Is quality or quantity of street trees having a protective effect against antidepressant prescriptions?

The harmful effects of urban life on physical and mental health have long been recognized, such as higher rates of cardiovascular and respiratory diseases. Depression is on the rise, particularly in urban areas, and is characterized by depressed mood and a feeling of helplessness.
In this post, we will focus on a specific type of the urban forest: individual street trees, and on how they can strengthen mental health. Street trees are an important component of the urban forest because they provide various ecosystem services for human health and wellbeing, such as improving air quality and reducing the urban heat island effect. They are public amenities laced throughout the urban matrix and they can be retrofitted into urban areas.
The potential impact of street trees on mental health was unknown and no previous study has investigated the relationship between ecological quality of street trees, the presence of street trees, and antidepressant prescriptions. An interdisciplinary research team from UFZ, iDiv and the University of Leipzig applied objective indicators (antidepressant medication and individual-level greenspace exposure, using NDVI) to analyse this relationship.
To calculate the amount of contact that an individual has with urban greenspace, the researchers applied geographic metrics, using Euclidean buffers of 100, 300, 500 and 1000 m around the residences. The sampling involved 596 people who were prescribed antidepressants. All the participants were living in the city of Leipzig, and represented a representative sample.
In the city of Leipzig, street trees are planted throughout the city, while concentrated more densely in some areas than in other, with a total of 66,179 street trees, comprised of 51 genera and 131 species (Figure 1). This figure shows the distributions of street trees and antidepressant prescriptions among the participants. The circles indicate the location of the participants within the city of Leipzig, Germany. The circles with a black outline represent individuals who have been prescribed antidepressants. The yellow-green colored circles reflect the density of street trees within 100m of the house. Tree density values are the number of trees per meter of road within a 100m buffer.

Figure 1. Distribution of street trees and antidepressant prescriptions amongst participants. The figure was created by D. Eichenberg, with ggplot2 available for R (Marselle et al., 2020)

Subsequently, Figure 2 shows that the people who have a higher risk of antidepressant prescriptions are women, overweight or obese, smoking, or have pessimistic thoughts; there is also a higher risk during winter and spring. By contrast, reduced risk of antidepressant prescriptions was associated with being young (18–39) or old (age 65 +), employed, and optimistic. People living in homes with greater density of street trees within 100 m were less likely to be prescribed antidepressants.

Figure 2. Effect size of covariates and street tree density and richness at 100 m around the home on antidepressant prescriptions  (Marselle et al., 2020)

Species richness was not significantly associated with antidepressant prescriptions at any distance, this is well explained in the next figure. The results of Figure 3 show that the quantity of street trees around the home may be more important for preventing depression than the ecological quality of street trees. This is in line with previous studies. For example, a Chinese study (Elsadek, M., Liu, B., Lian, Z. & Xie, J. The influence of urban roadside trees and their physical environment on stress relief measures: A field experiment in Shanghai. Urban For. Urban Green. 42, 51–60, 2019) walks along roads, each with a different species of street tree, resulted in better mental health compared to walks in a road without street trees, suggesting the mere presence of trees on streets, but not their species affiliation, is important. Given that most people cannot identify different plant species in general, benefits of street trees may rather be provided through people experiencing tree abundance.

Figure 3. Effect size of street tree density and species richness at different spatial distances around the participants’ home on antidepressant prescriptions (Marselle et al., 2020)

The net result showing the graphs in Figure 4 is that under low street tree density, individuals with low SES tend to have higher probabilities of antidepressant prescriptions. For the medium and high SES groups, the effect of street tree density at 100 m from the home did not significantly change the probability of being prescribed antidepressants.

Figure 4. (a) Probability of antidepressant prescriptions as a function of street tree density 100 m around the home and individual socio-economic status (SES). The black line is the mean and the shaded area are the 95% confidence intervals. (Marselle et al., 2020)
(b) Probability of antidepressant prescriptions as a function of low (0), medium (average) and high (max) street tree density 100 m around the home stratified by SES. The black dot is the mean and the black line is the 95% confidence interval. (Marselle et al., 2020)


The study shows that street trees, as an urban green space accessible to the public on a small scale, could contribute to an “equigenic environment”, as nature-based solutions that can help close the gap in health inequalities between individuals with low and high SES.
During the COVID-19 pandemic, forests in cities, green spaces, tree-lined streets, but also having a small green space in front of the house, has acquired more importance in the eyes of many citizens. So, as this study suggests, ‘unintentional’ contact with nearby nature in daily life is important for mental health, but also reduces social inequalities and contributes to multiple Sustainable Development Goals. The study thus calls for a different urban green planning, no longer aimed at designing green spaces for intentional or targeted visits for recreation, but above all targeting green planning for daily contact with nature.
Planting and maintaining street trees thereby provides a proactive public health measure that also meets conservation goals. Such information can inform health professionals, urban foresters, urban planners and urban planners about the necessary policy, planning and management decisions needed to ensure that urban forest has a positive impact on both public health and nature conservation.

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