How Will Changing Climate Conditions Affect Volatile Organic Compounds in Temperate Forests?

King, Tom and Ashworth, Kirsti and MacKenzie, Rob and Wild, Oliver and Dodd, Ian (2024) How Will Changing Climate Conditions Affect Volatile Organic Compounds in Temperate Forests? PhD thesis, Lancaster University.

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Abstract

The rise in atmospheric CO2 has far-reaching consequences for terrestrial ecosystems, particularly forests. This impact stems from direct CO2 effects, changes in precipitation patterns, and an increased likelihood of drought and heat waves. Forests, major contributors of biogenic volatile organic compounds (BVOCs), play a crucial role in this interaction. Among BVOCs, isoprene (C5H8) is the most abundant in temperate and tropical forests, with high reactivity in the atmosphere, leading to various oxidation products. Other BVOCs in lower quantities in temperate forests exhibit significant photochemical and ecological activity emanating from foliage and the forest floor. This doctoral thesis aims to unravel the impact of elevated CO2 on VOC emissions from a temperate forest. It focuses on isoprene emissions and concentrations from the canopy and BVOC concentrations from the soil. The study explores the short-term effects of drought and high-temperature events on isoprene emissions. It investigates the potential long-term "legacy" effects of drought on isoprene seasonality through changes in leaf phenology. The research, utilising Birmingham's BIFoR FACE facility, represents the first effort to collect isoprene data under long-term CO2 fumigation in a mature forest. Findings reveal decreased leaf-level emissions and canopy concentrations under elevated CO2, aligning with previous laboratory data. Conducting a pioneering field experiment at BIFoR FACE, the study explores the impact of elevated CO2 on soil VOCs. Results indicate a consistent decrease in soil chamber concentrations under elevated CO2, mirroring the canopy trend. Additional experiments on root exudates and leaf litter offer insights into key drivers influencing the decline in soil VOCs. The final study uses isoprene observations over four growing seasons to model isoprene response to drought and heat waves. It highlights the need for models to consider extreme heat and soil moisture effects. Examining Wytham Woods, the study finds no change in isoprene seasonality in 2019 following the 2018 moderate drought, suggesting no discernible long-term legacy effects of drought on emissions. These insights contribute to understanding the nuanced responses of isoprene emissions to environmental stressors.