Volatile organic compound emissions are varying a lot based on season and diurnal time since volatility of compound is regulated by radiation and temperature dependence in boreal zone. Volatile organic compounds are released from the vegetation, although the relative amounts are pretty small compared to the summer.
Canopy emits large amount of volatile organic compounds in spring but the mechanism between these emissions peaks has not been discovered yet. The future question is if we can connect these peaks for the air chemistry throught the oxidation products of VOC emissions (ELVOC, extremely low volatile organic compounds) which can be used to explain NPF (new particle formation) events during spring.
Bud bursting is a significant source of volatile organic compounds since plants protect them against insects and fungi. The grow rate is high in spring when plant cells are active and this is also the reason why these emissions increase.
The biggest emission peaks from soil have been discover in spring and other ones in fall when growing season ends. Part of VOC emissions are released in spring when snow melts and compounds bound into snow will be released. We can explain VOC peaks in fall through decomposition of organic litter, mainly conifer litter, which produces VOCs in soil. Tree canopy will not release large amount of VOCs in fall.