Characteristics of BVOC emissions from a Swedish boreal forest: Using chambers to capture biogenic volatile organic compounds (BVOCs) from trees and forest floor

Plant-emitted biogenic volatile organic compounds (BVOCs) are a large group of hydrocarbons released by plant leaves, bark, flowers and fruits into the atmosphere and from plant roots into the soil. BVOCs have important physiological and ecological functions, such as mediating within-plant and plant-plant communication, defense against herbivores attack, protection against heat or other oxidative stress, attracting pollinators etc., and BVOCs are also important for microorganisms’ communication and nutrient cycling in the soil. These emitted reactive BVOCs also have impact on atmospheric chemistry, such as affecting the oxidation rate and atmospheric concentration of other trace gases, and the oxidized BVOCs contribute to particles formation and growth. The most abundant BVOCs are terpenes, such as isoprene (C₅H₈), monoterpene (MT, C₁₀H₁₆) and sesquiterpene (SQT, C₁₅H₂₄), and woody plants tend to emit various blends of terpenes. Norway spruce and Scots pine are two dominant boreal species, which have been recognized as MT emitters, and Norway spruce is also known as a low isoprene emitter. The contribution of forest floor to the ecosystem BVOC emissions has not been thoroughly studied and evaluated.A dynamic branch chamber system was used in this thesis to quantify the BVOC emission rates and emission spectra (composition of compounds) from 20-m and lower canopy levels of a Norway spruce and from 20-m canopy of a Scots pine from June to September of 2013 and 2014. The observed BVOC emissions from 20-m canopy of Norway spruce peaked in August 2013 and July 2014, and the minimum was found in September in both two years. The total BVOC emission rates of 2013 were significantly higher than those of 2014, and these high emissions in 2013 were likely induced by insect attack. High induced MT emissions from Scots pine were also observed in September 2014. Besides the long-term observation including seasonal variations are needed for accurately estimating or scaling up BVOC emissions, the stress-induced BVOC emissions are necessary to be incorporated into the emission models given the observed high amount of induced emissions. No clear vertical distribution pattern of BVOC emission rates was found within-canopy of the Norway spruce, and the compounds detected on different canopy levels were quite similar.
A dynamic soil chamber was used to quantify the BVOC emissions from the forest floor from June to October of 2015. The peak emission was observed in October. Litterfall might be an important source for MT emissions, especially in autumn. Air temperature inside chamber and PAR (photosynthetically active radiation) were the most influential environmental variables affecting MT and SQT emissions. The understorey vegetation coverage and composition and soil moisture also have impact on the BVOC emissions from the forest floor.