[Simpraga2012] "Understanding the link between photosynthesis, growth and emissions of biogenic volatile organic compounds (BVOCs) in beech, oak and ash",
: Ghent University, 2012.
Gas exchange between vegetation and the atmosphere is very dynamic. In addition to gases such as carbon dioxide (CO2), water vapor, oxygen, nitrogen oxides (NOx), sulphur dioxide, ammonia and ozone (O3), also biogenic volatile organic compounds (BVOCs) are exchanged between the vegetation and the atmosphere. This PhD focussed on the exchange of CO2 and BVOCs, since net photosynthesis (Pn) and BVOC emission are two plant processes important in plant functioning. Vegetation, and forests in particular, acts as a major source of BVOCs. The importance of the study lays in understanding the link between Pn, BVOC emissions and tree growth. BVOC emissions indirectly affect climate change as BVOCs are in combination with atmospheric NOx the main precursors of photochemical O3 in the troposphere, where it acts as potential greenhouse gas, damaging vegetation and affecting human respiratory organs. BVOCs are therefore dominant reactive compounds in the troposphere and important in atmospheric chemistry and climatology. Understanding tree chemistry and ecophysiology is crucial to predict future changes in the Earth’s carbon balance as well as to update BVOC inventories and improve predictions in tropospheric air chemistry. Accordingly, the main goals of the PhD were to identify and quantify the effects of temperature, drought, seasonality and vertical canopy gradients on Pn and BVOC emissions. The general methodology consisted of developing and constructing enclosure systems for gas exchange measurements indoors and outdoors, where coupling of an infra-red gas analysis (IRGA), proton transfer reaction-mass spectrometry (PTR-MS) and thermal desorption gas chromatography/mass spectrometry (TD-GC/MS) represented a major challenge. With respect to tree species, the focus was on European beech (Fagus sylvatica L.), while additionally common ash (Fraxinus excelsior L.) and northern red oak (Quercus rubra L.) were examined in Chapter 4. The trees were examined in growth room conditions, at the campus and in the Aelmoeseneie experimental forest. The main variables measured were Pn and BVOC emissions, in particular of monoterpenoids (MTs). In addition, microclimatic variables (air temperature, photosynthetic photon flux density, soil water potential, and vapor pressure deficit) and leaf characteristics (specific leaf area, leaf temperature, leaf pigments, and leaf water potentials) were measured. In the growth room experiments, stem diameter variations and chlorophyll indices were measured to explain the behavior of MT emissions by young beech trees. In the forest, the experimental tower showed to be an important facility for adequate local characterization of adult beech Pn and BVOC chemistry. Leaf level studies showed to be crucial for unraveling the mechanisms behind the emission of BVOCs. The results indicated a large variability in BVOC emission patterns of different tree species. Temperature, drought, seasonality, vertical canopy gradients differently influenced Pn and BVOC emissions (and in particular MTs), as well as their ratio. Indoors and outdoors day-time Pn, MT emissions and MT/Pn carbon ratio varied in a systematic manner following light and temperature changes. The results indicated that not only light affected Pn, MT emissions and MT/Pn ratio, but also showed a pronounced temperature effect on MT emissions (and hence on the MT/Pn carbon ratio), with an increasing exponential trend with rising air temperatures. Furthermore, during drought stress MT emissions showed an increasing-decreasing trend depending on the drought severity. Linear variable displacement transducers (LVDTs) showed to be useful for stress quantification in BVOC studies. Another notable finding was that, under severe drought stress, two PTR-MS signals diverged from each other, indicating the possible presence of BVOC species other than MT such as green leaf volatiles (GLVs). Seasonal measurements on anatomically different trees indicated a strong temperature rather than light dependency when looking at total BVOC emission trends. Beside substantial quantities of MTs released from leaves into the atmosphere, driven by light and temperature, beside non-MTs, MTs also showed to play a role in plant-insect interactions. Detected stress compounds proved infestiation-based emissions. Consequently, plant-insect relationships require additional research, identifying individual MT species using the GC/MS speciation approach and looking at their relationships with ecophysiological parameters. In conclusion, the performed indoor and outdoor studies demonstrated that Pn and BVOC emissions are strongly interrelated. Proposed hypotheses were tested and confirmed. However, many unanswered questions remain, e.g. how the distribution of individual BVOC compounds correlated with temperature and drought stress as well as along the vertical canopy gradient.