Research PaperA dose of nature: Tree cover, stress reduction, and gender differences
Introduction
The demands and pressures of modern life are precursors to some of the most threatening medical problems we face today. Chronic stress can suppress the immune system (Cohen, Miller, & Rabin, 2001) and trigger cardiovascular disease, stroke, depression, asthma, and other critical health problems (e.g., Childs and Wit, 2009, Dimsdale, 2008, Gump et al., 2011, Russ et al., 2012, Steptoe and Brydon, 2009). There is mounting evidence, however, that exposure to nature enhances the resources necessary to manage the demands and pressures of modern life. Settings that include tress, grass, and open spaces have been shown to aid physiological stress reduction (e.g., Chang and Chen, 2005, Hartig et al., 1991, Ulrich et al., 1991, van den Berg et al., 2007).
Although it is well established that exposure to nature enhances stress reduction, the shape of the dose–response curve is entirely unclear. We do not know if exposure to a small amount of green space is enough to induce calming effects, whether increase in the density of vegetation produce additional calming effects, or even if the relationship between exposure to nature and stress reduction is linear. Lack of this knowledge prevents landscape architects and urban planners from making science-based design and management decisions that might improve the health and longevity of people in the communities they serve.
In this paper, we seek to describe the shape of the dose–response curve for how exposure to nearby nature impacts stress reduction. We begin by reviewing theory and evidence regarding stress and human health. Next we review recent evidence connecting exposure to nearby nature to lower levels of stress. Finally, reporting the results of an experiment involving 160 individuals, we describe a dose–response curve for each gender and discuss the implications of the findings for design and planning.
When we feel stress, our bodies respond via two physiological pathways: the sympathetic–adrenomedullary system (SA) and the hypothalamic–pituitary–adrenocortical axis (HPA) (Smith and Vale, 2006, Taylor, 1999). The SA activates what is often termed the fight or flight response. It causes the adrenal medulla glands to produce epinephrine and norepinephrine, which result in increased blood pressure, heart rate, sweating, and constricts peripheral blood vessels. The SA enhances our ability to physically engage with the stress or threat. The HPA axis, on the other hand, prepares our bodies for possible injury and helps bring our bodies back to normal after the threat is no longer present. In the HPA axis, the cerebral cortex sends a message to the hypothalamus, which activates the corticotrophin-releasing factor (CRF), and results in cortisol being released into the blood stream. Cortisol plays an important role in helping the body return to its normal state after the stress (Young, Abelson, & Lightman, 2004).
Cortisol responses differ within and among individuals. Cortisol levels change within healthy individuals each day, generally peaking shortly after waking in the morning and reaching a low shortly after falling asleep at night (Edwards et al., 2001, Kudielka et al., 2004a, Kudielka et al., 2004b). Men typically have stronger physiological responses to stress than do women, as indicated by greater increases in cortisol levels to stressful events (Bratman et al., 2012, Dedovic et al., 2009, Jackson, 2003). An individual's health status also can impact the levels of cortisol in their blood (De Rooij & Roseboom, 2010). Given this amount of variation within and among individuals, research that examines levels of cortisol must take gender and other confounding factors, such as measurement time, physical and mental health status, and intake of drugs, tobacco, or acohol, into consideration.
Together, these physiological responses to stress can be lifesaving. But if they are activated too often, if we spend significant parts of our daily lives feeling stress, these same physiological systems can be life threatening. People who experience chronic stress are at risk for immune dysfunction, cardiovascular disease including ventricular arrhythmias and stroke, depression, obesity, memory and concentration problems, and early death (Curtis and O’Keeffe, 2002, Lee et al., 2009, Taylor, 1999).
For centuries, philosophers, poets, and artists have suggested that people can reduce the stress they feel by escaping to nature. Emerson, Whitman, and Thoreau all wrote about the sense of peace and tranquility that comes with being in a wood, meadow, or other natural place. During the past two decades, scientists have shown that exposure to urban nature is related to a greater capacity to deal with difficult life problems (Kuo, 2001); increasing “peacefulness,” “tranquility,” and “relaxation” (Ulrich, 1993); and decreasing physiological indicators of stress (Chang and Chen, 2005, Parsons et al., 1998).
Ulrich's Stress Reduction Theory (SRT) is an important framework explaining why contact with nature might foster stress reduction (Bratman et al., 2012, Ulrich et al., 1991). Ulrich et al. (1991) postulated that landscapes containing water, vegetation, richness (or complexity), some visual depth, and a degree of curvilinearity would have aided human survival for hundreds of thousands of human generations. The idea is that in such settings, our ancestors could have spotted food or other resources, predators, and other humans that would have aided their survival. Ulrich argued that, given the impact such settings had on shaping our survival as a species, such settings should help moderate and reduce the physiological signs of stress in modern day humans.
SRT proposes that contact with such natural places will produce a relatively fast (within minutes) affective reaction at a subconscious level that can be measured through physiological pathways. In the last decade, scholars have measured physiological responses associated with various kinds of landscapes and have generally found that, in urban areas, the higher the level of vegetation, the greater the stress reduction (e.g., Alvarsson et al., 2010, Beil and Hanes, 2013, Lee et al., 2009, Roe et al., 2013, Ward Thompson et al., 2012).
