The Relationship Between Trees and Human Health

Background

Several recent studies have identified a relationship between the natural environment and improved health outcomes. However, for practical reasons, most have been observational, cross-sectional studies.

Purpose

A natural experiment, which provides stronger evidence of causality, was used to test whether a major change to the natural environment—the loss of 100 million trees to the emerald ash borer, an invasive forest pest—has influenced mortality related to cardiovascular and lower-respiratory diseases.

Methods

Two fixed-effects regression models were used to estimate the relationship between emerald ash borer presence and county-level mortality from 1990 to 2007 in 15 U.S. states, while controlling for a wide range of demographic covariates. Data were collected from 1990 to 2007, and the analyses were conducted in 2011 and 2012.

Results

There was an increase in mortality related to cardiovascular and lower-respiratory-tract illness in counties infested with the emerald ash borer. The magnitude of this effect was greater as infestation progressed and in counties with above-average median household income. Across the 15 states in the study area, the borer was associated with an additional 6113 deaths related to illness of the lower respiratory system, and 15,080 cardiovascular-related deaths.

Conclusions

Results suggest that loss of trees to the emerald ash borer increased mortality related to cardiovascular and lower-respiratory-tract illness. This finding adds to the growing evidence that the natural environment provides major public health benefits.

Introduction

There is increasing evidence from multiple scientific fields that exposure to the natural environment can improve human health.1, 2, 3, 4 However, existing research has often been hampered by cross-sectional study design and a failure to adequately address confounding factors.⁵ Quasi-experimental designs, such as the pioneering work by Ulrich,⁶ who showed that patients recovered faster from surgery in a room with a view of a natural scene than those without such a view, are needed to provide stronger evidence of a causal link between the natural environment and health.

To address this gap in the literature, a longitudinal study design was used to quantify the public health effects of an introduced forest pest, the emerald ash borer, which has killed tens of millions of ash trees since it was first detected in the U.S. in 2002. The spread of the borer is a unique natural experiment allowing the evaluation of the effect of changes in the natural environment on public health. The goal of the study was not to track the borer per se, but to use it as a proxy for tree loss.

Natural experiments approximate RCTs, as the mechanism determining exposure is independent of the outcome, and therefore, common prior causes of exposure and outcome are equally distributed between those exposed and those unexposed.⁷ The borer spreads directly from county to county, but it is spread also by accidental transport—typically on firewood—which results in satellite populations (Figure 1). This accidental spread adds an important random element to the current natural experiment. Nonetheless, natural experiments remain observational studies and cannot prove causality.

This study examined whether the spread of the emerald ash borer is associated with increased mortality related to cardiovascular and lower-respiratory-tract illness. These two types of health issues were chosen because they are the first and third most common causes of death in the U.S.,⁸ and there are plausible mechanisms linking these types of deaths with trees. Specifically, the natural environment has been shown to decrease stress,⁹ increase physical activity,¹⁰ and improve air quality.¹¹ In turn, stress,12, 13 lack of physical activity,14, 15 and poor air quality¹⁶ have been linked with cardiovascular and lower-respiratory-tract disease.

The pioneering work in the field by Ulrich⁶ found that patients recovering from gall bladder−removal surgery in a room with a view of a natural scene recovered faster and took fewer pain medications than patients in a room with a view of a brick wall. However, extending Ulrich's work has been problematic, because most health outcomes of interest have causes that long precede the short surgical recovery period, and most people spend little time in environments as controlled as a hospital room.

Observational studies of the relationship between the natural environment and health have examined a range of health outcomes. Mitchell and Popham¹⁷ found that, after controlling for SES, “greenness” was negatively associated with overall mortality in England. This relationship was particularly strong for cardiovascular-related mortality. Takona and colleagues¹⁸ studied the 5-year survival rate of 3144 senior citizens living in Tokyo. They found a positive association between survival rate and access to walkable green space.

In Holland, Maas et al.¹⁹ reported a positive association between greenness and self-reported health. In a later study, Maas et al.²⁰ analyzed the health records of 345,000 people. They found that those living in greener areas were less likely to be diagnosed with 15 of the 24 health outcomes examined. Results were particularly strong for anxiety and depression and for children and those with lower SES. Park and colleagues⁹ showed that walking in a forest reduced heart rate and cortisol levels. Finally, in New York City, Lovasi et al.²¹ found that children who lived in areas with more street trees were less likely to have asthma.

Two studies have examined the relationship between the natural environment and birth outcomes. Donovan et al.² found that mothers living in Portland OR with more tree canopy within 50 meters of their homes, or who lived closer to open space, were less likely to have a baby that was small for gestational age. Dadvand and colleagues¹ conducted a similar study in Spain. They found that women with more greenness within 100 meters of their homes, or who lived within 500 meters of a major green space, gave birth to heavier babies, although results only held for women with the lowest level of education.

The emerald ash borer, Agrilus planipennis, is a phloem-feeding borer native to East Asia. It was discovered in North America in 2002, when it was identified as the cause of widespread ash mortality (Fraxinus spp.) in Detroit MI and nearby Windsor, Ontario.²² By 2012, this borer had killed approximately 100 million trees in the U.S. (D. McCullough, Michigan State University, personal communication, 2012). However, its potential impact is much larger, as there are 7.5 billion ash trees in the country.²² In addition, the borer kills all 22 species of North American ash and virtually all infested trees, so it is a good proxy for ash tree death. For more information about this borer, see the video in Appendix A (available online at www.ajpmonline.org).