Hanna Valolahti:
Impacts of climate change induced vegetation responses on BVOC emissions from subarctic heath ecosystems

Date: 07-09-2015    Supervisor: Riikka Rinnan

The role of biogenic volatile organic compounds (BVOCs) affecting Earths’ climate system is one of the greatest uncertainties when modelling the global climate change. BVOCs presence in the atmosphere can have both positive and negative climate feedback mechanisms when they involve atmospheric chemistry and physics.

Vegetation is the main source of BVOCs. Their production is directly linked to temperature and the foliar biomass. On global scale, vegetation in subarctic and arctic regions has been modeled to have only minor contribution to annual total BVOC emissions. In these regions cold temperature has been regulating annual plant biomass production, but ongoing global warming is more pronounced in these regions than what the global average is. This may increase the importance of subarctic and arctic vegetation as a source of BVOC emissions in near future.

This thesis aims to increase the understanding of the controls of BVOC emissions from subarctic ecosystems under climate change by studying the responses to long-term manipulations from leaf level to small ecosystem scale. Leaf-level studies showed different anatomical responses for warming and shading manipulations between studied species, but no significant effects on BVOC emissions on plant individual level were found. The lack of changes in BVOC emissions after longterm exposure could be at least partially explained by long term-acclimation, which is supported by the observed anatomy responses. Whereas warming was not found to alter the BVOC emissions on plant individual level, emissions from ecosystem level was found to increase significantly. Anatomical acclimations and decrease in BVOC emissions on plant individual level have probably been compensated for by rapidly increasing foliar biomass on ecosystem level and have lead to increases in total BVOC emissions from subarctic ecosystems.

Ecosystem level emissions showed two-fold increase in monoterpene emissions and over five-fold increase in sesquiterpene emissions after a decade of warming. These increases were probably driven by the changes in vegetation composition. Increased autumnal leaf litter amount simulated by litter addition experiment was found to increase the plant biomass production, probably due to increased soil nutrient availability. The observed increase in BVOC emissions from litter addition treatment was therefore partially explained by increasing plant biomass and partially by increasing amount of BVOCs released from the decomposing litter.

According to the results presented in this thesis, I suggest the pronounced increase of BVOC emissions found after long-term exposure to climate manipulations is mainly carried indirectly via vegetation changes. Climate manipulations used in this study increased the ecosystem-scale BVOC emissions, supporting the prediction of increasing importance of subarctic regions for global BVOC emissions in near future.