@phdthesis{01J5Z5Z47Y85MZZXSY8PP077S7,
  abstract     = {{Although life-expectancies are rising, public health still faces a plethora of risk factors that vary in space and time. Across Europe and many other global regions, significant portions of populations are exposed to hazardous levels of heat and air pollutants, are malnourished and suffer from poor mental health. Urban environments aggravate many of these risks and some, especially heat, will worsen in the near future. To some extent, each of these can be mitigated by promoting different forms of contact with nature. So-called nature-based solutions can include anything from green roofs, flower meadows to (urban) forests, and have important ecological co-benefits. Expanding vegetated areas has been associated to improved mental wellbeing, reduced heat stress and even reduced premature mortality levels at the scale of cities. Whereas technological solutions such as air-conditioning or reflective surfaces address single risk factors, nature-based solutions are particularly compelling because they generate many benefits in parallel. Nature nevertheless presents risks as well, such as serving as sources of zoonotic and vector-borne diseases.
Yet, the vast differences among and within vegetation types often remain unaccounted for. For instance, low-stature vegetation such as grasslands offer expansive vistas that are conducive to mental restoration, whereas forests provide immersive environments with high air filtration capacity that shelter people from solar radiation. Within forest ecosystems, we also find large variability. Five decades of research on biodiversity’s impact on ecosystem functioning underscores the pivotal role of fine-tuned ecological characteristics in determining ecosystem services. Such characteristics can include species diversity, species composition and spatial structure. However, how this affects human health outcomes is largely unknown. 
In this PhD thesis, we studied how forest ecological characteristics affect multiple human health outcomes, with a focus on thermal stress mitigation. Thermal stress was monitored using microclimate stations under both rural and urban settings, and compared with subjective experiences of forest microclimates. Findings were then integrated with datasets pertaining to other forest-health effects to unveil general trends and potential synergies or trade-offs among health outcomes.
In a first study (chapter 2), we aimed to discern the dominant drivers of forest buffering effects. We deployed a vast international network of microclimate stations recording air temperature, relative humidity, wind speed and grey globe temperature, used to calculate human temperature perception. Forest ecological characteristics were quantified using recognised forest inventory methods. We showed that forests have very considerable buffering capacities, reducing the occurrence of days with strong to extreme heat stress by 84%. Under those hot conditions, both young plantations and mature forests provided cooling of at least 10°C in terms of perceived temperature. Large variation between forests stemmed primarily from stand structural variables, followed by factors related to species composition. Tree diversity only had a probable indirect effect.
Building on insights gained from rural forests, a similar methodology was applied to urban environments, comparing treed locations, low-stature vegetation, and fully anthropogenic settings (chapter 3). Whereas building shade could provide relief against daytime heat and low-stature vegetation had moderate cooling effects, trees were disproportionately effective, cooling perceived temperatures by 5.5°C on average over the monitoring period and fostering lower temperatures both day and night. Locations with particularly high tree canopy cover (50% or more) remained coolest by far, emphasizing the need to secure a minimal urban canopy cover to safeguard urbanites against increasingly frequent and intense urban heat islands. 
Recognising the influence of psycho-physiological factors on individuals’ perception of temperature, we conducted an experiment on subjective thermal comfort and mental wellbeing in peri-urban forests (chapter 4). Participants' thermal perceptions closely mirrored meteorological measurements as sampled in chapters 2-3. However, participants were almost three times as likely to feel cooler in forests compared to non-forest baseline conditions (i.e. partly paved locations with limited tree canopy cover) for the same perceived temperature, indicative of psychological mediation. Indeed, mental wellbeing of participants was also improved under forest environments, and associated positively to thermal comfort. In other words, forests improved thermal and mental wellbeing independently, but a synergistic effect enhanced wellbeing levels beyond what would be expected based on isolated observation. 
Data and understandings from chapter 2-4 were used to develop a large synthesis model that also included data from six other forest-health effects – covering physical and mental outcomes (chapter 5). Using Bayesian Belief Networking, we detected key forest ecological characteristics driving variation in all health outcomes. While canopy density and tree diversity emerged as key variables that can be targeted by management interventions, changes in canopy density can lead to trade-offs in health outcomes that prevent universally valid recommendations. For example, dense canopies may enhance the mitigation of heat and improve air quality, but may also lead to an increased risk of tick-borne Lyme disease and a reduce medicinal plant yield. In contrast, increasing tree diversity was less influential but provided more consistent beneficial effects. 
Our findings imply careful consideration of unwanted trade-offs when managing a forest for single or multiple human health outcomes. Maximising a forest’s heat buffering capacity entails efforts to obtain tall and dense canopies to foster microclimates that are strongly decoupled from ambient conditions. Incidentally, this may improve the air pollution filtering capacity of the forest, but inadvertently also reduce the yield of medicinal plants and increase Lyme risk prevalence. Forest managers and policy makers may therefore want to rank public health challenges according to the specific local context. Such a ranking can be defined by asking three questions: Which public health challenges are of priority? What is the local environmental and societal context? How strong and consistent is the expected forest effect? We conclude by attempting to provide a generalised priority list for European forests and suggest that, under the average scenario, the three most relevant health outcomes for forest management are heat mitigation, air quality improvement and mental health benefits. The risk of Lyme disease and the provisioning of medicinal plants and edible mushrooms can nonetheless be of great importance under local circumstances.
In conclusion, our findings corroborate the considerable health benefits of forests in general and therefore support the expansion of tree canopies in rural and urban areas. However, acknowledging the nuanced variations within forests and the inherent trade-offs is imperative when considering silvicultural interventions aimed at specific health outcomes. Nonetheless, increasing tree diversity is a particularly compelling strategy because it generates health benefits without heightening risks while fostering overall forest biodiversity.}},
  author       = {{Gillerot, Loïc}},
  isbn         = {{9789463577571}},
  keywords     = {{Biodiversity,forest,Nature-based Solution,Heat mitigation,Thermal Stress,Environmental health,Environmental psychology,Ecosystem functioning}},
  language     = {{eng}},
  pages        = {{198}},
  publisher    = {{Ghent University. Faculty of Bioscience Engineering}},
  school       = {{Ghent University}},
  title        = {{How forest biodiversity affects human thermal stress mitigation and other health outcomes}},
  year         = {{2024}},
}