None of these previous studies have reported gender differences in physiological responses after individuals have been exposed to various forms of nature. But a host of other studies that examine physiological responses to stressful conditions do report varying rates of recovery between males and females (e.g., Kudielka et al., 2004a, Kudielka et al., 2004b, Wang et al., 2007, Weekes et al., 2008). Both biological and social difference between men and women might explain gender difference in stress responses (e.g., Carrillo et al., 2001, Dedovic et al., 2009, Wang et al., 2007). Thus, in this study, we examine the extent to which gender differences exist in response to varying densities of nature.
Although previous studies demonstrate that exposure to nature, even urban nature, has calming effects, they do not help us understand the shape of the dose–response curve for the impact of nature on stress reduction. That is because none of the previous studies was able to examine the impacts of small, incremental increase in the density of nature have on stress outcomes. Previous findings show that exposure to natural environments is generally more beneficial to human well-being than exposure to predominantly built environments (Hartig et al., 2003, Laumann et al., 2003, Lee et al., 2009, Ulrich et al., 1991), but they do not help us understand the dose–response relationship between exposure to nature and stress reduction.
Thus, there is a critical gap in our knowledge regarding the shape of the dose–response curve for the effect of nearby nature on stress reduction. Is a little exposure to nearby trees and other forms of vegetation enough to produce calming effects from a stressful event? Do higher densities of vegetation produce more calming? Is the relationship linear, or does the effect lessen with greater and greater amounts of vegetation? Are there gender differences in these responses? This study begins to address these questions for one particular setting: the residential street in a single-family neighborhood.
Section snippets
2.1 Overview
To establish this dose–response curve, we recruited 160 individuals for a laboratory experiment. Participants engaged in the Trier Social Stress Test (TSST), which was designed to induce mental stress, and were then randomly assigned to view one of ten, 6-min, 3-D videos of neighborhood streets as the nature treatment. The density of tree cover in the videos varied from 1.7% to 62.0%. We measured stress reactions by assessing salivary cortisol and skin conductance levels.
2.2 Nature treatments
To simulate exposure to
3.1 Overview of the results
Results are presented in three parts. We begin by examining the extent to which the Trier Social Stress Test (TSST) induced acute stress. Next, we present analyses indicating that the relationship between tree cover and stress reduction should be investigated separately by gender. Finally, we examine the relationship between tree cover and stress reduction for men and women and present the resulting dose–response curves that best fit the data.
3.2 Did the stressor create stress?
To what extent did the TSST produce a stress
Discussion
This study identified the shape of the dose–response curve for the impact of tree cover density along residential streets on stress reduction measured by salivary cortisol and skin conductance levels. There are two central findings. First, there was a significant gender difference in physiological stress responses measured by salivary cortisol and skin conductance levels: men had changes in physiological stress that were significantly associated with varying densities of tree cover but women
Conclusions
This study is an initial attempt to describe the dose response curve for the impact of increasing densities of tree cover on stress reduction. Although we found no relationship between tree cover density presented through a 6-min, 3-D video and stress reduction in women, the findings do not necessarily mean that exposure to nearby nature does not impact women. To further investigate the dose–response curve of the density of tree cover and female's stress recovery, future research should use
Acknowledgments
The study was partially supported with two grants from the USDA Forest Service: One recommended by the National Urban and Community Forestry Advisory Council (Agreement #11-DG-11132544-333); the other from the US Forest Service Northern Research Station. We appreciate members of the Sustainability and Human Health Lab at Illinois for their participation in this research. Special thanks to David M. Buchner and Brian Deal from the University of Illinois at Urbana-Champaign.
Bin Jiang is an Assistant Professor in Landscape Architecture at the University of Hong Kong and an adjunct lecturer and postdoctoral scientist at University of Illinois at Urbana-Champaign. He holds a Ph.D. in Landscape Architecture from the University of Illinois at Urbana-Champaign, U.S., and a Master of Science in Landscape Architecture from Peking University where he worked with Kongjian Yu. His research work examines the impacts of built environment on human health.
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Bin Jiang is an Assistant Professor in Landscape Architecture at the University of Hong Kong and an adjunct lecturer and postdoctoral scientist at University of Illinois at Urbana-Champaign. He holds a Ph.D. in Landscape Architecture from the University of Illinois at Urbana-Champaign, U.S., and a Master of Science in Landscape Architecture from Peking University where he worked with Kongjian Yu. His research work examines the impacts of built environment on human health.
Chun-Yen Chang is Professor of Landscape Architecture and Director of Healthy Landscape Healthy People lab at National Taiwan University, Taiwan. He holds a Ph.D. from Pennsylvania State University. He examines the extent to which healthier landscapes help create healthier people.
William C. Sullivan is Professor of Landscape Architecture and Director of the Sustainability and Health Lab at the University of Illinois, Urbana-Champaign. He holds a Ph.D. from the University of Michigan where he worked with Rachel and Stephen Kaplan.