Methodological and mechanistic context for the interpretation of leaf-level spectral chlorophyll-a fluorescence

Abstract

Boreal forests assimilate a substantial fraction of global atmospheric CO2 and thus play a key role in the global carbon cycle. However, due to the prevalence of evergreen species, monitoring photosynthetic dynamics of boreal forests is challenging when using conventional greenness- or vegetation-indices. Fortunately, an increasing body of evidence suggests that chlorophyll-a fluorescence (ChlF) – a weak red-to-far-red radiation emitted by the chlorophyll a molecules nanoseconds after light absorption – can enhance our capacity to assess photosynthetic dynamics in evergreen-dominated ecosystems. However, before extracting complete information embedded in the ChlF, comprehensive understanding and quantitative characterization of the mechanisms that connect the measured ChlF to photosynthesis across various scales are essential. In this thesis, I discuss several challenges that we currently need to face to leverage the full potential of ChlF. I present a roadmap through these challenges, towards a more comprehensive interpretation of ChlF. The main focus is laid on the challenges concerning ChlF measured at a leaf-level in methodological and mechanistic contexts. In other words, this thesis contributes to the interpretation of ChlF by contextualizing the influence that methodological and mechanistic factors have on leaf-level spectral ChlF. An impact of methodological factors, measuring geometry and sample arrangements, on spectral ChlF was analysed. Results indicate that ChlF shape is less dependent on measuring geometry as compared to ChlF magnitude and that if needle-mats are used, measuring geometry does not lower the comparability between studies using different setups. Mechanical factors were investigated in terms of their effect on spatial and temporal variation in spectral ChlF. The diversity of species and light environments within an ecosystem was shown to generate a temporarily-invariant, baseline variation in leaf spectral ChlF, as well as contrasting seasonal photosynthetic acclimation patterns. Consequently, I suggest the need for considering both the methodological and mechanistic contexts in the interpretation of ChlF.