Understanding the role of abiotic stress in biosphere-atmosphere exchange of reactive trace gases

Otu-Larbi, Frederick and Wild, Oliver and Ashworth, Kirsti (2021) Understanding the role of abiotic stress in biosphere-atmosphere exchange of reactive trace gases. PhD thesis, Lancaster University.

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Abstract

The terrestrial biosphere removes carbon dioxide (CO 2 ) from the atmosphere through photosynthesis and is a major sink for atmospheric pollutants like ozone. It is also the main source of biogenic volatile organic compounds (BVOCs) in the atmosphere. In turn, atmospheric conditions such as temperature, precipitation and photosynthetically active radiation regulate the growth and functioning of plants. The role of some environmental factors like temperature and solar radiation in regulating plant physiological processes are well understood and parameterised in land surface models (LSMs) but others like drought stress and ozone damage are poorly understood and hence poorly represented in LSMs. Yet, these LSMs are integral to climate modelling and climate change mitigation measures as they provide estimates of carbon stored in forests currently and how this will change in future. This thesis seeks to bridge the gap between the latest scientific knowledge about the effects of drought stress and ozone damage on plant physiological processes, and how these stressors are currently modelled. The response of plant isoprene emissions and gas exchange to drought stress and ozone damage was investigated using various model parametrisations, and long-term measurements of isoprene mixing ratios and fluxes, carbon dioxide and water fluxes made in a broad range of forest ecosystems. We find that current isoprene emission models are unable to account for stress-induced isoprene emissions during mild or moderate drought stress leading to underestimation of observed isoprene mixing ratios and fluxes. New methods for modelling isoprene emissions during moderate drought were developed and shown to improve model reproduction of observations. Further, it is shown that both drought stress and ozone damage act to reduce plant productivity and gas exchange. However, the impact of drought stress on vegetation was found to outweigh that of ozone damage at individual forest sites in both present day and future climate scenarios but accounting for the impact of both stress factors provided the best model-observation fit. As global climate changes, abiotic stress factors will become increasingly important in regulating biosphere-atmosphere interactions with potentially negative impacts on plant productivity and hence climate mitigation efforts. These findings highlight the need for more observations, especially in remote and data-sparse regions of the world such as the tropics, to improve understanding of how plants will respond to future stress, and how to better model the impacts.