Walking under fir trees, or bathing in forests in natural environments, helps reduce stress through several interconnected mechanisms and has numerous health benefits. One of the key benefits is the production of serotonin, a neurotransmitter that contributes to feelings of well-being and happiness.

Here are the key ways this natural activity aids in stress reduction:

Inhalation of Phytoncides

Trees, including fir trees, release organic compounds called phytoncides. Inhaling these substances has been linked to increased activity of natural killer cells, which play a role in the immune response and can influence overall mental well-being.

• Immune System Boost
  Phytoncides can enhance the activity and number of natural killer (NK) cells, which play a crucial role in the body's defence against infections and tumours. This immune boost contributes to overall well-being and reduces stress.
  
• Reduced Stress Hormones
  Exposure to phytoncides has been linked to lower levels of cortisol, a hormone associated with stress. Lower cortisol levels are indicative of reduced stress and improved relaxation.
  

Improved Air Quality

Pine forests often have cleaner air due to the presence of trees that filter out pollutants and produce oxygen. Breathing in fresher, cleaner air can improve respiratory health and overall physical well-being, which can contribute to a reduction in stress.

Sensory Stimulation

The natural environment of a pine forest provides a rich sensory experience:

• Visual
  The sight of greenery and the natural landscape can be soothing and reduce mental fatigue. Studies have shown that views of nature can lower stress and improve mood.
  
• Olfactory
  The scent of pine has been found to have calming effects. Aromatic compounds from pine needles and sap can promote relaxation and reduce anxiety.

• Auditory
  The sounds of rustling leaves, birds, and other natural forest noises can have a calming effect, reducing stress and promoting a sense of peace.

Physical Activity

Walking itself is a form of physical exercise, and engaging in moderate physical activity has well-documented benefits for stress reduction.

• Endorphin Release
  Physical activity stimulates the production of endorphins, the body’s natural mood elevators, which can alleviate stress and improve mood.
• Improved Sleep
  Regular physical activity can improve sleep quality, which in turn reduces stress and enhances overall mental health.
  

Psychological Benefits

Spending time in nature, particularly in a forest setting, can provide significant psychological benefits.

• Mindfulness and Presence
  Walking in nature encourages mindfulness, allowing individuals to be present in the moment and engage with their surroundings.
  This mindfulness practice can help reduce anxiety and stress.
  
• Mental Break
  Being in nature provides a mental break from daily routines and stressors.
  This detachment can help clear the mind and reduce stress levels.

Physiological Effects

Several studies have shown that spending time in forest environments can lead to measurable physiological changes.

• Lower Blood Pressure
  Forest environments, including pine forests, have been shown to lower blood pressure, which is a physical marker of reduced stress.
  
• Reduced Heart Rate
  Time spent in nature can lead to a lower heart rate, indicating a more relaxed state.
  
• Improved Parasympathetic Nervous System Activity
  Forest exposure enhances parasympathetic nervous system activity, promoting relaxation and reducing the fight-or-flight response associated with stress.
  

Supporting Studies

1. "The physiological effects of Shinrin-yoku (taking in the forest atmosphere or forest bathing): evidence from field experiments in 24 forests across Japan" by Qing Li et al. (2008) found significant reductions in cortisol levels, pulse rate, and blood pressure among participants, indicating reduced stress.
   
2. "Forest bathing enhances human natural killer activity and expression of anti-cancer proteins" by Qing Li et al. (2010) demonstrated that forest bathing increased NK cell activity and the expression of anti-cancer proteins, suggesting enhanced immune function and reduced stress.
   
3. "Reduction in stress hormone levels and blood pressure by forest bathing: Results of field experiments in 19 forests across Japan" by Tsunetsugu et al. (2007) showed lower cortisol levels and blood pressure in participants who engaged in forest bathing, attributed to the calming environment and the inhalation of phytoncides.
   

In summary, walking under pine trees reduces stress through a combination of inhaling beneficial phytoncides, experiencing improved air quality, engaging the senses, benefiting from physical activity, and experiencing physiological and psychological relaxation.
These elements together create a holistic and powerful stress-reducing experience.