Completed PhD Theses
Physiological responses of six temperate tree species to water limitation
By Lars Dietrich
Supervisor: Prof. Dr. Ansgar Kahmen
Direct evidence for the link between stem diameter variations (SDV) and the daily canopy water status, i.e. daily water potentials (Ψ), is rare, particularly for tall trees. It thus remains unclear up to what degree SDV readings are useful to estimate daily canopy Ψ. We measured SDV with point dendrometers at the stem base of tall, mature individuals of six European forest tree species in a near-natural temperate forest and compared them to daily canopy Ψ during the growing seasons of 2014 (wet) and 2015 (dry). SDV were de-trended for growth with two different approaches leading to the socalled tree water deficit (TWD). We found that midday Ψ can be predicted from TWD, independent of the growth-de-trending procedure to obtain TWD from SDV. Further, daily TWD was a better indicator for daily midday Ψ, particularly under dry conditions, than maximum daily shrinkage (MDS), another common quantity derived from SDV. Based on data from six temperate tree species, we conclude that TWD measured at the stem base is a consistent proxy for daily canopy midday Ψ of tall trees over the entire range of measured conditions.
Link to full text (download restricted until 1 June 2020): https://baselbern.swissbib.ch.
Biochemical and metabolic effects on hydrogen isotope composition of organic compounds in plants
By Marc-André Francis Cormier
Supervisors: Prof. Dr. Ansgar Kahmen
Despite a large unexplained variability in the values, the analysis of the hydrogen (H) isotope composition (δ2H) of plant organic compounds is confidently applied to assess ecohydrological processes in the environment. This is possible because most studies that use the stable H isotopes from plant-derived biomarkers consider the H isotope fractionation that occurs during the biosynthesis of any given compound (2H-εbio) to be constant within a species. Consequently, δ2H values in plant organic compounds are assumed to be mainly driven by the plant's source water δ2H values and the leaf water evaporative 2H-enrichment.There are, however, several indications that 2H-εbio of plant organic compounds can vary and that the δ2H values are also related to the plant's metabolism.
In this thesis, the elucidate the puzzling variability in the δ2H values of plant organic compounds, the influence of the plant's metabolism on the variability of 2H-εbio is specifically explored with regards to the possiblebiochemical mechanisms underlying this variability. In a first study, based on empirical data produced in two separate experiments, it is shown that the 2H-εbio of different compounds in plants is tightly coupled to a plant's carbonand energy metabolism. Based on these data, we develop a conceptual biochemical model that explains how and where 2H-fractionation occurs in the biosynthesis of major plant organic compound classes such as carbohydrates and lipids and what the isotope fractionation processes are that introduce a metabolic signal in δ2H values to these compounds.
In a second study, with δ2H analyses from heterotrophic plants, it is shown that the hydrogen isotope fractionation occurring during the biosynthesis of different organic compounds in plants can explain part of the variability observed in δ2H values across species. Metabolic effects on δ2H values between heterotrophic plants and their autotrophic hostplants differed for different compound classes. The remarkable consistency of the compound 2H as a plant metabolic proxy specific isotope effects between autotrophic host or reference plants and the heterotrophic parasitic or mycoheterotrophic plants points towards a general physiological mechanism thatdetermines these effects and support the model developed in the first study.Finally, with this model, it is mechanistically illustrated that information recorded inthe δ2H values of plant organic compounds goes beyond hydrological signals, but alsocontains important information on the carbon and energy metabolism of a plant. As such we provide the mechanistic basis to introduce hydrogen isotopes in plant organic compounds as new metabolic proxy for the carbon and energy metabolism of plants. Such a new metabolic proxy has the potential to be applied in a broad range of disciplines, including plant and ecosystem physiology, plant breeding, biogeochemistry, paleoecology and Earth system sciences.
Thesis submitted to ETH Zürich.
Ecophysiological responses of four common Swiss tree species to global environmental change
By Nadine Brinkmann
Supervisors: Prof. Dr. Ansgar Kahmen; Prof. Dr. Nina Buchmann
Being a substantial part of the global water cycle, forest ecosystems play an important role in the Earth's climate as they return about 60% of the annual land precipitation back to the atmosphere via transpiration. Predictions of global environmental changefor Europe and Switzerland encompass, e.g., an increase in the frequencies of heatwaves and drought events, such as the European heat wave in 2003 and changes inprecipitation patterns. Temperate forests (including Swiss forests) are expected to be particularly vulnerable to droughts as they are not adapted to such conditions and perform best in mesic environments. Water is a key resource for physiological processesin trees (i.e., transpiration, photosynthesis and growth) and a deeper understanding inhow far different tree species are able to respond in their tree water relations to globalenvironmental changes allows conclusions about a species' sensitivity to such changes. Previous work found temperate tree species to differ in their ecophysiological responseto changes in their environment, however, evidence about the causes of species-specific responses is still rare.
