The volatile chemical substances released into the air by forest trees and plants are called phytoncides. The primary substances are a-pinene and isoprene and, more specifically, the terpenoids C10 to C15 monoterpenes, sesquiterpenes and C5 hemiterpenes. The amounts released by the various parts of the trees vary during the daytime and during the seasons. The amounts tend to collect near the centre of a forest and close to the ground.
It is claimed that phytoncides act as insect repellents, anti-microbal, antiseptic and possibly analgesic agents (to wit, aspirin is found white birch bark) and inhibitors of other plants’ growth. Phytoncides are claimed to exert a refreshing effect on humans, for instance via tree essential oils, notably frankincense.
Japanese research has shown that the amounts of volatile substances in the forest air are too small to have any direct chemical impact on human health.
The effect of tree aroma on humans, specifically its placebo effect, has not been ascertained save by dodgy (because mightily biased) aroma therapy researchers.
It is likely that the placebo effects (to wit, benign mental misdirections) of olfactory, visual and auditory flooding of the brain, and which produces an intense positively experienced altered state, produce the beneficial affects of forest bathing so often (subjectively) reported, even from ancient times.
For more detail see:
‘Forest Bathing’, Ed. Qing Li (2013)
‘Die sanfte Medizin der Bäume’, Moser & Thoma (2014)