Dis­ser­ta­tion: The forest BVOC source is more vari­able than has been as­sumed

12.12.2015
Juho Aalto presented his dissertation, Seasonal and spatial variation of VOC emissions from boreal Scots pine forest, for public examination on 10 December 2015. Carlo Calfapietra from the National Research Council/Institute of Agro-Environmental & Forest Biology, Italy, acted as opponent. The dissertation will be published as part of the Dissertationes Forestales series.

Boreal forests are the most significant source of volatile organic compounds (VOCs) in Northern Europe, emissions originating both from trees and forest floor. The VOCs are reactive trace gases that participate in chemical reactions in the atmosphere, thus affecting aerosol formation and climate.

The overall aim of the thesis was to characterize the temporal and spatial variability of VOC emissions and explain the processes and phenomena affecting those. Extensive field measurements were used, including both gas chromatograph and mass spectrometer as VOC analyzers. A dynamic enclosure method was utilized in measuring VOC fluxes from the forest floor and emissions from Scots pine shoots.

The genetic background determines the blend of terpenoids emitted by Scots pine, thus having an effect on the composition of the atmosphere. The forest floor and soil also have a substantial effect on VOC fluxes on the ecosystem scale.

In addition to the considerable spatial variation in VOC fluxes from the forest floor, there is variation of VOC emissions from Scots pine shoots; differences were associated with needle age, seasonality and growth processes. New foliage dominates the VOC emissions from Scots pine foliage during spring and early summer, when growth processes release significant amounts of VOCs, especially monoterpenes. Scots pine shoots are a strong source of monoterpenes during the early stages of photosynthetic recovery; these periods last from a couple of days to about one week and are likely related to the protection of evergreen foliage against photo-oxidative stress.

The studies challenge the presumption of constant emission capacities, which is currently a common presumption in VOC emission inventories. Atmospheric concentrations of VOCs result from an output of the existing sources and their seasonal and spatial variation. This underlines the relevance and importance of details on the large scale. The findings provide new opportunities for developing VOC emission models based on underlying physico-chemical processes.

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