Blog post contributed by Loïc Gillerot (WP2, WP5) and Katriina Kilpi (WP6)
Forests have made it into the spotlight. They are frequently mentioned in both scientific and popular media outlets, often as the solution to solve climate change. While human technologies are racing towards finding the smartest and most advanced technologies to halt the global warming trend, the carbon naturally sequestered by forests seems to provide a simple answer to the problem. Unfortunately, this complex problem is not as simply answered by planting new forests. Young forests store much less carbon than their more mature counterparts, with the latter also being crucial to safeguard biodiversity which takes decades to develop. Therefore, one of the most important measures we should strive for is the protection of existing forests. But even this doesn’t seem evident, as a net loss of 4.7 million ha of forest was recorded on average every year on a global scale during the last decade (FAO 2020, Mitchard 2018). However, if crucial aspects like biodiversity, forest management, site conditions and the destiny of wood products are carefully considered, the vast quantities of carbon stored in forest soils and biomass can indeed play a keystone role in at least partly mitigating our emissions (e.g. Krankina et al. 2006, Liu et al. 2009, McKinley et al. 2011).
But forests represent even more than a silver bullet to solve the climate issues we have caused ourselves. Besides their relevance to this global and relatively abstract phenomenon, they also have more tangible benefits for human health and wellbeing. This notion is probably most concretely applied in Japan, where the concept of ‘shrinrin yoku’ or forest bathing has been promoted by the government since the 80s and studied since the beginning of the century for its physiological and psychological health benefits (Rajoo et al. 2020, Song et al. 2016). Forest bathing means spending time in the forest environment to relax while mindfully using our five senses (Park et al. 2010). It can be done individually or as part of a guided group. In commodifying the practice, some guided walks have been created around a sequence of sensory practices, though the experience can be modulated in order to accommodate the preferences of the individual or group in question.
Although unequivocally proving its welldoings is far from straightforward (Oh et al. 2017), the current body of literature suggests that forest-based relaxation activities hold a lot of (untapped) potential for, amongst others, reducing stress, increasing relaxation, and, more generally, serving as preventive medicine (Park et al. 2012, Rajoo et al. 2020). In Japan, much of the focus of the benefits of shinrin yoku has been centered around the wellbeing effects of the forest and especially on the nervous system and stress reduction theories (Richardsson et al. 2016). A specific aspect within forest health effect studies that is gaining attention are the high concentrations of beneficial volatile substances such as negative air ions and, especially concerning studies on shinrin yoku, phytoncides (Franco et al. 2017). Though this may be one of the more abstract processes, phytoncides may benefit our immune system and mental wellbeing through relaxation, improved cognitive performance and mood, decreased mental fatigue and stress (Franco et al. 2017, Antonelli et al. 2020) and increased NK activity (Li., et al 2009). The research is currently mostly limited to Japan and South Korea, where phytoncides of certain tree species have been proven effective (Antonelli et al. 2020, Li et al. 2009). Health effects generated by forests elsewhere in the world could therefore be expected to have a contrasting outcome. However, since studies in forest types with different species compositions lack, such propositions can hardly be supported. This uncertainty is of little consequence when taking a step back to consider the bigger picture. Even though the phytoncide effects of other forests are not well-known yet, they certainly do not represent the sole pathway leading to improved health (Antonelli et al. 2020). They merely offer a contribution to the plethora of health effects that can be generated by forests (Karjalainen et al. 2010).
Many of those health effects act universally, that is, they are not restricted to certain species or forest types. Exactly what constitutes the harmonious, energized yet peaceful feeling that ensues after spending time in a forest environment is a sum of a complex combination of health benefits. If we want a holistic and accurate answer to what forests represent for human health and wellbeing, drilling into a single health effect only, such as the phytoncide pathway, will not cut it. Let’s rather try deconstructing that complexity by trying to unravel the effects acting in parallel.
