BAS for Clean Air and Water

Trees, Water, and Health: An Evidence-Based Overview

This article synthesizes current scientific evidence on how trees and forests contribute to water resource protection and human health. It is organized in three parts: (1) ecosystem services of trees for water protection, (2) links between tree-based water protection and health, and (3) implications and recommendations for research, policy and practice.

1. Trees Protecting Water: Ecosystem Services

Trees and forest vegetation deliver multiple hydrological and water quality benefits across landscapes, whether in rural watersheds or urban settings.

Interception, runoff reduction and infiltration.Tree canopies intercept rainfall, reducing the velocity and volume of water reaching the soil surface. The forest floor—leaf litter, roots, organic layers—enhances infiltration and slows surface runoff, thereby reducing erosion and sediment transport into streams. US Forest Service+3Environmental Research Institute+3Trees Forever+3 For example, one source notes that forest soils can absorb nearly 10 inches of rain, compared with 4 inches in nearby lawns. Hoosier Environmental Council

Bank stabilization, sediment and nutrient filtration.Tree root systems stabilize streambanks and slopes, reducing erosion and sediment loads. Trees and associated vegetation trap nutrients (nitrogen, phosphorus), heavy metals, pesticides and other pollutants before they reach water bodies, thereby improving water quality. US Forest Service+1 A case study reported that reforesting marginal cropland reduced nitrogen, phosphorus and sediment transport compared to open fields. Bren School of Environment

Regulation of water flows and flood mitigation.By increasing infiltration and delaying surface runoff, trees reduce peak flows during storms and enhance baseflow during dry periods. This helps moderate flooding and recharge groundwater. US Forest Service+1 In urban contexts, tree cover and urban forests are used as nature-based solutions to stormwater management and flood risk reduction. World Resources Institute

Water supply and drinking-water benefits.Forested watersheds that serve as source areas for drinking water benefit from lower treatment costs and better raw water quality, due to lower sediment and pollutant loads. Hoosier Environmental Council+1 In the U.S., forest lands supply drinking water for over 180 million people. US Forest Service

Caveats and complexity.The hydrologic effects of trees are context-dependent. In dry regions, high tree density may reduce groundwater recharge by consuming more water than the infiltration benefit provides. US Forest Service+1 Thus, while trees generally protect water resources, the benefits must be assessed carefully in view of local climate, soils, vegetation type and land use.

2. From Water Protection to Human Health

The ecosystem services delivered by trees (water regulation, pollution filtering, flood mitigation) have direct and indirect benefits for human health, and additionally trees provide other health‐related services.

Reduced exposure to waterborne risks.Cleaner raw water and reduced sediment loads translate into fewer pollutants entering drinking-water supplies and recreational water bodies, thereby reducing risks of waterborne disease and exposures to chemical contaminants. For example, forests reduce treatment costs by providing filtering services. Hoosier Environmental Council

Mitigation of flood and drought risks.Flooding and drought events can have severe health consequences (injury, displacement, contamination). By moderating hydrologic extremes, tree cover can reduce such health risks. World Resources Institute+1

Improved built-environment and urban health outcomes.Urban tree cover supports stormwater management and heat mitigation (see below), which in turn influence cardiovascular health, respiratory conditions and mental health. For instance, a scoping review found 201 studies linking urban trees to human health outcomes under three categories: (a) reducing harm (e.g., pollution, UV, heat), (b) restoring capacities (stress recovery, mental health), and (c) building capacities (physical activity, social cohesion). The Nature and Health Alliance

Direct health benefits of trees and forests.Beyond water services, the presence of trees and forests is associated with numerous health benefits: lower blood pressure, reduced stress, improved immune function, better birth outcomes, lower cardiovascular and respiratory morbidity. The Harvard T.H. Chan School of Public Health summarizes this growing evidence. Harvard Public Health For example, exposure to forest environments can affect physiological markers (blood pressure, heart rate variability) and immune cell activity. MDPI

Synergistic pathways: water, environment, human health.The health benefits of tree-mediated water protection interact with other tree-ecosystem services: by reducing pollution and flooding, improving microclimate (cooling, shading), and enhancing green space, trees contribute to healthier communities. For example, one fact sheet notes that trees help reduce respiratory disease, skin cancer risk (via shade/UV reduction) and promote active outdoor lifestyles. Massachusetts Government

3. Implications & Recommendations

For research.

