Trees have a significant impact on our lives, and science backs that up. On this Arbor Day, April 25, 2025, I hope you’ll think about all the benefits that trees provide.
Trees are some of my favorite beings on Earth. That’s why I look forward to Arbor Day every year, the national holiday that recognizes the beauty and the importance of trees. While many states observe Arbor Day on different dates throughout the year based on the best tree-planting times in their areas, National Arbor Day in the U.S. is celebrated on the last Friday in April, which this year is April 25.
Trees have a significant impact on our lives, whether we recognize it or not; and science backs that up. Recently, a long-term study conducted in Switzerland found that neighborhoods with numerous, well-arranged trees exhibit lower mortality risks than other areas. And another new investigation has found that people living in places where the number of shrubs and trees was more than doubled showed lower levels of an inflammation blood marker than those living outside the planted areas. General inflammation is an important risk indicator for heart disease and other chronic disorders.
Unfortunately, there’s been a long-standing attitude in many farming communities that trees and agriculture don’t mix. But there’s pro-tree news on this front, too: agroforestry can provide a multitude of benefits to farmers and landscapes. A new study provides a foundation for strategic plans that integrate economic, environmental and social considerations of trees in agricultural landscapes.
In a recent Swiss study, researchers found that in residential neighborhoods, both tree quantity and positioning matter.
The layout of trees and human health
Trees create a serene atmosphere in urban areas. There are various reasons for this: trees filter pollutants out of the air, provide shade, lower the ambient temperature in hot weather and encourage people to spend more time outdoors. Many governments—local, national and worldwide—have set ambitious tree-planting targets for the decades ahead, partially in response to climate change and rising temperatures. In densely developed cities, however, space is at a premium. So, the key question is how to plant trees in existing green spaces to optimal effect.
Solving that problem occupies urban planning researchers and practitioners, because any answer must take into account the specific, local spatial circumstances and climatic conditions. Researchers from ETH Zurich, a public research university in Switzerland; from Future Cities Lab operated in Singapore by ETH Zurich; and from the National University of Singapore are tackling this issue—not only in Switzerland but also in Asia. In a paper published in the journal The Lancet: Planetary Health in March 2025, the scientists report that they discovered interesting links between tree management and the health of urban residents.
To begin with, the researchers examined high-resolution tree canopy data to determine the structure of tree-covered green spaces within a radius of 1,640 feet of a person’s place of residence. In addition to recording the total area covered by all tree clusters, they also identified the proximity and connectedness of tree clusters, their geometrical complexity and the fragmentation level.
Tree canopy cover correlates with mortality. There’s a significantly lower mortality risk for people living in neighborhoods with large, contiguous and well-networked areas of tree canopies than for people residing where there are fewer and fragmented areas of tree canopies with complex geometries.
The scientists then linked this information with the survival times of more than 6 million adults in their respective neighborhoods, looking exclusively at natural-cause deaths due to illness and old age. This data, supplied by the Swiss Federal Statistical Office, covered a 10-year period (2010-2019).
An analysis showed that both the tree canopy cover in residential areas and the spatial arrangement correlate with mortality. The study identified a significantly lower mortality risk for people who live in neighborhoods with large, contiguous and well-networked areas of tree canopies than for those who live in areas with fewer, fragmented areas of tree canopies with complex geometries. This correlation is particularly evident in densely developed suburban and urban areas with poor air quality and high temperatures: if such areas feature well-structured forested green spaces, the residents received more health benefits than those in the other areas.
Yet, while this study represents an important first step, it is still not possible to draw conclusions regarding the causes, state the scientists. They are not yet able to say with precision the pathways through which tree canopy configuration influences human health. Nevertheless, the study’s findings at the individual level are generally consistent with the results of similar studies done at the community level in Philadelphia, Taipei and Tehran.
To fully exploit trees’ potential to support human health, cities should not only increase the number of trees but also connect isolated green spaces—including by creating tree-lined boulevards.
Although they can’t yet define a direct causal link, the researchers found that when they addressed factors such as age, gender and socioeconomic status, the data showed clear correlations. The results provide plausible indications that human health may be influenced not only by the quantity of trees but also by their spatial distribution.
The findings underline the importance of carefully considering the layout of forested green spaces and adopting a targeted approach to tree placement. To fully exploit trees’ potential to support human health, conclude the scientists, cities should strive to not only increase the number of trees but also to connect isolated green spaces—including by creating tree-lined boulevards.
The study also suggests that compact, geometrically simple areas of tree canopy—including circular and rectangular forms—could have a greater positive effect on health than irregular, fragmented tree coverage. One possible explanation is that simply structured areas offer a larger core area, promote biodiversity and consequently attract residents to use these spaces.
The groundbreaking Green Heart Louisville Project found that people living in neighborhoods where the number of trees was more than doubled showed lower levels of a blood marker of inflammation—a risk indicator for heart disease and other chronic illnesses—than those living outside the planted areas.
The addition of trees and reduced inflammation
In 2018, the Christina Lee Brown Envirome Institute at Kentucky’s University of Louisville, in partnership with The Nature Conservancy, Washington University in St. Louis, Missouri, and others, launched a first-of-its-kind project to study whether and how living among more densely greened surroundings contributes to better heart health. The design of the study closely mirrored clinical trials which test whether medical treatments are effective. The team applied the treatment—the addition of shrubs and large trees—to some participants’ neighborhoods but not to others. They then compared residents’ health data to see how the addition of the trees affected their well-being.