To start, a part of the complexity already stems from the sheer amount of different health benefits that can be attributed to forests. As touched upon above, mental effects are plenty and acting simultaneously, but benefits can also be more palpable. For example, forests have an important provisioning function: their wood has been an excellent building material for millennia and the energy stored in it has fuelled human progress until today. Forests provide berries, medicinal herbs and mushrooms, finer materials for crafting as well as game for hunters. Besides provisioning, they regulate our environment by filtering the air, purifying the water, mitigating global warming but also buffering weather extremes, protecting us against floods, tsunamis, avalanches, rockfall and landslides (Krieger et al. 2001). All of those services forests provide for us would amount to extraordinary sums if we were to monetize them (Krieger et al. 2001), but the health benefits are equally impressive.
You may already have guessed it: a second reason that makes the story so complex is that the beneficial aspects can both act directly and (very) indirectly. Let’s take the example of the health risk posed by heatwaves. Not only will they leave many feeling uncomfortable, but they can also lead to heat strokes and even death in more vulnerable individuals. One of the more dramatic examples includes the 2003 heatwaves, estimated to have caused over 70000 deaths in Europe (Robine et al. 2008). On the one hand, the direct effect of the forest will be one of sheltering the visitor. The canopy protects from the scorching sunlight, while the air temperature is lowered through the plant’s evaporative cooling, which can cool the air by multiple degrees Celsius. On the other hand, the forest can benefit us very indirectly by storing carbon, which will reduce greenhouse gas concentrations, mitigating climate change and ultimately reducing the frequency and intensity of such heatwaves.
Third, effects operate over different spatial and temporal scales. Take the example of heatwaves again: the direct sheltering effect takes place at the scale of a single forest, and manifests itself in a matter of minutes to hours. In contrast, the indirect effect leading to less heatwaves happens at the global scale over decades. Similarly, psychological benefits can be nearly immediate once visiting a forest, but long-term improvements require frequent and regular use of forest areas over many years.
Fourth, the forests wellbeing effects can be had both passively (i. e. by being exposed to the forest air while jogging through the forest) and actively (i. e. actively engaging with the millions of details of the forests environment via an engaging activity such as forest bathing). In fact, there is indication that forest bathing could facilitate a larger impact on some wellbeing effects, for instance in having an impact on a person’s nature connection. Connectedness with nature refers to “an individual’s experiential sense of oneness with the natural world’’ (Mayer & Frantz, 2004, p. 504), which has been associated with increased life satisfaction, experiencing more positive affect and vitality (Capaldi et al., 2014). What’s more, nature connectedness has been linked as a predictor to pro-environmental behaviour (Geng et al., 2015; Mayer et al., 2009). Though, to date, forest bathing research has not addressed nature connection, nature connectedness researchers predict forest bathing to be able to activate the pathways to nature connection (Lumber at al. 2017 in Kotera 2020b). Furthermore, forest bathing has been indicated to have psychological effects such as alleviating depression, anxiety, anger, stress, and therefore possibly also addiction (Kotera a, 2020, Park et al, 2012), as well as enhancing relaxation, promoting gratitude and selflessness (Park et al, 2012; Pritchard, 2019).
Health effects act simultaneously and can even mutually interact. Let’s use the example of phytoncides again. As said before, phytoncides alone do not offer a quick solution for our wellbeing. Administering phytoncides indoors is unlikely to yield the same health benefits as the ones reaped from moving about in the forest air (Li et al. 2009), because there’s so much more happening at the same time outdoors. When one breathes in the forest air, one takes up negative ions, forest microbes, an abundance of oxygen as well as phytoncides (Forest Europe 2019, Franco et al. 2017). This physical process goes in pair with mental effects. What happens in the body and brain of the walker is a complex string of events that are rooted in our evolutionary history and hence starts almost at once at entering a green space with the calming down of the sympathetic nervous system and activation of the parasympathetic nervous system (van den Berg et al, 2015). The experience is shaped by the recognition of the smells of the forest and subconsciously connecting those with one’s previous experiences (Bowring, 2006); registration and reaction to the natural soundscape of e.g., the rustling of leaves in the wind and birdsong (Ratcliffe et al., 2013); feeling at ease with the type of environment (Ulrich,1993; Purcel et al., 2011), one’s level of nature connection (Mayer & Frantz, 2004); the visual and other sensory cues matching the state of wellbeing of the walker (Kaplan & Kaplan, 1982) as well as what the walker finds appealing in the environment (Hartig & Staats, 2006; Grahn & Stigsdotter, 2010; Richardsson, 2016). This complex brew of conscious and unconscious aspects mediates the changes in health and wellbeing state that one can reach at the end of the forest walk. A great number of physical and psychological effects is thus at play, but some studies indicate that both can affect and even mutually reinforce each other. Taking thermal comfort studies to illustrate this, Park et al. (2016) suggest that the improved thermal conditions found in a forest leads to more positive emotions while, conversely, Jeong et al. (2016) found that beneficial psychological effects of the forests could extend the range of temperatures people would feel comfortable with. Although more research is needed on these interactions, such synergies could be more widespread and help to explain provided health benefits.