• More quantitative studies are needed to link specific tree/forest interventions to measurable water and health outcomes (e.g., quantifying how much sediment reduction leads to decreased exposure).

• Contextual studies: understanding how benefits vary by climate zone, vegetation type, soil, land-use history.

• Longitudinal and intervention studies linking tree/forest restoration to human health metrics (birth outcomes, cardiovascular incidence, mental health).

  
  

For policy and land management.

• Protecting and restoring forests—especially in watershed headwaters that supply drinking water—should be a public-health priority.

• Integrate urban forestry into stormwater, flood and health planning: tree canopy goals, riparian buffer zones, permeable surfaces.

• Consider equity: lower-income communities often have less access to tree cover, and therefore less of the health and water protection benefits.

  
  

For practice and community action.

• Maintain existing mature trees and plant trees in strategic locations: riparian buffers, slopes, urban streetscapes.

• Choose appropriate tree species for the site: native species generally provide more reliable ecosystem services.

• Ensure interdisciplinary collaboration: forestry, hydrology, public health, urban planning.

Conclusion

The scientific evidence indicates that trees and forests are foundational to protecting water resources—through interception, infiltration, erosion and pollutant control—and that the resulting environmental services extend to significant human-health benefits, both directly and indirectly. By integrating tree- and forest-based solutions into water management and public-health strategies, societies can achieve multiple benefits: cleaner water, reduced flood risk, healthier communities, and greater resilience. As climate and land-use pressures increase, leveraging these nature-based solutions becomes increasingly important.

References

International Science Council. (2023). Unveiling the health benefits of forests and trees. https://council.science/blog/unveiling-the-health-benefits-of-forests-and-trees/ International Science Council

Keller, A. et al. (2019, June 27). How do trees improve water quality? UCSB Current. https://bren.ucsb.edu/news/study-shows-reforestation-can-improve-water-quality Bren School of EnvironmentLivesley,

S. J., McPherson, E. G., & Calfapietra, C. (2016). The urban forest and ecosystem services: Impact on urban water, heat, and pollution cycles at the tree, street, and city scale. Journal of Environmental Quality, 45, 119-124. https://doi.org/10.2134/jeq2015.11.0567 USDA Forest Service Research

Nowak, D. J., et al. (2023). Chapter 10: Trees and water in Forest ecosystem services: Global change and emerging issues. U.S. Forest Service. https://doi.org/10.xxxx (PDF) US Forest ServiceTrees

Forever. (2022, January 5). Trees and water quality. https://treesforever.org/2022/01/05/trees-and-water-quality/ Trees Forever

Wolf, K. L., Lam, S. T., McKeen, J. K., Richardson, G. R. A., van den Bosch, M. A., & Bardekjian, A. C. (2020). Urban trees and human health: A scoping review. International Journal of Environmental Research and Public Health, 17(12), 4371. https://doi.org/10.3390/ijerph17124371 The Nature and Health Alliance

Harvard T.H. Chan School of Public Health. (2021, September 15). The health benefits of trees. https://hsph.harvard.edu/news/the-health-benefits-of-trees/ Harvard Public Health

Massachusetts Department of Public Health. (2023). Trees and our health: Explore current research on how trees improve public health. (Fact Sheet) Massachusetts Government

Resilient Landscapes. (2022). Agroforestry supports ecosystem services and biodiversity conservation: Trees on farms protect waterways, lessen the impact of drought and increase biodiversity. https://resilient-landscapes.org/mf_publication/agroforestry-supports-ecosystem-services-and-biodiversity-conservation-trees-on-farms-protect-waterways-lessen-the-impact-of-drought-and-increase-biodiversity/ Resilient Landscapes (RL)

World Resources Institute. (2023). How forests benefit cities’ water, health, climate and biodiversity. https://www.wri.org/insights/forests-benefit-cities World Resources Institute

Mississippi Watershed Management Organization. (n.d.). How do trees improve water quality & reduce water pollution? https://www.mwmo.org/learn/preventing-water-pollution/trees/ MWMO