In this doctoral thesis, two studies aim to increase the knowledge on species-specific responses and sensitivities - above and belowground - of four common Swiss tree species (with European distribution) in their tree water relations to environmental changes such as changes in soil water availability. Build on the data and knowledge gained from the second study, a third study focused on resolving the temporal origin of precipitation insoil and xylem water and on understanding how precipitation δ2H and δ18O signals are coupled to soil and xylem water δ2H and δ18O values in a temperate forest, which will further help to interpret plant water isotope models that aim to reconstruct climate.
All three studies in this thesis have been carried out in the framework of the iTREE project funded by the Swiss National Foundation.
Thesis submitted to ETH Zürich.
Partial transfer of leaf water ²H-enrichment and variable biosynthetic fractionation affect the leaf wax n-alkane δ²H values in grasses
By Bruno Gamarra
Supervisors: Prof. Dr. Ansgar Kahmen
Leaf wax n-alkanes are long chained hydrocarbons contained in the cuticle of terrestrialplants. Leaf wax n-alkane δ2H values have been successfully used to study the hydrological cycle, plant-water and plant-carbon relationships. However, the physiological and biochemical drivers that shape leaf wax n-alkane δ2H values are not completely understood. It is particularly unclear, why n-alkanes in grasses are typically 2H-depleted compared todicotyledonous plants and why C3 grasses are 2H-depleted compared to C4 grasses. Also, the timing of n-alkane synthesis and the de novo synthesis of n-alkanes in fully matured leaves are still matter of debate. On top, calibration studies designed to resolve sources of heterogeneity in n-alkane δ2H values have exclusively focused on n-alkanes derived from leaves. To solve these uncertainties three studies aimed to explore how leaf wax n-alkane δ2Hvalues in grasses are influenced by plant physiological and biochemical processes.
In the first study the effect of leaf water evaporative 2H-enrichment (LW Δ2H) on n-alkane δ2H valueswas quantified for a range of C3 and C4 grasses that were grown in climate chambers undercontrolled environmental conditions. It was found that only a fraction of the LW Δ2H isimprinted on the n-alkane δ2H values in C3 and C4 grasses. It was also detected that thebiosynthetic hydrogen isotope fractionation (εbio) was different for C3 and C4 grasses. As such, differences between leaf wax n-alkane δ2H values of C3 and C4 grasses would not bedriven by LW Δ2H but largely the result of systematic differences in εbio between these two plant groups.
In the second study the timing of the leaf wax n-alkane synthesis was investigated.Specifically the secondary leaf wax n-alkane synthesis was tested in mature leaf blades of C3 grass species. The experiments showed an incorporation of hydrogen from highly 2H enrichedirrigation water into the wax n-alkanes from mature leaves. Mature grass leavescontinued the synthesis of wax n-alkanes after leaf emergence. The rate of secondary nalkanessynthesis was, however, relatively low and varied among species from 0.09% to1.09% per day. As such, leaf wax n-alkane δ2H values would be determined mainly byenvironmental and physiological conditions in the beginning of the life of a leaf.
In the third study n-alkane concentration and δ2H values of different grass organswere surveyed. Leaves, sheaths, stems, inflorescences and roots were sampled from a totalof 15 species of C3 grasses in temperate and alpine grasslands in Switzerland. It wasdetected that inflorescences had typically much larger n-alkane concentrations compared toother organs while roots had very low n-alkane concentrations. The δ2H values of the carbonautonomous plant organs such as leaves, sheaths and stems were in general more negativecompared to the non-carbon autonomous organs such as inflorescences and roots. Variablen-alkane δ2H values in different plant organs could be the result of differences in the HNADPHbiosynthetic origin in response to the carbon autonomy of the plant organ.Overall, this thesis brings new insights into the natural variability of leaf wax n-alkane δ2H values in grasses. The incomplete transfer of LW Δ2H to the leaf wax n-alkanes δ2Hvalues in grasses can explain why grasses are typically 2H-depleted compared todicotyledonous plants. The low secondary leaf wax n-alkane synthesis in grass leaves aftermaturity suggests that in general leaf wax lipid δ2H values do not record environmental andplant physiological processes beyond leaf maturity. Finally, a different εbio between C3 andC4 grasses and also between grass organs suggests that n-alkane δ2H values have a great potential as indicator of changes in plant carbon autonomy. This has important implicationsfor the interpretation of n-alkane δ2H values in plant physiology and paleoecology.
Thesis submitted to ETH Zürich.