Actions where trees are a central feature to address environmental challenges have the potential to provide significant health benefits but community needs must be carefully considered, according to a new study.

The first review of its kind, published in The Lancet Planetary Health, found compelling evidence that tree-based interventions in low- and middle-income countries (LMICs) could improve agricultural yields, dietary diversity, household income, child growth, and self-reported wellbeing, while encouraging the sustainable use of forest resources.

Examples of conservation solutions involving trees could include tree intercropping (growing trees and crops together in the same area); planting native trees to improve soil fertility; payments for ecosystem services; establishing or strengthening protected areas; reducing deforestation; and restoration of mangrove ecosystems.

However, the findings of the review suggest that without careful consideration of how communities are impacted, some activities aimed at protecting or restoring the natural environment may compromise the wellbeing and livelihood of the implementing or surrounding communities. For example, people could lose income due to a reduction in timber production, although this loss could be offset by income from non-timber forest products.

The review also found potential benefits varied by the type of intervention. Solutions with a strong focus on livelihood improvement such as agroforestry (which involves adding trees to farms) had more positive impacts compared to solutions where the main objective is to reduce carbon emissions or conserve biodiversity, such as REDD+ (Reducing Emissions from Deforestation and forest Degradation) and protected areas.

The study was led by researchers from the Pathfinder Initiative research team at the London School of Hygiene & Tropical Medicine (LSHTM), in collaboration with experts from Muhimbili National Hospital and LEAD Foundation in Tanzania, and Conservation International in Kenya.

Nature-based solutions (NbS) aim to protect, sustainably manage, and restore natural and modified ecosystems, and their role in addressing environmental and societal challenges is increasingly being recognised. Trees are often a key component of NbS. Vital for supporting life on Earth, trees supply food and nutrition, provide natural resources for building and energy, can improve air and water quality, help mitigate climate change by storing and sequestering carbon, and buffer societies against climate impacts.

Understanding of the health and societal impacts of these solutions is currently limited. This is particularly important to inform policy and practice in LMICs, where agricultural production may be the main income source and there could be trade-offs from poorly implemented interventions.

The researchers identified over 23,400 studies, of which 372 underwent full-text review and 54 were included in the final analysis. The review covered diverse regions, with the largest number of studies from China (9 studies), followed by Kenya and Vietnam (5 studies each). The researchers pooled the results from multiple studies that reported the health and wellbeing outcomes of the tree-based interventions and assessed their effectiveness by reporting the effect sizes, which estimate the difference between implementing and non-implementing communities.

Dr Peninah Murage, Assistant Professor at LSHTM and lead author of the study, said: “Integrating health and wellbeing objectives alongside climate and ecosystem restoration goals is crucial to unlocking the full potential of nature. Our findings show that tree-based solutions could bring major benefits to implementing communities, but stronger evidence is needed to improve our understanding of the effects of interventions on health and wellbeing, both to maximise the benefits and minimise the risk of unintended negative impacts.

“This review corroborates the criticism of poorly implemented REDD+ solutions. Although there is growing interest in these solutions as a climate mitigation measure and a means of generating local and national revenue, commercialisation and privatisation of natural resources mean that the benefits are often not accessible to local communities. In some cases, these projects can deprive community access to forest resources or land for cultivation and threaten wellbeing and livelihoods.”

The authors noted several limitations, including the fact that the studies they reviewed varied in terms of context and evaluation methods. In addition, they highlighted gaps in available evidence relating to health-related outcomes such as heat adaptation, air and water quality, mental health, and waterborne diseases, as well as a lack of evidence showing impacts on illness, mortality, or life expectancy.

The study builds on ongoing work by the Pathfinder Initiative to synthesise evidence on climate and health, accelerate implementation and support with evaluation of actions that bring the greatest benefits to health and the environment. Case studies gathered as part of the first phase of the Pathfinder Initiative, such as a community-led intervention in Indonesia to address lack of healthcare access as a potential driver of illegal logging, and the Ntakata Mountains Forest Protection Programme in Tanzania, show that well-designed and implemented NbS that prioritise delivering benefits to local communities, can support health and livelihoods while contributing to climate mitigation.

Further research highlights the health benefits of work to integrate trees into agricultural landscapes in Tanzania, supported by the LEAD Foundation, which provides another example of the potential benefits of community-owned and locally-driven solutions.