To understand the state of a community’s health at the start of the study, the researchers took blood, hair, nail and urine samples and documented health data from 745 people living in a four-square-mile area of south Louisville. The researchers also took detailed measurements of tree coverage and levels of air pollution in the area.
Following this baseline data collection, the Envirome Institute worked with The Nature Conservancy and a host of local partners to plant more than 8,000 large trees and shrubs in designated neighborhoods within the project area. Those living in the greened area were considered the treated population; and the results obtained from this population were compared with residents of adjacent neighborhoods, where the project team did not plant any trees.
Trees contribute more to our lives than beauty and shade. They can improve the health of the people living around them. As more is discovered about the benefits of increased tree cover, expanding greening in cities may emerge as a key method for boosting public health.
After the plantings, the research team reassessed residents’ health. They found that those living in the greened area had 13% to 20% lower levels of a biomarker of general inflammation, a measure called high-sensitivity C-reactive protein (hsCRP) than those living in the areas that did not receive any new shrubs or trees. Higher levels of hsCRP are strongly associated with a risk of cardiovascular disease and are an even stronger indicator of heart attack than cholesterol levels. Higher CRP levels also indicate a higher risk of diabetes and certain cancers. A reduction of hsCRP by this range of percentages corresponds to a nearly 10% to 15% reduction in the risk of cancer, heart attacks or dying from any disease.
These results, presented at the 36th Annual Conference of the International Society for Environmental Epidemiology in Santiago, Chile, on August 25, 2024, indicate that trees contribute more to our lives than beauty and shade. They can improve the health of the people living around them. Although several previous studies have found an association between living in areas of high surrounding greenness and health, this is the first study to show that a deliberate increase in greenness in a neighborhood can improve health. The researchers hope their work will bolster the push to increase urban green spaces.
The benefits of trees and agroforestry
There has been a lot of research on the agronomy and ecology side of agroforestry, which is the intentional integration of shrubs and trees in agricultural systems—such as growing trees between annual crops, integrating trees in pastures or planting trees as windbreaks—and includes the environmental benefits these practices can offer. But according to University of Illinois Urbana-Champaign researchers, we don’t know a lot about agroforestry’s economic and social impacts, and those factors can dramatically shift priorities for implementing agroforestry in certain areas.
Planting trees as windbreaks is an agroforestry practice. The design of a windbreak should be based on the individual property.
The goal of the University of Illinois Urbana-Champaign scientists was to create a tool to target agroforestry where it would provide the greatest environmental advantages while also being economically viable, socially acceptable and suited to areas where agroforestry-relevant trees are most likely to thrive. Mapping social attitudes and economic feasibility together with environmental data is no easy task, but the researchers managed to do just that.
Ultimately, the team developed a map identifying areas where agroforestry could deliver the biggest impacts in terms of climate change mitigation, soil erosion, water quality and profitability for lands that are not viable for other crops. They also arrived at a high-level estimate of potential carbon sequestration if agroforestry was adopted on the most suitable lands in the Midwest.
The results, published in the journal Environmental Research Letters in February 2025, demonstrated that expanding these practices across just 5% of suitable Midwestern agricultural land could store 43 million tons of carbon dioxide equivalent (CO2e) per year. That’s a major advantage over cover crops, which are estimated to store about 8.4 million tons of CO2e per year on the same lands. So far, in the Midwest, these bonuses have gone unrealized, with vanishingly small adoption rates.
Riparian buffers, another agroforestry practice, are established by planting native grasses, shrubs or trees next to water bodies. The buffers provide erosion control, flood protection, pollution reduction, wildlife value and make the land-water transition a healthy one.
The researchers are making their mapping tool freely available for conservation scientists, landowners and policymakers. There are many ways to tailor agroforestry to the types of benefits sought, and the tool allows users to tweak all the parameters for their suitability analysis. For example, an assessment could be run on only one tree species, only considering economics or setting economics as five times more important than environmental factors.
The researchers note that each agroforestry practice has its own set of spatial opportunities, such as edge-of-field practices—like windbreaks and riparian buffers—and whole- or partial-property practices, like alley cropping (planting annual crops between widely spaced rows of trees or woody plants) and silvopasture (combining forage [like grasses and legumes], livestock and trees in a single, managed system).
Agroforestry also assists farmers by allowing them to generate income from multiple sources: from livestock products like meat, milk and wool to timber. There’s an appropriate practice for nearly every region, conclude the researchers, and adopting agroforestry does not have to be a whole-farm transformation.
The friendship of trees and thankfulness
I think of trees as my friends. Like good friends, just their presence provides comfort, reducing our stress and improving our overall well-being. And like steadfast companions, they remember their experiences (through epigenetic changes that alter how the genetic instructions written in their DNA are carried out).
I think of trees as my friends. The concept isn’t new; in Greek mythology, the olive tree is associated with Athena, the goddess of strategic warfare and wisdom, and the olive branch is a symbol of friendship and peace.
There’s even a historical precedent that validates my thinking: the olive tree is a symbol of friendship, an association that began as early as the fifth century in ancient Greece.
This Arbor Day, I hope you’ll think of your favorite tree. I bet you have one. Please give it thanks for absorbing carbon dioxide to regulate our planet’s climate; for beautifying our surroundings; for helping us to learn; for preventing erosion and flooding; for producing our oxygen; for providing our foods, materials to build our shelters and medicines; for harboring countless animals and insects to create a rich and diverse ecosystem; and for bestowing us with shade.
Here’s to finding your true places and natural habitats,
Candy
