards increasing carbon stocks in existing forests (Mo et al. 2024), as well as restoring forests that have been degraded or destroyed by disturbances caused by climate change, is therefore a more effective and more urgent course of action than establishing new plantations.
What are the merits of natural forests and old-growth forests?
Planting trees to mitigate the effects of climate change
Natural forests can store carbon over long periods and provide many other biodiversity-related ecosystem services. Thanks to their meristematic tissues, trees grow continuously and without limit, renewing juvenile structures each year as they regenerate their vegetative organs (Pasques and Munné-Bosch 2023). Even at a very great age they retain the capacity to grow and to accumulate carbon (Gilhen-Baker et al. 2022). In this context, old-growth forests, which retain high biodiversity and store significant carbon stocks in both biotic and abiotic components, can play a very important part in mitigating climate change (Curtis and Gough 2018).
What about forest management?
Reconciling active forest management with the need to preserve and increase current carbon stocks is a complex problem. There are many variables to consider in balancing the overall effectiveness of interventions: the growth stimulated by thinning (Zhang et al. 2024), the carbon removed during intermediate cuttings, the carbon stored in wood products, the soil respiration caused by reducing canopy cover, the fossil fuel used for the interventions and so on. The impact of management is site-specific and hard to quantify.
Conclusions
Planting millions and billions of trees, or planting them everywhere, are easily understood and inspiring slogans, but if applied unwisely the result may cause greater harm (increased warming, reduced biodiversity and ecosystem services) than the hypothetical benefits. Planting is no substitute for policies to cut emissions and decarbonise.
Measures must be carefully planned and carried out, taking into account what (Di Sacco et al. (2021)) call the “10 golden rules”:
– protect and improve existing forests;
– involve local communities in forest management and conservation;
– maximise biodiversity in natural forests in order to achieve varied and differentiated objectives;
– choose suitable areas for each intervention;
– use natural regeneration wherever possible;
– select species suited to afforestation in order to promote diversity;
– use resilient planting material with appropriate genetic diversity and local provenance;
– plan adequate infrastructure for seed supply and seedling production;
– learn by doing (take experience into account);
– ensure the economic sustainability of the project.
We may conclude (along with Brancalion and Holl (2020)) that tree planting, combined with other strategies for increasing forest area in suitable places, can make a valuable contribution to the ecological and social well-being of our planet in the decades ahead, but only if these efforts are seen as one element among broad and multifaceted solutions to complex environmental problems, and if they are carefully planned, implemented and monitored over a sufficiently long period, involving stakeholders and taking socio-ecological complexity into account.