Stress-induced BVOC emissions from forests in Sweden

Sammanfattning: A changing climate is expected to lead to more extreme weather events, such as an increased frequency of droughts and forest fires. Changing forest conditions can offer benefits to species such as insects, including bark borers that thrive in warm conditions. Warmer temperatures also favor the acceleration of ozone (O3) production, which can cause stress on vegetation. Vegetation stress can have negative impacts on growth and can also result in increased tree mortality, negatively impacting global forests with deforestation, loss of biodiversity and associated ecosystem services. This thesis aimed to quantify the effect of forest stress from the European spruce bark beetle (Ips typographus), elevated O3 concentrations and forest fires by analyzing the biogenic volatile compound (BVOC) emissions from both healthy and stressed trees.The results of this thesis revealed that bark beetle infestations appear as a critical stressor, increasing the total BVOC emissions by 7 900%. Recovery after forest fires, on the other hand, indicated a 54% to 90% reduction in emission rates. Similarly, elevated O3 concentrations revealed an 80% reduction in BVOC emissions from mountain birch leaves. However, after exposure the emissions were instead boosted with 16% higher emissions compared to healthy leaves. In addition to the total emission rates, the importance of the individual compounds were highlighted in this thesis. Bark beetle infestations caused significant increases for all measured compounds, plus the addition of eucalyptol not seen from healthy bark. Even if there was a reduction in the total emission rates from the forest fire recovery and the O3 exposure, increases were seen for individual compounds. This can have substantial implications for atmospheric chemistry as certain compounds found to increase have a higher capacity for secondary organic aerosol (SOA) formation. Scaling up the impacts to cover all of Sweden, the study estimates a 2% increase in BVOC emissions when considering all stresses concurrently.This thesis provides an insight into how stress from drought, bark beetle and forest fire can have long-term impacts on BVOC emissions. The quantified emissions are an important contribution to our knowledge on vegetation response to stress. These results can be used as a basis for better prediction of future emission changes in atmospheric and ecosystem models. Additional applications of the thesis results can be to monitor forest status by using the quantified stress emissions as indicators of forest stress.