At last, those many benefits should be weighed against the health risks forests can represent. This will improve the accuracy and thus the credibility of scientific studies. For example, in the immediate sense, while a forest may protect the village below it from rockfalls and avalanches, they are a dangerous place to walk in during a storm: falling branches may instantly (and ‘directly’) cause physical harm. They are also a place where ticks may transmit tick-borne encephalitis or Lyme borreliosis, or where pollen allergies and the urticating hairs of processionary caterpillars may render a forest visit a perilous endeavour during risk seasons. When promoting forests for their health benefits, it is thus paramount to consider which types of forests offer the most benefits compared to the risks, as well as what type of activity is most suitable to specific types of forest environment. For example, during risky seasons, forest bathing could be relocated or replaced by an activity that keeps the participants at safe distance from the oaks possibly infested with oak processionary caterpillars.
Considering this intricate patchwork of interwoven health impacts, it is very likely that there will be a lot of variation among forest types. The relative importance of the array of health benefits provided by a conifer forest will not be exactly the same as a neighbouring broadleaved forest. One of the potentially decisive factors for the provision of those benefits, and simultaneously the forest’s health itself, is biodiversity, i.e. the diversity of and within species in any given ecosystem. Forest ecosystems happen to be the keepers of biodiversity in human-dominated landscapes within Europe. To this end, the Dr. FOREST project, funded by the Biodiversa research funding network, aims to quantify the impacts of forest biodiversity on human health in order to better combine biodiversity conservation with ecosystem management in a way that supports human health and well-being. It thereby aims to address the following question: which tree species composition is most suited for human wellbeing?
Despite the health risks and high complexity behind forest health effects, one thing is certain: evidence shows that health benefits are manifold and dwarf potential risks. An increasing number of studies are converting those health benefits to avoided health care costs for society, often resulting in impressive amounts of potential savings (e.g. Kardan et al. 2015, Song et al. 2018). Considering that roughly 15% of the world population (1.1 billion people) has been estimated to suffer from mental health or substance use disorder (Ritchie & Roser, 2018). With the rise in anxiety and shortage of mental health services available during the pandemic, this number is expected to soar post-pandemic. Nature-based, or in this case, forest-based practices such as forest bathing that seek to actively engage the participants with the forest environment, provide a cost-effective manner to prevent ill health and treat our overall health. As long as forests remain accessible, active and passive ways to enjoy the forest atmosphere can mean relatively approachable means for wellbeing promotion, complementing the increasing calls for environmental preservation (Kotera, 2020a).
Finally, by realizing how an individual’s wellbeing is linked to the wellbeing of forests near and far, incentivizing the protection and restoration of forests worldwide should be prioritized in nations around the world. Forests are not only pivotal to mitigate global warming and, by consequence, human wellbeing in the long term, but are also crucial for our wellbeing right now and in the future.
Loïc Gillerot is a PhD student working on two of Dr. Forest’s themes. The first focuses on the forest microclimate and thermal comfort, while his second goal will be to study how forest biodiversity and its various health effects interact using holistic models. He previously studied carbon sequestration and drought tolerance of forests in, respectively, Kenya and Switzerland.
Katriina Kilpi works for BOS+, a Belgian pro-forest organization where she is active in projects that deal with forests and human health. She also conducts applied research under her NatureMinded consultancy and organizes the annual International Forest Therapy Days which has been held in Finland since 2018.
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